A specimen is categorized as Non-Diagnostic when it lacks sufficient well-preserved, interpretable pancreaticobiliary epithelial cells for evaluation; this may be due to various quantitative or qualitative limitations.

Cells

─ Insufficient number of diagnostic epithelial cells (ductal, acinar), or cells are absent altogether

─ Presence of only gastrointestinal contaminants (eg, gastric or duodenal mucosa not accompanied by pancreatic elements)

─ Presence of only inflammatory cells and benign debris (unless clinical-radiologic context suggests an abscess and no mass lesion is targeted)

─ Presence of only benign, non-mucinous cyst contents (eg, histiocytes, macrophages) without an epithelial lining

─ Acellular specimen (eg, only blood, acellular mucin, or clear fluid)

Cytoplasm

─ Cytoplasmic features are obscured by artifact, degeneration, or there are insufficient cells for assessment

Nuclei

─ Nuclear features are obscured by artifact, degeneration, or there are insufficient cells for assessment

Background

─ Markedly obscured by excessive blood, thick mucus, extensive inflammation, or widespread necrosis, preventing adequate visualization of any epithelial cells

─ Significant preparation artifacts (eg, extensive air-drying, crush artifact, poor fixation, or inadequate staining) preclude interpretation

─ Composed solely of non-mucinous cyst fluid without a diagnostic cellular component

─ Presence of only ultrasound gel precipitate

Absent

─ An adequate, well-preserved, and interpretable pancreaticobiliary epithelial component

─ Specific features of a benign lesion when only non-specific cyst contents are seen (eg, background mucin for a mucinous cyst, or hematoidin with mixed inflammation for a pseudocyst, if only histiocytes are present)

─ Definitive features of malignancy

Note

─ Common examples of Non-Diagnostic specimens include: acellular samples; those with only blood or scant cellular elements; specimens composed solely of gastrointestinal contaminants; non-mucinous cyst fluid lacking epithelium; or samples severely obscured by artifact or necrosis

─ Any sample with significant cellular atypia (classified as Atypical or higher) is generally considered adequate for evaluation, even if cellularity is limited

─ Specimens composed exclusively of normal pancreatic elements may be deemed Non-Diagnostic if a discrete mass or lesion was targeted (indicating a potential sampling miss)

─ Clinical and imaging correlation is crucial

─ Repeat FNA, often with EUS-guidance and ideally with Rapid On-Site Evaluation (ROSE), is typically recommended for a definitive diagnosis

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Benign / Negative (for Malignancy)

A specimen containing adequate cellularity and unequivocally benign cytologic features, which may or may not be diagnostic of a specific benign entity.

Cells

─ Normal pancreatic elements: acinar cells, ductal cells, +/- islet cells

─ Acinar cells: cohesive grape-like clusters, +/- attached to fibrovascular stroma; round, eccentric nuclei; small nucleoli; abundant granular cytoplasm

─ Ductal cells: cohesive flat sheets, honeycomb pattern; round/oval nuclei; inconspicuous nucleoli; non-mucinous cytoplasm

─ Inflammatory cells: neutrophils, lymphocytes, plasma cells, histiocytes, giant cells (depending on etiology)

─ Contaminants: Gastric or duodenal epithelium (must be accompanied by pancreatic elements to be considered diagnostic)

Cytoplasm

─ Acinar: abundant, granular, bichromatic

─ Ductal: moderate, non-mucinous, well-defined borders

Nuclei

─ Acinar: round, eccentric, fine chromatin, small nucleolus

─ Ductal: round/oval, evenly spaced, fine chromatin, inconspicuous nucleolus

─ Minimal atypia, no significant enlargement, pleomorphism, or irregularity

Background

─ Clean or may show inflammation, fat necrosis, calcific debris (chronic pancreatitis)

─ May contain extracellular mucin (if GI contaminant present)

─ May contain cyst fluid elements (histiocytes, debris) but with identifiable benign epithelium

Absent

─ Significant cytologic atypia

─ Features suspicious for or diagnostic of malignancy

Note

─ Includes normal pancreatic tissue (only if no mass lesion targeted), chronic pancreatitis, autoimmune pancreatitis, pseudocysts (if epithelium present), lymphoepithelial cysts, serous cystadenoma, schwannoma, lymphangioma

─ Benign neoplasms like serous cystadenoma are included here

─ Normal findings alone may be nondiagnostic if a specific mass was targeted

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Normal and Contaminants

This section details normal cellular components of the pancreas and common contaminants encountered in pancreatic FNA specimens.

Normal Pancreatic Acinar Cells

Cells derived from the exocrine acini of the pancreas.

Cells

─ Cellularity often high, few single cells

─ Cohesive, grape-like (acinar) clusters, often attached to fibrovascular stroma

Cytoplasm

─ Abundant, granular (zymogen granules), often bichromatic (basophilic at base, eosinophilic at apex)

─ Cell borders indistinct within clusters

Nuclei

─ Round to oval, typically eccentric

─ Finely granular, evenly distributed chromatin

─ Single, small, but often distinct/prominent nucleolus (can be larger if reactive)

─ Some naked nuclei may be present

Absent

─ Significant pleomorphism, hyperchromasia, or nuclear membrane irregularity

─ Mucinous features

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Normal Pancreatic Ductal Cells

Cells lining the pancreatic ductal system.

Cells

─ Cohesive flat sheets, often in a honeycomb arrangement

─ May form small tubules or clusters

Cytoplasm

─ Scant to moderate, pale, non-mucinous, or finely vacuolated

─ Well-defined cell borders

Nuclei

─ Round to oval, relatively uniform in size and shape, evenly spaced

─ Finely granular, evenly distributed chromatin

─ Inconspicuous nucleoli

─ Smooth nuclear membranes

Absent

─ Significant crowding, stratification, or loss of polarity (unless reactive)

─ Prominent atypia

Note

─ Unlike larger mesothelial cells, typically lack prominent intercellular "windows"

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Duodenal Epithelium (Contaminant)

Epithelial cells from the duodenum, a common contaminant in EUS-FNA of the pancreatic head.

Cells

─ Cohesive flat sheets (honeycomb) or papillary-like groups (representing villi)

─ Biphasic population: enterocytes and goblet cells

Cytoplasm

─ Enterocytes: columnar, non-mucinous, with an apical brush border (cilia-like, but shorter, denser)

─ Goblet cells: distended with mucin, "fried egg" appearance in sheets, nucleus pushed to base

Nuclei

─ Enterocytes: oval, basally located, uniform

─ Goblet cells: small, compressed, basal

─ Intraepithelial lymphocytes ("sesame seeds") often scattered within the epithelium

Background

─ May see detached brush borders, mucin

Absent

─ True pancreatic acinar or islet cells (unless mixed sample)

Note

─ Presence of brush border and goblet cells are key diagnostic features

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Gastric Epithelium (Contaminant)

Epithelial cells from the stomach, a common contaminant in EUS-FNA.

Cells

─ Small sheets, strips, or isolated cells; may see intact gastric pits/crypts

─ Predominantly foveolar (surface mucous) cells; chief and parietal cells less common on FNA

Cytoplasm

─ Foveolar cells: columnar, with distinct apical mucin caps (neutral mucin)

─ Chief cells (if present): granular, basophilic

─ Parietal cells (if present): larger, eosinophilic, often fried-egg appearance

Nuclei

─ Foveolar cells: small, round to oval, basal, often bland; may see nuclear grooves in naked foveolar nuclei

─ Chief/Parietal cells: round, central

Absent

─ Brush border, goblet cells (helps distinguish from duodenal)

─ True pancreatic acinar or islet cells (unless mixed sample)

Note

─ Apical mucin caps are characteristic of foveolar cells

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Mesothelium (Contaminant/Rare Primary)

Cells lining serosal surfaces, may be sampled if the lesion abuts the peritoneum or during transephrenic approaches.

Cells

─ Flat sheets, clusters, or single cells

─ Characteristic intercellular "windows" or clefts between cells in sheets

Cytoplasm

─ Moderate amount, dense or somewhat pale, may be "two-toned"

─ Well-defined cell borders

Nuclei

─ Round to oval, often centrally located

─ Smooth nuclear contours

─ Finely granular chromatin

─ Nucleoli can be small to prominent, especially if reactive

─ Binucleation or multinucleation can be seen, especially in reactive states

Absent

─ True glandular formation, mucin production (vacuoles can be present)

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Hepatocytes (Contaminant)

Liver cells, may be sampled if the lesion is near or involves the liver, or during transhepatic FNA.

Cells

─ Large, polygonal cells; may be in small groups, trabeculae, or single

Cytoplasm

─ Abundant, granular, eosinophilic

─ May contain lipofuscin (brown pigment) or bile pigment

Nuclei

─ Round to oval, often centrally located

─ Prominent, single, centrally located nucleolus is characteristic

─ Binucleation common

─ Chromatin can be vesicular

Absent

─ Ductal structures with honeycomb pattern (unless bile ductules are sampled)

─ Acinar formations

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Acute Pancreatitis

Acute inflammatory condition, often associated with gallstones or alcohol abuse.

Cells

─ Mostly neutrophils

─ Scant reactive/reparative ductal cells may be present

─ Foamy histiocytes

─ Fat necrosis (ghost-like outlines of adipocytes, saponification)

Cytoplasm

─ Ductal cells (if present): May show reactive changes, vacuolization

Nuclei

─ Ductal cell nuclei (if present): May be enlarged, have irregular nuclear membranes, and conspicuous nucleoli (reactive atypia)

Background

─ "Dirty" background with abundant inflammatory debris, necrotic material

─ Fibrin strands

─ Hemorrhage

─ Calcifications may be present, especially in necrotizing pancreatitis

Absent

─ Significant number of lymphocytes or plasma cells (unless evolving to chronic)

─ Granulomas (unless specific etiology like TB)

─ Malignant cells

Note

─ Cytologic findings reflect the acute inflammatory process and tissue damage

─ Reactive atypia in ductal cells should not be overinterpreted as malignancy in this context

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Cholangitis

Inflammation of bile ducts, often due to obstruction (e.g., stones) or infection.

Cells

─ Reactive bile duct epithelial cells, often in cohesive sheets or small groups

─ Neutrophils, lymphocytes, plasma cells, eosinophils (variable depending on etiology)

─ Histiocytes

Cytoplasm

─ Bile duct cells: May show vacuolization, bile staining

Nuclei

─ Bile duct cells: May be enlarged, show variable size (mild anisonucleosis), irregular contours, and prominent nucleoli (reactive atypia)

─ Chromatin is often finely granular but may be slightly coarse

Background

─ Inflammatory debris

─ Bile pigment may be present

Absent

─ Unequivocal features of malignancy (though reactive atypia can be marked)

─ Pancreatic acinar or islet cells (unless sampling pancreatic head or tail lesions extending to hilum)

Note

─ Reactive atypia in bile duct cells can be significant and mimic adenocarcinoma (cholangiocarcinoma); features favoring malignancy include significant loss of honeycomb pattern, marked nuclear irregularity, high N:C ratio, and numerous single atypical cells

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Chronic Pancreatitis

Progressive inflammatory condition leading to irreversible pancreatic damage, fibrosis, and loss of exocrine/endocrine function.

Cells

─ Often a hypocellular smear

─ Atrophic acinar cells (may be sparse or absent)

─ Ductal epithelial cells, often showing reactive atypia

─ Fibroblasts and myofibroblasts (spindle cells), may be in cellular stromal fragments

─ Chronic inflammatory cells: Lymphocytes, plasma cells, histiocytes

─ +/- Islet cells (may appear relatively prominent due to acinar atrophy)

Cytoplasm

─ Ductal cells: May show reactive changes, vacuolization, squamous metaplasia

─ Acinar cells (if present): Often atrophic, cytoplasm may be pale or vacuolated

Nuclei

─ Ductal cells: May be enlarged, show variable size, irregular contours, and prominent nucleoli (reactive atypia)

─ Acinar cells (if present): May be small and pyknotic, or show reactive enlargement

─ Fibroblasts/Myofibroblasts: Elongated, bland nuclei

Background

─ Fibrous stromal fragments, sometimes cellular

─ Calcifications (may be grossly gritty during FNA)

─ Fat necrosis

─ Inspissated proteinaceous material (protein plugs in ducts)

Absent

─ Unequivocal features of malignancy (though reactive atypia can be a significant pitfall)

─ Abundant neutrophils (unless there is an acute exacerbation)

Note

─ Reactive atypia in ductal cells can be marked and mimic adenocarcinoma

─ Cellular stromal fragments with spindle cells may mimic a mesenchymal neoplasm but typically lack significant atypia or mitoses

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Groove & Paraduodenal Pancreatitis

A specific form of chronic pancreatitis affecting the anatomical "groove" between the pancreatic head, duodenum, and common bile duct.

Clinical

─ Often mimics pancreatic head carcinoma clinically and radiologically

Cells

─ Similar to chronic pancreatitis: Often hypocellular

─ Reactive ductal cells, may show significant atypia

─ Spindle cells (fibroblasts, myofibroblasts) often prominent, may form clusters

─ Chronic inflammatory cells (lymphocytes, plasma cells)

─ +/- Duodenal epithelium (columnar cells with brush borders, goblet cells) if sampled

─ +/- Brunner gland hyperplasia (clusters of bland cuboidal cells with pale cytoplasm) if sampled

Cytoplasm

─ Ductal cells: May show reactive vacuolization

─ Spindle cells: Elongated, pale

Nuclei

─ Ductal cells: May show reactive enlargement, irregular contours, prominent nucleoli

─ Spindle cells: Typically bland, elongated, may appear plump or crowded

Background

─ Fibrous stroma, may be dense and hyalinized

─ Chronic inflammation

─ +/- Cystic degeneration, pseudocyst formation

─ +/- Calcifications

Absent

─ Unequivocal features of malignancy

Note

─ Prominent spindle cell proliferation (myofibroblastic) is a characteristic feature, often more pronounced than in typical chronic pancreatitis

─ Reactive atypia in ductal cells entrapped in fibrosis can be marked and mimic adenocarcinoma

─ Presence of Brunner gland hyperplasia or duodenal elements helps confirm the anatomical location

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Autoimmune and IgG4-related Pancreatitis

An inflammatory condition of the pancreas, often part of systemic IgG4-related disease, characterized by a dense lymphoplasmacytic infiltrate and often storiform fibrosis.

Clinical

─ May present as a discrete pancreatic mass (mimicking carcinoma) or diffuse pancreatic enlargement

─ Can cause obstructive jaundice

─ Often associated with elevated serum IgG4 levels

─ May involve other organs (e.g., bile ducts, salivary glands, kidneys, retroperitoneum)

─ Typically responds well to steroid therapy

Cells

─ Often hypocellular on FNA

─ Lymphocytes and plasma cells, sometimes numerous and forming aggregates

─ +/- Eosinophils

─ Ductal cells, may show reactive atypia

─ Acinar cells, often atrophic or absent

─ Spindle cells (fibroblasts from stroma)

Cytoplasm

─ Ductal cells: May show reactive changes

─ Plasma cells: Typically show eccentric nuclei, basophilic cytoplasm, and a perinuclear hof

Nuclei

─ Ductal cells: May show reactive atypia (enlargement, prominent nucleoli)

─ Lymphocytes: Mostly small, round, with dark, condensed chromatin

─ Plasma cells: Eccentric, with characteristic clock-face chromatin

─ Fibroblasts: Bland, elongated

Background

─ Cellular stromal fragments composed of fibroblasts, lymphocytes, and plasma cells

─ Storiform fibrosis (a swirling, "cartwheel" pattern of fibrosis) is a key histologic feature but often not apparent on FNA

─ +/- Obliterative phlebitis (inflammation and fibrosis of veins) is another histologic feature rarely seen on FNA

Absent

─ Unequivocal features of malignancy

─ Granulomas

─ Abundant neutrophils (unless there is a secondary acute pancreatitis component)

Ancillary studies

─ Serum IgG4 levels are often elevated (but can be normal)

─ IgG4 immunostaining (on cell block or biopsy, if sufficient material) can show an increased number of IgG4-positive plasma cells (e.g., >10/HPF or IgG4/IgG ratio >40% are common thresholds, but vary); sensitivity on FNA material is limited

DDx

─ Pancreatic ductal adenocarcinoma (especially when reactive ductal atypia is prominent)

─ Other forms of chronic pancreatitis (the lymphoplasmacytic infiltrate can be nonspecific)

─ Lymphoma (especially if the lymphoid infiltrate is very dense and appears monotonous)

Note

─ FNA findings are often nonspecific, resembling chronic pancreatitis with a prominent lymphoplasmacytic component

─ Diagnosis usually relies on a combination of clinical presentation, serologic findings (IgG4 levels), imaging characteristics, cytologic/histologic features, and response to steroid therapy

─ Key histologic features like storiform fibrosis and obliterative phlebitis are difficult to assess reliably on FNA alone

─ Reactive ductal atypia can be a significant diagnostic pitfall for adenocarcinoma

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Lymphoepithelial Cyst

Benign cyst, unilocular or multilocular; squamous lining plus lymphoid tissue in the cyst wall.

Clinical

─ Can occur in peripancreatic lymph nodes or intrapancreatic

─ May be associated with HIV infection or other causes of lymphoid proliferation

Cells

─ Mature squamous cells, often anucleated (squames) or with small pyknotic nuclei

─ Lymphocytes (small, mature, in aggregates or germinal centers)

─ Histiocytes, including multinucleated forms

Background

─ Proteinaceous fluid, often thick

─ Abundant keratinous debris

─ +/- Cholesterol crystals

Absent

─ Significant atypia

─ Mucin-producing cells

─ Features of malignancy

Ancillary studies

─ Cyst fluid CEA very low (unlike mucinous cysts)

Note

─ Cytologic diagnosis is usually straightforward with the characteristic triad of squamous cells, lymphocytes, and keratin debris

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Pseudocyst

A localized collection of fluid, rich in pancreatic enzymes, lacks true epithelial lining; arises after acute pancreatitis, trauma, or in setting of chronic pancreatitis.

Clinical

─ Most common cystic lesion of pancreas

Cells

─ Predominantly inflammatory cells: Neutrophils (especially if acute/infected), lymphocytes, histiocytes (often numerous and foamy)

─ Epithelial cells absent

Background

─ "Dirty" proteinaceous debris; necrotic material

Absent

─ True epithelial lining (by definition); malignant cells

─ Thick, viscous mucin (characteristic of mucinous neoplasm)

Ancillary studies

─ Cyst fluid amylase high ( >1000 U/L) and CEA low (<192 ng/mL, often <5 ng/mL)

DDx

─ Mucinous cyst (mucin in the background)

─ Serous cystadenoma (clear fluid and bland cuboidal cells)

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Splenule (Accessory Spleen)

Ectopic splenic tissue; an incidental finding; mimics pancreatic neoplasm on imaging.

Cells

─ Polymorphous lymphoid cells, endothelial cells (lining sinusoids), histiocytes/macrophages

Background

─ Bloody with lymphoglandular bodies (cytoplasmic fragments of lymphocytes)

Ancillary studies

─ IHC: CD8 highlights sinusoidal lining cells; CD20/CD3 for B/T lymphocytes; CD68 for histiocytes

Note

─ Key is recognizing the mixed lymphoid population and characteristic vascular structures (sinusoids) if visible

─ Can be confused with intrapancreatic lymph node or lymphoma

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Serous Cystadenoma

Benign cystic neoplasm of small, uniform, glycogen-rich cuboidal cells forming microcysts.

Clinical

─ More common in women, body or tail of pancreas

─ Microcystic variant has "spongy" appearance on imaging; central stellate scar is classic

Cells

─ Often hypocellular or acellular (cyst fluid aspirate)

─ If cellular: Bland, cuboidal epithelial cells arranged in flat sheets or loose clusters

Cytoplasm

─ Scant to moderate, clear, pale, or finely vacuolated/granular (due to glycogen)

Nuclei

─ Small, round to oval, centrally or eccentrically located, monomorphic

─ Smooth nuclear contours, even chromatin, nucleoli inconspicuous, minimal to no atypia

Background

─ Colloid-like material (proteinaceous) may be present, not prominent or thick like mucin

─ Hemosiderin-laden macrophages

Absent

─ Significant atypia, pleomorphism, or mitotic activity

─ Thick, viscous mucin (distinguishes from mucinous cysts)

─ Lymphoid component (distinguishes from lymphoepithelial cyst)

─ Squamous cells or keratin debris

Ancillary studies

─ Cyst fluid: Low CEA (<5 ng/mL) & low amylase (variable)

─ PAS-D will show cytoplasmic glycogen (PAS positive, diastase sensitive)

─ IHC: (+) inhibin & GLUT-1 (membranous); (-) CEA, MUC1, MUC5AC

─ Molecular: VHL mutations

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Schwannoma

Benign nerve sheath tumor of Schwann cells, rare in the pancreas.

Cells

─ Spindle cells in cohesive fascicles or clusters, sometimes loosely arranged

─ Verocay bodies (acellular areas rimmed by palisading nuclei) – less common on FNA

─ Cellular (Antoni A) and hypocellular, myxoid (Antoni B) areas may be present

Cytoplasm

─ Indistinct, fibrillary, pale eosinophilic

Nuclei

─ Elongated, wavy, or buckled ("fish-hook" or comma-shaped) with pointed or tapering ends

─ Finely granular and even chromatin, nucleoli inconspicuous

─ Minimal atypia; Degenerative (ancient) atypia can occur but lacks mitoses

Background

─ Variable, may be clean or myxoid

─ +/- Cystic degeneration (common in pancreatic schwannomas)

─ +/- Lymphoid aggregates (especially in cystic variants)

Absent

─ Significant pleomorphism (except ancient change), mitoses, or necrosis (unlike MPNST)

─ Mucin

─ Epithelial cells (ductal, acinar) unless contaminant

Ancillary studies

─ IHC: (+) S100 & SOX10; (-) cytokeratins, neuroendocrine markers, CD117, DOG1

─ Cyst fluid (if present) low CEA & low amylase

Note

─ Very rare in pancreas, can mimic other spindle cell lesions or cystic neoplasms if cystic

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Lymphangioma

A benign congenital malformation of lymphatic channels, rare in the pancreas.

Clinical

─ More common in females, wide age range; often distal pancreas

─ Imaging: Well-circumscribed, thin-walled, uni- or multilocular cystic lesion

Cells

─ Often hypocellular

─ Predominantly small, mature lymphocytes

─ +/- Endothelial cells (flat, bland, lining lymphatic spaces – often difficult to identify)

─ +/- Histiocytes

Cytoplasm

─ Lymphocytes: Scant

─ Endothelial cells: Attenuated, difficult to see

Nuclei

─ Lymphocytes: Small, round, dark, mature-appearing

─ Endothelial cells: Flat, elongated, bland

Background

─ Clear, watery, chylous (milky), or serosanguinous fluid (proteinaceous)

─ +/- Fat globules

─ +/- Cholesterol crystals

Absent

─ Significant epithelial component (unless contaminant)

─ Atypia in lymphoid or endothelial cells

─ Mucin

─ Features of malignancy

Ancillary studies

─ Cyst fluid: Typically low CEA and low amylase; triglycerides may be elevated if chylous

─ IHC: Endothelial cells (+) D2-40, CD31, CD34

Note

─ MRI can be useful to exclude ductal communication (seen in some other cystic lesions)

─ Cytologic diagnosis can be challenging due to paucicellularity; main finding is often proteinaceous fluid with lymphocytes

─ DDx includes other paucicellular cysts like simple cysts, pseudocysts (if inflammation is scant), or cystic degeneration in other neoplasms. Lymphoid component can mimic lymphoepithelial cyst, but lymphangiomas lack squamous cells and keratin.

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Atypical

A specimen with cytologic atypia that falls short of being suspicious for malignancy; this includes both epithelial and non-epithelial atypia. The atypia exceeds definite reactive changes but lacks features to definitively classify as neoplastic or suspicious.

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Pancreaticobiliary Neoplasm (Low-Grade and High-Grade)

Per Papanicolaou Society encompasses neoplasms with features of ductal or biliary differentiation that are cytologically low-grade or high-grade but lack definitive features of invasion on FNA; includes precursor lesions (PanIN & BilIN) as well as some intraductal and cystic neoplasms

Note

─ This is a broad cytologic category, specific entities within it (like IPMN, MCN) have distinct clinicopathologic features and are detailed below; The distinction between low-grade and high-grade is based on the degree of cytologic and architectural atypia

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Pancreatic Intraepithelial Neoplasia (PanIN)

A microscopic, flat or micropapillary, non-invasive epithelial proliferation confined to pancreatic ducts; Graded low-grade (PanIN-1A, -1B, -2) to high-grade (PanIN-3)

Clinical

─ Incidental finding in resected specimens

Cells

─ Rarely specifically identified on FNA unless high-grade and exfoliating

─ Low-Grade PanIN: Small, crowded groups of ductal cells; columnar or cuboidal; minimal atypia

─ High-Grade PanIN: Crowded, disorganized groups; increased nuclear atypia, pleomorphism, hyperchromasia, prominent nucleoli; loss of polarity; may show micropapillary tufts

Cytoplasm

─ Low-Grade PanIN: May be mucinous or non-mucinous

─ High-Grade PanIN: Often scant, N:C ratio increased

Nuclei

─ Low-Grade PanIN: Round to oval, uniform or mildly enlarged, smooth contours, inconspicuous nucleoli

─ High-Grade PanIN: Enlarged, pleomorphic, irregular contours, hyperchromatic, coarse chromatin, prominent nucleoli; mitoses may be seen

Background

─ Clean (unless associated with pancreatitis or obstruction)

Absent

─ Stromal invasion

─ Features of other specific neoplasms (e.g., abundant extracellular mucin of IPMN, ovarian stroma of MCN)

Ancillary studies

─ Not typically performed on FNA for PanIN specifically due to diagnostic difficulty

─ Molecular: KRAS mutations common and early; TP53, CDKN2A/p16, SMAD4 mutations accumulate with progression to high-grade PanIN and invasive cancer

DDx

─ Reactive ductal atypia

─ IPMN (especially main duct or branch duct with minimal cystic change)

─ Invasive ductal adenocarcinoma (high-grade PanIN can be difficult to distinguish from well-differentiated adenocarcinoma on cytology alone if invasion isn't seen)

Note

─ Cytologic diagnosis of PanIN, especially low-grade, is very challenging and often not possible on FNA

─ High-grade PanIN cells may be indistinguishable from invasive adenocarcinoma cells

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Biliary Intraepithelial Neoplasia (BilIN)

A preinvasive neoplastic lesion of the biliary tract epithelium, analogous to PanIN in the pancreas, considered a precursor to cholangiocarcinoma; Graded from low-grade (BilIN-1, -2) to high-grade (BilIN-3)

Clinical

─ Incidental microscopic finding

Cells

─ Rarely specifically identified on FNA unless high-grade and exfoliating

─ Cells would likely be categorized as "Atypical" or "Pancreaticobiliary Neoplasm"

─ Low-Grade BilIN: Crowded groups of biliary-type cells; columnar; minimal atypia

─ High-Grade BilIN: More disorganized, increased nuclear atypia, pleomorphism, hyperchromasia; loss of polarity

Cytoplasm

─ Low-Grade BilIN: May be scant or slightly mucinous

─ High-Grade BilIN: Scant, increased N:C ratio

Nuclei

─ Low-Grade BilIN: Mildly enlarged, slightly irregular, inconspicuous nucleoli

─ High-Grade BilIN: Enlarged, pleomorphic, hyperchromatic, prominent nucleoli

Absent

─ Stromal invasion

Note

─ Cytologic diagnosis is very challenging on FNA

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Pancreatic Intraductal Papillary Mucinous Neoplasm (IPMN)

Intraductal, grossly visible ( >1 cm) mucin-producing epithelial neoplasm arising in the main pancreatic duct or its branches, characterized by papillary proliferation of mucinous cells

Clinical

─ Symptoms depend on location and duct involvement

─ Main duct IPMN (MD-IPMN) and mixed-type have higher risk of malignancy than branch duct IPMN (BD-IPMN)

─ Sendai and Fukuoka guidelines are used for risk stratification and management

Cells

─ Variable cellularity

─ Cohesive groups, sheets, clusters, and true papillary fronds with fibrovascular cores

─ Columnar mucinous cells characteristic

─ Single cells may be present, esp in higher-grade lesions

Cytoplasm

─ Abundant, clear to pale, foamy or finely vacuolated (mucin-rich)

─ Apical mucin caps may be seen

─ Oncocytic, intestinal, or pancreatobiliary epithelial types can occur, influencing cytoplasmic appearance

Nuclei

─ Low-Grade (esp gastric-type): Round, basally located, uniform, smooth contours, inconspicuous nucleoli

─ High-Grade (intestinal, pancreatobiliary, or oncocytic): Enlarged, pleomorphic, irregular contours, hyperchromatic, coarse chromatin, prominent/irregular nucleoli, loss of polarity, stratification, mitoses

Background

─ Abundant, thick, viscous, colloid-like extracellular mucin is a hallmark feature ("string sign" on aspiration)

─ Inflammatory cells, histiocytes

Absent

─ Ovarian-type stroma (unlike MCN)

─ Stromal invasion (if present, categorized as IPMN with an associated invasive carcinoma)

Ancillary studies

─ Cyst fluid: CEA elevated (>192 ng/mL, often >800 ng/mL); Amylase variable (high if duct communication, low if obstructed)

─ Molecular: KRAS and GNAS mutations common; RNF43, TP53, SMAD4 mutations with progression

DDx

─ Mucinous Cystic Neoplasm (MCN): Lacks ductal communication, has ovarian-type stroma (not seen on FNA), almost exclusively in women

─ Pancreatic Ductal Adenocarcinoma (PDAC): More discohesion, single malignant cells, and infiltrative pattern

─ Chronic pancreatitis with mucinous metaplasia/hyperplasia: Less architectural complexity, less atypia, background of chronic pancreatitis

─ Simple mucinous cyst/retention cyst: Thin mucin, bland flat lining, low CEA

Note

─ Grading (low vs high) is crucial for management

─ Type of epithelium (gastric, intestinal, pancreatobiliary, oncocytic) influences morphology and risk

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Intraductal Papillary Neoplasm of the Bile Duct (IPNB)

Intraductal growth of biliary epithelium with papillary or villous architecture; a rare papillary neoplasm arising in the intrahepatic or extrahepatic bile ducts, considered a precursor to invasive cholangiocarcinoma

Clinical

─ Can cause biliary obstruction, cholangitis, or be an incidental finding

Cells

─ Similar to pancreatic IPMN but arising in bile ducts

─ Papillary clusters, sheets of columnar cells

─ Four epithelial subtypes described: Pancreatobiliary, intestinal, gastric, oncocytic

Cytoplasm

─ Variable depending on subtype; may be mucinous or eosinophilic (oncocytic)

Nuclei

─ Range from low-grade to high-grade atypia, similar to IPMN

Background

─ May see mucin (less consistently thick or abundant as in IPMN)

─ Inflammatory cells

Absent

─ Features of invasion into stroma (on cytology sample)

Note

─ Analogous to pancreatic IPMN

─ Cytologic features depend on the grade and histologic subtype

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Mucinous Cystic Neoplasm (MCN)

Mucin-producing cystic epithelial neoplasm of the pancreas that does not connect with the pancreatic duct system and is characterized by an ovarian-type stroma beneath the epithelium

Clinical

─ Almost exclusively occurs in women in the body or tail; large, unilocular or multilocular cysts

Cells

─ Variable cellularity; paucicellular if only cyst fluid aspirated

─ Columnar mucinous epithelial cells in flat sheets, strips, clusters, or papillae

─ Ovarian-type stromal cells (spindle cells) are diagnostic but rarely sampled by FNA

Cytoplasm

─ Epithelial cells: Abundant, clear to pale, foamy or vacuolated (mucin-rich)

Nuclei

─ Epithelial cells: Range from bland (low-grade) to markedly atypical (high-grade)

─ Low-Grade: Small, round to oval, basally located, uniform, smooth contours

─ High-Grade: Enlarged, pleomorphic, irregular contours, hyperchromatic, prominent nucleoli, mitoses

Background

─ Thick, viscous, extracellular mucin

─ Inflammatory cells, histiocytes, debris

Absent

─ Communication with the pancreatic duct system

─ Stromal invasion (if present, categorized as MCN with an associated invasive carcinoma)

Ancillary studies

─ Cyst fluid analysis: CEA elevated (>192 ng/mL); Amylase low

─ Molecular: KRAS mutations; RNF43 mutations also seen; TP53 mutations with progression

DDx

─ Intraductal Papillary Mucinous Neoplasm (IPMN): Connects with duct system, lacks ovarian stroma

─ Pseudocyst: Inflammatory background, high amylase, low CEA, no true epithelial lining

─ Serous Cystadenoma: Clear fluid, glycogen-rich cells, low CEA, low amylase

─ Simple cyst/retention cyst: Thin mucin, bland flat lining, low CEA

Note

─ Presence of ovarian-type stroma is pathognomonic but rarely seen on FNA; Diagnosis often presumptive based on clinical (female, body/tail cyst), imaging, and cyst fluid (high CEA, low amylase) with mucinous epithelium

─ All MCNs are considered to have malignant potential and are usually recommended for resection

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Intraductal Oncocytic Papillary Neoplasm (IOPN)

Intraductal papillary neoplasm with complex arborizing papillae lined by oncocytic cells with eosinophilic cytoplasm and prominent nucleoli, involves main pancreatic duct or major branch ducts

Cells

─ Hypercellular smears

─ Large, complex papillary fronds, often with delicate fibrovascular cores

─ Sheets and clusters of oncocytic cells

─ Cells are typically large, polygonal to columnar

Cytoplasm

─ Abundant, dense, granular, eosinophilic (oncocytic)

─ Intracytoplasmic lumina or vacuoles may be present

Nuclei

─ Round to oval, eccentrically placed, enlarged, with vesicular or finely granular chromatin

─ Prominent, centrally located, eosinophilic macronucleoli are characteristic

─ Mild to moderate anisonucleosis; high-grade atypia can be seen

Absent

─ Significant extracellular mucin

─ Ovarian-type stroma

Ancillary studies

─ Molecular: Rearrangement of PRKACA or PRKACB; KRAS & GNAS mutations absent

DDx

─ IPMN subtypes (gastric, intestinal, pancreatobiliary): Lack diffuse oncocytic change and prominent macronucleoli; often have more extracellular mucin

─ Solid-pseudopapillary neoplasm: Lacks true oncocytic features; characteristic nuclear grooves, fine chromatin, and β-catenin nuclear positivity

─ Acinar cell carcinoma: More discohesive, prominent acinar formations, different nuclear features (zymogen granules)

─ Pancreatic neuroendocrine tumor: Different chromatin (salt-and-pepper), less prominent nucleoli, positive neuroendocrine markers

─ Serous cystadenoma: If oncocytic change is focal in an SCA, but SCA lacks true papillae and has clear, glycogen-rich cells predominantly

Note

─ Cytologically appears high-grade, but behavior can be relatively indolent if non-invasive

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Intraductal Tubulopapillary Neoplasm (ITPN)

Intraductal neoplasm with tubular and tubulopapillary growth, cuboidal to columnar cells with minimal or no overt mucin production

Clinical

─ Rare, distinct from IPMN and MCN, can occur in main or branch ducts

─ May cause duct obstruction, pancreatitis, or be an incidental finding

Cells

─ Cellular smears

─ Cohesive, complex, 3D groups, back-to-back tubular structures or short/stubby papillae

─ Cells cuboidal to low columnar

Cytoplasm

─ Scant to moderate, eosinophilic or amphophilic, non-mucinous (or only focal apical mucin)

Nuclei

─ Round to oval, uniform, with smooth contours; chromatin granular to moderately coarse

─ Nucleoli usually conspicuous, no macronucleoli

─ High-grade atypia common

Background

─ Clean; some necrotic debris in higher-grade lesions

Absent

─ Abundant extracellular mucin (unlike IPMN)

─ Ovarian-type stroma (unlike MCN)

─ Prominent oncocytic features (unlike IOPN)

Ancillary studies

─ Molecular: KRAS, GNAS, and RNF43 mutations are absent

DDx

─ Pancreatic ductal adenocarcinoma (PDAC): ITPN is intraductal; PDAC is invasive with desmoplasia; ITPN cells often more uniform with less pleomorphism than typical PDAC, though high-grade ITPN can mimic PDAC

─ IPMN, particularly pancreatobiliary subtype: IPMN usually has more overt mucin production and often cystic change; ITPN is more solid/tubular

─ PanIN: PanIN is a microscopic flat lesion, ITPN is grossly visible and forms complex structures

Note

─ High-risk precursor lesion, frequently associated with high-grade dysplasia and invasive carcinoma

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Suspicious for Malignancy

A specimen that contains cells with cytologic features highly worrisome for, but not diagnostic of, malignancy; this category is used when the atypia is qualitatively or quantitatively insufficient for an unequivocal diagnosis of malignancy

Clinical

─ This interpretation carries a high risk of malignancy (ROM), often >70-80%, most commonly for pancreatic ductal adenocarcinoma (PDAC)

─ Clinical and imaging correlation is crucial; further investigation (e.g., repeat FNA, core biopsy, or surgical exploration) is usually warranted

Note

─ This category is used sparingly

─ The Papanicolaou Society of Cytopathology guidelines suggest this category should account for a small percentage of pancreatic FNA diagnoses

─ Scant sampling of a well-differentiated malignancy is a common reason for this interpretation

─ Cases with rare atypical ductal cells in the setting of indwelling stents or inflammatory conditions like primary biliary cholangitis may fall here if reactive changes cannot be definitively distinguished from malignancy

Malignant

A specimen with unequivocal cytologic features of malignancy; further classification as to the type of malignancy (e.g., ductal adenocarcinoma, neuroendocrine carcinoma, lymphoma, metastasis) should be attempted whenever possible

Note

─ This category is used when there is sufficient qualitative and quantitative evidence for a definitive diagnosis of malignancy

─ The Papanicolaou Society of Cytopathology guidelines provide specific criteria for various malignant pancreaticobiliary neoplasms

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Cholangiocarcinoma (Intrahepatic, Hilar, Extrahepatic)

Adenocarcinoma arising from biliary epithelium lining the intra or extrahepatic bile ducts

Clinical

─ Risk factors include primary sclerosing cholangitis, choledochal cysts, liver flukes (Clonorchis, Opisthorchis), hepatolithiasis, and chronic biliary inflammation

─ Intrahepatic cholangiocarcinoma may present as liver mass

─ Hilar (Klatskin tumor) and extrahepatic cholangiocarcinoma presents with obstructive jaundice

─ Serum CA 19-9 and CEA elevated

Cell patterns

─ Malignant glandular cells in sheets, clusters, acini, or papillae, often single cells; cuboidal to columnar, pleomorphic; moderate to high cellularity

Cytoplasm

─ Scant to moderate, basophilic, pale, or vacuolated (mucin-containing)

─ Signet-ring cells may be present

Nuclei

─ Enlarged, irregular contours, notching, and grooves

─ Hyperchromatic, coarse or clumped chromatin; parachromatin clearing may be seen

─ Nucleoli prominent and irregular; macronucleoli can be seen

─ Anisonucleosis and pleomorphism apparent

─ Mitotic figures may be present

Background

─ "Dirty" with necrotic debris and inflammation; bile pigment if obstructed; desmoplastic stromal fragments

Absent

─ Features specific to HCC (endothelial wrapping, tumor cell bile production, HepPar-1); features specific to other pancreatic neoplasms

Ancillary studies

─ IHC: (+) CK7, CK19, CEA, MUC1, MUC5AC; CK20 variable

─ IHC: (-) Hepatocellular markers (HepPar-1, Arginase-1, Glypican-3), neuroendocrine markers (synaptophysin, chromogranin), often TTF-1 (hilar cholangiocarcinomas can be (+) TTF-1)

─ SMAD4/DPC4 loss common, similar to PDAC

─ FISH for polysomy of chromosomes 3, 7, 17 and 9p21 (CDKN2A/p16) deletion can be helpful in biliary brushings, esp in PSC patients

DDx

─ Pancreatic ductal adenocarcinoma: Similar or identical; distinction based on location

─ Metastatic adenocarcinoma

─ Reactive biliary atypia: Can be very difficult, esp in the setting of stents or PSC; reactive cells usually show less architectural disarray, smoother nuclear contours, finer chromatin, and lack numerous atypical single cells

─ Hepatocellular carcinoma: esp poorly differentiated variants; HCC typically shows trabecular patterns, sinusoidal capillarization, and positive hepatocellular markers

Note

─ Cytologic diagnosis challenging, particularly in well-differentiated tumors or with reactive changes

─ Bile duct brushings high specificity but variable sensitivity; FNA higher sensitivity for mass lesions

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Solid Pseudopapillary Neoplasm (SPN)

A low-grade malignant epithelial neoplasm with solid and pseudopapillary architecture, composed of relatively uniform cells with round to oval nuclei, often with nuclear grooves, and eosinophilic or clear cytoplasm

Clinical

─ young women (mean age 20s-30s)

─ large, well-circumscribed, encapsulated mass, frequently in the tail

─ many are incidental findings

─ Excellent prognosis after complete surgical resection, even with invasion or metastases (primarily to liver or peritoneum)

Cell patterns

─ Highly cellular; monomorphic cells in branching papillary-like fronds with fibrovascular cores, solid sheets, loose clusters, or single; round to polygonal/plasmacytoid

Cytoplasm

─ Moderate, finely granular, eosinophilic, or clear/vacuolated

─ Cytoplasmic hyaline globules (PAS-D positive) may be present

─ Indistinct cell borders

Nuclei

─ Round to oval, eccentrically placed; Finely granular, "powdery" or "salt-and-pepper"

─ Longitudinal nuclear grooves characteristic

─ Nucleoli inconspicuous or small

─ Mitotic figures sparse

Background

─ Often hemorrhagic; myxoid or hyalinized material (stroma of fibrovascular stalks); cholesterol crystals, foamy macrophages, necrotic debris (esp if cystic degeneration)

Absent

─ Significant pleomorphism; overt glandular or acinar differentiation; abundant extracellular mucin

Ancillary studies

─ IHC: (+) β-catenin (nuclear and cytoplasmic), CD10 (membranous), CD56, vimentin, α1-antitrypsin, progesterone receptor

─ IHC: (-) Chromogranin (or only focal), trypsin, CK7 (usually)

─ Molecular: CTNNB1 (gene encoding β-catenin) mutations

DDx

─ Pancreatic neuroendocrine tumor (PanNET): Can have plasmacytoid cells and granular cytoplasm; PanNETs more salt-and-pepper chromatin, lack nuclear grooves, and (+) chromogranin/synaptophysin, (-) nuclear β-catenin

─ Acinar cell carcinoma: granular cytoplasm (zymogen granules), prominent nucleoli, (+) trypsin/BCL10

─ Pancreatoblastoma: Occurs in children, squamous morules, may show acinar or neuroendocrine differentiation

─ Serous cystadenoma: If SPN is predominantly cystic; serous cystadenoma has clear, glycogen-rich cells and lacks the nuclear features or IHC profile of SPN

Note

─ combination of delicate papillary fronds, monomorphic cells with nuclear grooves, and characteristic IHC (nuclear β-catenin) is diagnostic

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Pancreatoblastoma

rare malignant epithelial neoplasm of childhood, acinar differentiation and squamous morules

Clinical

─ Most common pancreatic neoplasm in children ( <10 years old); very rare in adults

─ associated with Beckwith-Wiedemann syndrome or familial adenomatous polyposis (FAP)

─ presents as a large abdominal mass; serum AFP may be elevated

Cell patterns

─ Highly cellular; dual population (primitive cells in clusters/sheets/rosettes; larger cells with acinar differentiation); characteristic squamous morules

Cytoplasm

─ Primitive cells: Scant, basophilic

─ Acinar cells: Moderate, granular (zymogen granules), eosinophilic or basophilic

─ Squamous cells: Dense, eosinophilic/orangeophilic

Nuclei

─ Primitive cells: Round to oval, hyperchromatic, coarse chromatin, inconspicuous nucleoli

─ Acinar cells: Round, eccentric, vesicular chromatin, prominent nucleolus

─ Squamous cells: Bland, often pyknotic

Background

─ May be clean or contain blood, necrotic debris

Absent

─ Significant extracellular mucin; ovarian-type stroma; prominent neuroendocrine features (focal neuroendocrine differentiation can occur)

Ancillary studies

─ IHC: (+) Trypsin, chymotrypsin, BCL10 (acinar component); Keratins (squamoid corpuscles); β-catenin (nuclear, often)

─ IHC: (+) Synaptophysin, chromogranin (focally in some cases)

─ AFP can be positive

DDx

─ Pancreatic neuroendocrine tumor (PanNET): Lacks squamoid corpuscles and true acinar differentiation; diffusely (+) neuroendocrine markers

─ Acinar cell carcinoma: Predominantly acinar differentiation, lacks squamoid corpuscles and primitive component ( some overlap exists, esp in adults)

─ Solid pseudopapillary neoplasm (SPN): Lacks squamoid corpuscles and acinar differentiation; characteristic nuclear grooves and nuclear β-catenin

─ Hepatoblastoma (metastatic): Can have squamoid morules and AFP production; clinical context and liver imaging important

Note

─ Squamoid corpuscles are the most distinctive cytologic feature

─ Prognosis better in children than in rare adult cases

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Pancreatic Ductal Adenocarcinoma (PDAC)

An invasive malignant epithelial neoplasm showing glandular (ductal) differentiation, accounts for majority of pancreatic cancers

Clinical

─ older adults ( 60s-70s), presents late with painless jaundice (head lesions), abdominal/back pain, weight loss, anorexia

─ Most common in head of pancreas

─ Serum CA 19-9 often elevated but not specific

─ Poor prognosis, high rate of local invasion and distant metastases

Cell patterns

─ Variable cellularity; malignant ductal cells in disorganized crowded 3D clusters, poorly formed glands, sheets, chains, or single; cell-in-cell arrangements; marked anisonucleosis

Cytoplasm

─ Variable, scant to moderate, pale, basophilic, or vacuolated (mucin)

─ N:C is high

Nuclei

─ Enlarged, pleomorphic (variable size and shape), with irregular nuclear contours (notching, angulation, membrane thickening)

─ Hyperchromasia, coarsely granular, clumped, or irregularly distributed chromatin

─ Parachromatin clearing prominent

─ Nucleoli enlarged, irregular, and multiple; macronucleoli can be present

─ Mitotic figures, including atypical forms, may be seen but are not always numerous

Background

─ Often "dirty"; fragments of desmoplastic stroma; extracellular mucin may be seen

Absent

─ Features of specific differentiation (e.g., neuroendocrine, acinar, squamous, unless a variant); ovarian-type stroma

Ancillary studies

─ IHC: (+) CK7, CK19, CEA (often strong cytoplasmic/luminal), CA19-9, MUC1, MUC5AC; p53 (aberrant expression - strong diffuse positivity or complete absence often)

─ IHC: (-) SMAD4/DPC4 (loss in ~55% of cases, highly specific)

─ Molecular: KRAS mutations in >90%; TP53, CDKN2A/p16, SMAD4 mutations also common

DDx

─ Reactive ductal atypia (pancreatitis, stents): Can be very challenging; reactive changes usually show less architectural disarray, smoother nuclear contours, finer chromatin, less prominent nucleoli, and lack numerous single atypical cells

─ Cholangiocarcinoma: Morphologically and immunophenotypically very similar/identical; distinction based on primary site

─ High-grade PanIN or dysplasia in IPMN/MCN: Lack stromal invasion (though this is hard to assess on FNA alone)

─ Other adenocarcinomas (metastatic or other primary pancreatic types): Clinical history and IHC panel crucial

Note

─ Diagnosis relies on a combination of architectural disarray and high-grade nuclear atypia

─ Well-differentiated PDAC can be particularly difficult to distinguish from reactive changes

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Pancreatic Neuroendocrine Tumor (PanNET)

Neoplasm showing neuroendocrine differentiation, arising from islet cells or ductal pluripotent stem cells; graded by WHO as G1, G2, or G3 based on mitotic rate and Ki-67 index

Clinical

─ functional (hormone-secreting) or non-functional (most common)

─ Non-functional tumors often present due to mass effect or incidentally

─ May be part of MEN1 syndrome

─ Prognosis varies widely depending on grade, stage, and functionality

Cell patterns

─ Highly cellular; dispersed single cells, loose clusters, trabeculae, rosettes, or acinar-like arrangements; monomorphic round, oval, or plasmacytoid cells

Cytoplasm

─ Moderate amount, finely granular ("salt-and-pepper" on Romanowsky stains, eosinophilic to amphophilic on Pap), or clear

Nuclei

─ Round to oval, often eccentric (plasmacytoid appearance)

─ Finely stippled, "salt-and-pepper" or "powdery" chromatin characteristic

─ Nucleoli inconspicuous or small; can be more prominent in higher-grade lesions

─ Nuclear molding may be seen in cohesive groups

─ Minimal anisonucleosis in low-grade tumors; more pleomorphism in higher grades

─ Mitoses rare in G1, frequent in G2/G3

Background

─ Often bloody (high vascularity); lymphoglandular bodies (stripped cytoplasm); amyloid (in some types e.g., insulinoma)

Absent

─ Significant extracellular mucin; true glandular lumina or well-formed acini (unless mixed tumor); squamoid corpuscles

Ancillary studies

─ IHC: (+) Synaptophysin, Chromogranin A (Chromogranin may be focal or negative in poorly differentiated NETs/NECs), CD56, INSM1; Specific hormones (insulin, glucagon, gastrin, etc, if functional)

─ Ki-67 index: G1: <3%; G2: 3-20%; G3 PanNET: >20%

DDx

─ Acinar cell carcinoma: More cohesive, true acinar structures, coarser chromatin, prominent nucleoli, (+) trypsin/BCL10

─ Solid pseudopapillary neoplasm (SPN): Papillary structures, nuclear grooves, (+) nuclear β-catenin, CD10

─ Pancreatoblastoma: Squamoid corpuscles, acinar differentiation, typically in children

─ Well-differentiated PDAC: Glandular structures, mucin, different IHC

─ Lymphoma: Usually more pleomorphic lymphoid cells, (+) lymphoid markers

Note

─ "Salt-and-pepper" chromatin is a key feature

─ Grading (G1, G2, G3 PanNET) is essential for prognosis and management and relies on mitotic count and Ki-67 index, ideally assessed on histologic material or cell block

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Pancreatic Neuroendocrine Carcinoma (PanNEC)

high-grade, poorly differentiated neuroendocrine neoplasm, morphologically similar to small cell carcinoma or large cell neuroendocrine carcinoma of the lung

Clinical

─ Rare, aggressive tumors with poor prognosis; large at presentation, early metastases

─ May or may not be associated with a well-differentiated PanNET component

Cell patterns

─ Highly cellular; Small cell type (sheets/clusters/dispersed small cells, scant cytoplasm, molding, crush, mitoses/apoptosis) or Large cell type (sheets/clusters larger cells, more cytoplasm, vesicular nuclei, prominent nucleoli, mitoses/necrosis)

Cytoplasm

─ Small cell type: Very scant, often indistinct

─ Large cell type: Moderate, eosinophilic or amphophilic

Nuclei

─ Small cell type: Round to oval or spindled, hyperchromatic, finely granular to smudged chromatin, inconspicuous nucleoli, marked nuclear molding

─ Large cell type: Large, round to oval, vesicular or coarse chromatin, prominent nucleoli

─ High mitotic rate and extensive necrosis/apoptosis are characteristic of both

Background

─ Often necrotic, bloody, extensive crush artifact (esp small cell type)

Absent

─ Features of well-differentiated PanNET (e.g., plasmacytoid cells, salt-and-pepper chromatin throughout); glandular or acinar differentiation (unless combined tumor)

Ancillary studies

─ IHC: (+) Synaptophysin (often strong), CD56, INSM1, Cytokeratins (dot-like pattern can be seen); Chromogranin A (may be focal or negative)

─ Ki-67 index is very high (typically >50-60%, WHO defines NEC as >20% but most are much higher)

─ TP53 and RB alterations are common

DDx

─ Poorly differentiated pancreatic ductal adenocarcinoma: May show some neuroendocrine marker expression but usually has glandular features and different cytomorphology

─ Metastatic small cell or large cell neuroendocrine carcinoma (esp from lung): Clinically indistinguishable; IHC (TTF-1 may be positive in lung origin) and clinical history essential

─ Lymphoma/Leukemia: Dispersed atypical cells; (+) lymphoid markers

─ Acinar cell carcinoma: Can be poorly differentiated but usually shows some acinar features/markers

Note

─ Diagnosis requires neuroendocrine marker expression and high-grade morphology (high mitotic rate, necrosis)

─ Distinction from G3 PanNET can be challenging; PanNECs are generally more poorly differentiated with small cell or large cell neuroendocrine morphology, while G3 PanNETs still resemble their lower-grade counterparts but have higher proliferation

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Acinar Cell Carcinoma

A malignant epithelial neoplasm showing predominantly acinar differentiation

Clinical

─ Rare, accounting for ~1-2% of exocrine pancreatic neoplasms

─ Can occur at any age, slight male predominance

─ May present with nonspecific abdominal symptoms, mass effect, or paraneoplastic syndromes (e.g., panniculitis due to lipase hypersecretion - Schmid's triad: subcutaneous fat necrosis, polyarthritis, eosinophilia)

─ Serum lipase may be elevated

─ Generally aggressive with poor prognosis, though better than PDAC for localized disease

Cell patterns

─ Highly cellular; cohesive clusters, acinar structures, sheets, or dispersed single cells; medium to large polygonal cells

Cytoplasm

─ Abundant, finely granular to coarsely granular (zymogen granules), eosinophilic or basophilic

─ Apical cytoplasmic granularity may be prominent in well-formed acini

─ Cell borders often distinct

Nuclei

─ Round to oval, often eccentric

─ Vesicular or coarsely granular chromatin

─ Single, prominent, centrally located nucleolus is characteristic

─ Anisonucleosis and pleomorphism can be variable; some tumors are quite monomorphic, others more pleomorphic

─ Mitotic figures may be present

Background

─ May be clean, hemorrhagic, or show necrosis; stripped nuclei with intact cytoplasm (acinar "ghosts") can be seen

Absent

─ Significant extracellular mucin; neuroendocrine features (unless mixed acinar-neuroendocrine carcinoma); ovarian-type stroma or squamoid corpuscles

Ancillary studies

─ IHC: (+) Trypsin, Chymotrypsin, BCL10 (nuclear and cytoplasmic), Pancreatic lipase, Cytokeratins (CK8/18, CAM5,2)

─ IHC: (-) Chromogranin, Synaptophysin (unless mixed tumor), nuclear β-catenin

DDx

─ Pancreatic neuroendocrine tumor (PanNET): Plasmacytoid cells, salt-and-pepper chromatin, (+) neuroendocrine markers

─ Solid pseudopapillary neoplasm (SPN): Papillary structures, nuclear grooves, (+) nuclear β-catenin

─ Pancreatoblastoma: Squamoid corpuscles, primitive component, typically in children

─ Well-differentiated PDAC with oncocytic or clear cell features: Lacks true acinar differentiation and zymogen granules; different IHC

─ Islet cell hyperplasia or normal acinar tissue: Less cellularity, no atypia, no discohesion

Note

─ Prominent nucleoli and granular cytoplasm are key features

─ Cytologic features can overlap with PanNET, requiring IHC for definitive classification

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Main Document

Thyroid

Table of Contents

Non-Diagnostic

Benign

Follicular Nodular Disease (FND) (Adenomatoid Nodule, Colloid Nodule)

Graves' Disease

Thyroiditis

Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis)

Subacute (Granulomatous, De Quervain) Thyroiditis

Acute Suppurative Thyroiditis

Riedel Thyroiditis

Thyroglossal Duct Cyst

Branchial Cleft Cyst

Atypia of Undetermined Significance (AUS)

Follicular Neoplasm / Suspicious for a Follicular Neoplasm (SFN)

Parathyroid lesions (DDx for Follicular Neoplasm)

Follicular Neoplasm – Oncocytic (Hürthle Cell Neoplasm / Suspicious for Hürthle Cell Neoplasm)

Suspicious for Malignancy

Suspicious for Papillary Thyroid Carcinoma

Suspicious for Medullary Thyroid Carcinoma

Suspicious for Lymphoma

Suspicious for Malignancy, Not Otherwise Specified (NOS)

Malignant

Papillary Thyroid Carcinoma (PTC)

NIFTP (Non-Invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features)

Papillary Thyroid Carcinoma – Encapsulated Follicular Variant (EFVPTC) and NIFTP (for historical context/comparison)

Papillary Thyroid Carcinoma – Follicular Variant (FVPTC) with Infiltrative Growth

Papillary Thyroid Carcinoma – Macrofollicular Variant

Papillary Thyroid Carcinoma – Cystic Variant

Papillary Thyroid Carcinoma – Oncocytic Variant

Papillary Thyroid Carcinoma – Warthin-Like Variant

Papillary Thyroid Carcinoma – Tall Cell Variant

Papillary Thyroid Carcinoma – Columnar Cell Variant

Papillary Thyroid Carcinoma – Solid / Trabecular Variant

Papillary Thyroid Carcinoma – Diffuse Sclerosing Variant

Papillary Thyroid Carcinoma – Hobnail Variant

Medullary Thyroid Carcinoma (MTC)

High-Grade Follicular Cell-Derived Non-Anaplastic Thyroid Carcinoma

Anaplastic Thyroid Carcinoma (ATC)

Lymphoma (Primary Thyroid Lymphoma)

Bethesda ROM & Management

Other Tumors

Cribriform-Morular Thyroid Carcinoma

Hyalinizing Trabecular Tumor (HTT)

Non-Diagnostic

Insufficient for diagnosis due to limited cellularity, obscuring factors, or poor preservation
Clinical ─ Not applicable for this category in terms of patient specifics; specimen may be re-aspirated
Cytology
Cells
─ Scant or poorly visualized
─ Or mostly cyst macrophages
─ Fewer than six groups of well-preserved follicular cells
─ Each group having at least ten cells
Cytoplasm
─ Obscured by blood or artifact
─ Or poorly preserved
Nuclei
─ Show obscuring artifact or degeneration
─ Or are too few to assess
Background
─ Shows mostly blood
─ Or thick colloid obscuring cells
─ Or preparation artifact
─ Or cyst fluid only
Absent
─ Adequate well-preserved follicular epithelium for evaluation
Ancillary studies
─ Molecular: Not typically performed
DDx
─ Cyst fluid from a benign nodule (if cyst macrophages only, but no epithelium)
─ True aspiration of a sparsely cellular benign nodule (sampling error vs actual lesion)
─ Technical issues (poor smear, fixation, obscuring blood)
Note
─ Repeat FNA, preferably ultrasound-guided, is recommended
─ Exceptions: abundant colloid only, or inflammation consistent with thyroiditis, may be diagnostic
─ Criteria: < 6 groups of 10 well preserved follicular cells each
─ Or cyst fluid only (with/without histiocytes)
─ Other examples: poorly prepared/stained smears
─ Significantly obscured follicular cells
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Benign

Encompasses a group of non-neoplastic thyroid conditions; specific entities detailed below
Clinical ─ Overall risk of malignancy (ROM) for benign category is ~5-15%; follow-up is typically clinical and/or ultrasound
Note ─ Specific features, ancillary studies, and DDx are best considered for each sub-entity
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Follicular Nodular Disease (FND) (Adenomatoid Nodule, Colloid Nodule)

Benign proliferation of follicular cells forming nodules with colloid; includes colloid-rich adenomatoid nodules and more cellular ones (aka colloid nodule, hyperplastic nodule, adenomatous nodule, benign follicular nodule)
Clinical ─ Common cause of thyroid nodules; usually euthyroid; may be solitary or multiple in multinodular goiter
Cytology
Cells
─ Follicular cells present
─ Arranged in flat honeycomb sheets (monolayered)
─ Or arranged in macrofollicles
─ Occasionally 3D but retain polarity
─ Minimal crowding or overlapping observed
─ Oncocytes (Hürthle cells) can be present
Cytoplasm
─ Follicular cells: moderate amount, clear to granular
─ Follicular cells: delicate, ill-defined cell borders
─ Oncocytes: abundant, dense, eosinophilic
Nuclei
─ Round to oval shape, monomorphic appearance
─ Uniform finely granular chromatin
─ Smooth nuclear contours
─ Slight anisonucleosis allowed
─ Nucleoli inconspicuous or absent
─ Oncocyte nuclei: central, round
─ Oncocyte nucleoli: prominent
─ Oncocytes: +/- large cell dysplasia
Background
─ Abundant colloid present
─ Colloid may be watery, forming folds/lacunae
─ Or colloid may be thick, with hyaline quality
─ "Stained-glass cracking" appearance of colloid
─ Cyst macrophages common
─ +/- Hemosiderin or lipofuscin pigment
─ +/- Papillary hyperplasia
Absent
─ Significant nuclear atypia (no PTC features)
─ True papillae
─ Psammoma bodies
─ Predominant microfollicular pattern
Ancillary studies
─ IHC (+): Thyroglobulin (can confirm follicular origin if needed)
DDx
─ Follicular neoplasm/AUS (more cellular, microfollicles, scant/absent colloid, nuclear crowding)
─ Papillary thyroid carcinoma, follicular variant (PTC nuclear features present, even if focal)
─ Nondiagnostic aspirate (FND requires adequate cellularity and/or colloid)
Note
─ Spectrum from colloid-rich (macrofollicular) to more cellular (normofollicular) nodules

Graves' Disease

Autoimmune disorder causing hyperthyroidism due to diffuse thyroid hyperplasia
Clinical ─ Hyperthyroidism symptoms (tachycardia, tremor, exophthalmos); diffuse goiter; typically young to middle-aged women
Cytology
Cells
─ Follicular cells often hypercellular
─ Arranged in flat sheets or loosely cohesive groups
─ Typically not hypercellular (can be variable)
Cytoplasm
─ Abundant, foamy, pale
─ Finely vacuolated or granular
─ "Fire flares" (marginal vacuolization/scalloped colloid at cell edges)
Nuclei
─ Round to oval shape
─ Often enlarged & vesicular but smooth contours
─ Finely granular chromatin
─ Nucleoli can be prominent
Background
─ Variable colloid, often scant and watery
─ Lymphocytes usually not prominent
─ If lymphocytes present, grooves and chromatin clearing are non-diffuse
Absent
─ Definitive PTC nuclear features
─ True papillae
DDx
─ Cellular FND (less atypia, no fire flares usually)
─ Papillary thyroid carcinoma (PTC nuclear features, true papillae if present)
─ Toxic adenoma (FNA may be similar; clinical/scan correlation)
Note
─ Cytologic features can overlap with other hyperplastic states
─ Diagnosis relies heavily on clinical context
─ Treated Graves' may show prominent microfollicular architecture
─ Treated Graves': nuclear overlapping/crowding, considerable anisonucleosis

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Thyroiditis

Inflammation of the thyroid gland, includes autoimmune and other etiologies

Hashimoto Thyroiditis (Chronic Lymphocytic Thyroiditis)

Autoimmune disease leading to thyroid destruction and often hypothyroidism
Clinical ─ Common cause of goiter and hypothyroidism; often middle-aged women; increased risk of PTC and lymphoma
Cytology
Cells
─ Mixed population present
─ Polymorphic lymphoid cells
─ Benign follicular cells
─ Hürthle cells (oncocytes)
─ Lymphocytes (varied stages), plasma cells
─ Occasional multinucleated giant cells
Cytoplasm
─ Follicular: scant to moderate
─ Hürthle cells: abundant, granular, eosinophilic
─ Lymphoid cells: scant to moderate
Nuclei
─ Follicular: round, some size variation
─ Hürthle cells: large, round, can show atypia
─ Hürthle cell nucleoli: prominent
Background
─ Prominent lymphoid infiltrate
─ (Lymphocytes, plasma cells, tangible body macrophages)
─ Colloid often scant or absent
─ Germinal center fragments possible
Absent
─ Definitive PTC features in follicular cells
─ Sheets of atypical lymphoid cells (unlike lymphoma)
Ancillary studies
─ IHC: If lymphoma suspected, lymphoid markers (CD45, CD20, CD3, light chains) useful
DDx
─ Lymphoma (monomorphic atypical lymphoid population; consider flow cytometry/IHC if suspicious)
─ Papillary thyroid carcinoma, esp Warthin-like variant (true PTC nuclear features)
─ Hürthle cell neoplasm (predominance of Hürthle cells without significant lymphoid infiltrate)
Note
─ Hürthle cell atypia can be marked but is usually benign in this context
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Subacute (Granulomatous, De Quervain) Thyroiditis

Self-limited thyroid inflammation, often post-viral, causing thyroid pain
Clinical ─ Painful, tender thyroid; fever; transient hyperthyroidism then hypothyroidism, then recovery
Cytology
Cells
─ Multinucleated giant cells, often engulfing colloid
─ Epithelioid histiocytes forming granulomas
─ Follicular cells, may be degenerated or reactive
─ Mixed inflammatory cells
─ Early: many neutrophils and eosinophils
─ Later: hypocellular, scant degenerated follicular cells
Cytoplasm
─ Follicular cells: variable, can show reactive changes
Nuclei
─ Follicular cells: can be reactive, enlarged
─ Giant cells: multiple bland nuclei
Background
─ Colloid, may be scant or engulfed by giant cells
─ Inflammatory cells, cellular debris
Absent
─ PTC nuclear features
─ Extensive fibrosis (unlike Riedel's)
─ Malignant giant cells
─ Involutional stage: absent inflammatory cells and giant cells (often insufficient for eval)
DDx
─ Palpation thyroiditis (milder features, history of recent palpation)
─ Riedel thyroiditis (rare, extensive fibrosis, paucicellular)
─ Anaplastic carcinoma (marked pleomorphism, mitoses, necrosis; giant cells are malignant)
Note
─ FNA can be painful; cytologic features vary with stage of disease
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Acute Suppurative Thyroiditis

Rare bacterial or fungal infection of the thyroid, often in immunocompromised or pre-existing thyroid disease
Clinical ─ Acute onset, fever, severe neck pain, erythema, fluctuance; may form abscess
Cytology
Cells
─ Predominantly neutrophils
─ Few reactive follicular cells
─ Bacteria or fungi may be visible (special stains helpful)
Cytoplasm
─ Follicular cells: may show reactive changes or degeneration
Nuclei
─ Follicular cells: may be reactive or obscured by inflammation
Background
─ Abundant acute inflammatory cells
─ Necrotic debris, fibrin, macrophages, blood
─ Occasionally background bacteria or fungi
Absent
─ Features of granulomatous thyroiditis
─ Typical Hashimoto's features
DDx
─ Anaplastic carcinoma with inflammatory infiltrate (malignant cells present)
─ Abscess from other source fistulizing into thyroid
Note
─ FNA is useful for diagnosis and obtaining material for culture

Riedel Thyroiditis

Rare chronic inflammatory condition characterized by dense fibrosis replacing thyroid parenchyma and extending into adjacent tissues (rarest form of thyroiditis)
Clinical ─ Rock-hard, fixed, painless goiter; compressive symptoms (dysphagia, dyspnea, hoarseness); often associated with IgG4-related disease (manifestation of IgG4-RD); fibrosing in other organs
Cytology
Cells
─ Often scant (paucicellular aspirate) or acellular
─ Few bland spindle cells (fibroblasts)
─ Rare follicular cells or inflammatory cells
Cytoplasm
─ Spindle cells: elongated
─ Follicular cells: may be atrophic
Nuclei
─ Spindle cells: bland, elongated
─ Follicular cell nuclei: small, regular
Background
─ Dense collagenous stroma (collagen strands)
─ May be difficult to aspirate
─ Scattered lymphocytes or plasma cells
Absent
─ Significant atypia
─ Colloid & follicular cells (usually)
─ Features of other thyroiditis types
DDx
─ Fibrosing variant of Hashimoto thyroiditis (more lymphocytes, Hürthle cells)
─ Anaplastic carcinoma (malignant cells, pleomorphism, mitoses)
─ Paucicellular benign nodule (lacks extensive fibrosis)
Note
─ Diagnosis often requires surgical biopsy due to paucicellularity of FNA

Thyroglossal Duct Cyst

Cystic remnant of the thyroglossal duct
Clinical ─ Midline neck mass (anterior midline, below hyoid, above thyroid isthmus), children/young adults; moves with swallowing or tongue protrusion
Cytology
Cells
─ Squamous cells (benign, anucleated, nucleated)
─ Degenerated squamous cells
─ Columnar ciliated respiratory-type cells
─ Mucinous cells
─ Inflammatory cells (macrophages, lymphocytes)
─ +/- Thyroid follicular cells (thyroglobulin + if present)
Background
─ Predominantly proteinaceous material & inflammatory cells
─ Mucoid/proteinaceous material
─ Cholesterol crystals, cellular debris
Ancillary studies
─ IHC: Thyroglobulin (+) if follicular cells present (confirms thyroidal origin of those cells)
DDx
─ Branchial cleft cyst (lateral neck, lacks respiratory/thyroid elements)
─ Cystic metastatic carcinoma (e.g. SCC, PTC; shows malignant features)
─ Dermoid cyst (more adnexal structures, keratin only)
Note
─ Carcinoma (PTC or SCC) can rarely arise within a TGDC

Branchial Cleft Cyst

Cystic remnant of embryonic branchial arches, lateral neck
Clinical ─ Lateral neck mass, anterior to sternocleidomastoid; any age, often young adult
Cytology
Cells
─ Predominantly benign squamous cells
─ (Mature squamous cells and anucleated squames)
─ +/- Columnar ciliated cells (less common than TGDC)
─ Lymphocytes, macrophages
Cytoplasm
─ Variable squamous type
Nuclei
─ Benign squamous features
Background
─ Proteinaceous fluid, cellular debris
─ Cholesterol crystals
─ Often prominent lymphoid component
─ (May include germinal center fragments)
Absent
─ Thyroid follicular cells
─ Significant atypia (unless inflamed or rare carcinoma)
DDx
─ Cystic metastatic squamous cell carcinoma (atypia, necrosis, dyskeratosis)
─ Cystic metastatic papillary thyroid carcinoma (PTC nuclear features, thyroglobulin +)
─ Thyroglossal duct cyst (midline, often respiratory/thyroid elements)
─ Lymphoepithelial cyst of salivary gland (intraglandular, associated salivary tissue)
Note
─ Inflammation can cause reactive squamous atypia; carcinoma can rarely arise

Atypia of Undetermined Significance (AUS)

Heterogeneous category; cytologic atypia present but not definitive for a benign or malignant diagnosis (atypia insufficient for FN/SFN or Suspicious categories); used sparingly (ideally <7-10% of cases per Bethesda, though rates can vary)
Clinical ─ ROM 10-40%; management involves repeat FNA, molecular testing, or diagnostic lobectomy
Cytology
General pattern
─ Predominantly benign pattern with focal, minor atypia
─ OR features suggestive but not diagnostic of neoplasm/malignancy
─ OR atypia cannot be further classified
Specific patterns often include one or more of the following:
─ AUS with nuclear atypia: Focal nuclear changes suggestive of PTC (e.g., enlargement, pallor, irregular contours, grooves) but not diffuse or classic enough for Suspicious for Malignancy or Malignant; often seen in Hashimoto's thyroiditis (common in patients with Hashimoto's)
─ AUS with architectural atypia: Predominantly microfollicular pattern with scant/absent (minimal) colloid, but cellularity or extent is insufficient for SFN; or crowded, disorganized groups with some nuclear overlap (may represent limited sampling of FN)
─ AUS with oncocytic (Hürthle cell) atypia: Predominantly oncocytic population (sparse, almost exclusively oncocytic) but insufficient cellularity for Hürthle cell neoplasm, or oncocytic cells with some nuclear atypia (e.g., size variation, prominent nucleoli) not meeting criteria for Suspicious for Hürthle Cell Neoplasm; often in Hashimoto's or multinodular goiter; minimal colloid
─ AUS with atypia of cyst lining cells: Cyst lining cells (reparative follicular cells &/or mesenchymal cells) show some nuclear atypia (elongation, grooves, prominent nucleoli, "pulled-out" cytoplasm, rare INPIs) often due to reparative changes, but malignancy cannot be excluded
─ Atypia, not otherwise specified (NOS): Atypia that does not fit neatly into the above patterns (e.g., psammoma bodies with scant bland cells; extensive Hürthle cell change in a sparsely cellular aspirate with some atypia)
Cells
─ Follicular or oncocytic (Hürthle cells)
─ Atypia is the defining feature but is limited in extent or degree
Cytoplasm
─ Variable; may be scant (microfollicles)
─ Or abundant and granular (oncocytic atypia)
Nuclei
─ Show mild to moderate changes:
─ Slight enlargement, contour irregularities
─ Chromatin clearing/pallor, or small grooves
─ Nucleoli may be visible but not overtly malignant
Background
─ Variable; may have scant colloid (esp with microfollicular pattern)
─ Or cystic changes
Absent
─ Definitive features of papillary carcinoma
─ (e.g., widespread, classic nuclear changes, true papillae, pseudoinclusions)
─ Definitive features of medullary carcinoma, or anaplastic carcinoma
Ancillary studies
─ Molecular: May be useful for risk stratification (e.g., ThyroSeq, Afirma GSC); presence of high-risk mutations (BRAF V600E, TERT) increases ROM
DDx
─ Benign follicular nodular disease (FND) (AUS has more concerning atypia or microfollicular architecture)
─ Follicular Neoplasm/Suspicious for Follicular Neoplasm (SFN) (AUS has less extensive microfollicular pattern or less crowding/overlap)
─ Non-invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features (NIFTP) (AUS may have focal PTC-like nuclear changes but not diffuse or convincing enough)
─ Papillary Thyroid Carcinoma (PTC) (AUS lacks definitive or widespread PTC nuclear features)
Note
─ This category should prompt further investigation due to the increased ROM compared to benign
─ Subclassification into patterns of atypia can be helpful for management discussions and guiding molecular testing
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Follicular Neoplasm / Suspicious for a Follicular Neoplasm (SFN)

Cytologic features suggestive of a follicular neoplasm (follicular adenoma or follicular carcinoma); definitive distinction requires histologic examination of capsule/vascular invasion
Clinical ─ ROM ~15-45%; diagnostic lobectomy is usual management
Cytology
Cells
─ Follicular, uniform, highly cellular
─ Arranged predominantly in microfollicular pattern (mc)
─ Microfollicles: flat or 3D groups
──── Circumference < 15 cells
──── In circle > 2/3 complete
──── Crowding & overlapping
──── Or arranged in crowded/trabecular, or solid pattern
─ Single cells infrequent
Cytoplasm
─ Scant to moderate, often ill-defined borders
Nuclei
─ Round to oval shape, normal size or some enlargement
─ Mild hyperchromasia, crowding/overlapping
─ Chromatin may be granular or clumpy (mild hyperchromasia)
─ Nucleoli usually inconspicuous or absent
Background
─ Scant or absent colloid
─ Stripped nuclei may be present
Absent
─ Definitive PTC nuclear features (though some overlap with FVPTC/NIFTP)
─ For potential NIFTP/FVPTC: nuclei larger, irregular contours/grooves, chromatin clearing
─ Lack true papillae, absent or very rare INPI
─ Significant oncocytic change (>75%)
─ Multinucleated cells
Ancillary studies
─ Molecular: May help in risk stratification; RAS mutations common
DDx
─ Cellular follicular nodular disease (FND) (SFN is more cellular, more microfollicular, less colloid)
─ AUS with architectural atypia (SFN shows more extensive and convincing neoplastic features)
─ Papillary thyroid carcinoma, follicular variant (FVPTC) or NIFTP (SFN lacks clear PTC nuclear features; however, this is a major area of diagnostic challenge and overlap)
─ Parathyroid lesions (can mimic follicular lesions; GATA3+, PTH+, thyroglobulin-)
Note
─ The term SFN is preferred over "follicular lesion" by Bethesda as it implies a higher likelihood of neoplasm
─ Cystic degeneration is uncommon
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Parathyroid lesions (DDx for Follicular Neoplasm)

Clinical ─ Hypercalcemia
Cytology
Cells
─ Crowded, overlapping
─ In microfollicular arrangement
─ Triangular clusters (aka “wedge pattern”)
Nuclei
─ Salt and pepper chromatin
Background
─ Colloid absent
Ancillary studies
─ IHC (+): GATA3, PTH (confirmatory)
─ IHC (-): Thyroglobulin, TTF-1 Note
─ PTH in the needle washout elevated

Follicular Neoplasm – Oncocytic (Hürthle Cell Neoplasm / Suspicious for Hürthle Cell Neoplasm)

Neoplasm composed predominantly (>75%) of oncocytic (Hürthle) cells; distinction between Hürthle cell adenoma and carcinoma requires histologic assessment of invasion
Clinical ─ ROM ~10-40% for Hürthle cell neoplasms; management usually surgical
Cytology
Cells
─ Almost exclusively (>75%) oncocytic (Hürthle cells)
─ Arranged in sheets, clusters, microfollicles, or dispersed
─ (Isolated cells, sheets, or crowded groups)
─ Moderate to high cellularity
Cytoplasm
─ Abundant, finely granular, eosinophilic
─ (Pink on H&E, green on Pap, blue or gray-pink on Wright/Giemsa)
Nuclei
─ Large size, round shape, often eccentric placement
─ Binucleation common
─ Atypia may be present:
─ Small cell atypia: high N:C
─ Large cell atypia: 2x anisonucleosis
Nucleoli
─ Prominent, central
Background
─ Scant or absent colloid
─ Lymphoid infiltrate usually sparse (unlike Hashimoto's)
─ Transgressing vessels sometimes seen
─ Intracytoplasmic "colloid" inclusions (lumens) sometimes seen
Absent
─ Definitive PTC nuclear features (distinguishes from oncocytic PTC)
─ High-grade features (necrosis, increased mitoses)
─ Abundant lymphs & plasma cells (excluding blood)
Ancillary studies
─ Molecular: Mitochondrial DNA mutations, chromosomal aneuploidies common; RAS mutations less frequent; PPARG rearrangements and BRAF V600E absent
DDx
─ Hashimoto thyroiditis with extensive oncocytic metaplasia (SFHCN has less lymphoid background, more monotonous oncocytic population)
─ Follicular nodular disease with oncocytic change (SFHCN is more cellular, predominantly oncocytic, lacks significant colloid)
─ Oncocytic variant of papillary thyroid carcinoma (SFHCN lacks definitive PTC nuclear features)
─ Medullary carcinoma (some variants can be oncocytic; calcitonin+, neuroendocrine markers+)
Note
─ Category excludes oncocytes with PTC nuclear features
─ "Small cell dysplasia" (small oncocytes with high N:C) or "large cell dysplasia" (marked anisonucleosis) can be seen and may be associated with higher risk
─ Transgressing vessels may be seen in cell block
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Suspicious for Malignancy

Cytologic features are suspicious for, but not diagnostic of, malignancy; this category is subclassified based on the suspected type of malignancy
Clinical ─ ROM ~45-85% (highly variable depending on subcategory and institutional rates); management typically involves surgical intervention, often total thyroidectomy for suspicious for PTC or MTC
Cytology
General pattern
─ Features present are highly suggestive of a specific malignancy
─ But are quantitatively or qualitatively insufficient for a definitive diagnosis
─ (e.g., focal changes, limited cellularity, obscuring factors like cyst debris)
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Suspicious for Papillary Thyroid Carcinoma

Features are suggestive of PTC, but fall short of a definitive diagnosis; often due to limited extent of nuclear features, sparse cellularity, or cystic degeneration obscuring features
Cytology
Cells
─ Follicular, may be in sheets, clusters
─ Or show microarchitectural abnormalities
Cytoplasm
─ Variable, may be dense
─ Or "histiocytoid" in cystic cases
─ (Abundant vacuolated cytoplasm in histiocytoid cells)
Nuclei
─ Show some features of PTC:
─ Enlargement, pallor, grooves
─ Irregular contours, pseudoinclusions
─ But these are either focal, not fully developed, or seen in limited cells
Background
─ May be clean, cystic (with macrophages, hemosiderin-laden macrophages)
─ Or show scant colloid
Absent
─ Widespread classic PTC features
─ Sufficient atypical cells to definitively diagnose PTC
─ Psammoma bodies, INPIs (or rare), true papillary architecture usually absent
Patterns:
─ Patchy nuclear changes pattern: Nuclear features of PTC present but only in some cell groups or focally; at least moderately cellular
─ Incomplete nuclear changes pattern: Nuclear features generalized but mild (e.g., some enlargement and pallor but few grooves or irregular contours); variable cellularity; nuclear membrane irregularity or molding absent
─ Sparsely cellular pattern: Classic PTC nuclear features present but on very few cells
─ Cystic degeneration pattern: Atypical cells with PTC-like nuclear features in cyst fluid; often "histiocytoid" appearance; features may be obscured
DDx
─ AUS with nuclear atypia (Suspicious has more convincing/extensive, albeit still limited, PTC features)
─ Papillary thyroid carcinoma (Suspicious lacks sufficient quantity/quality of features for definitive diagnosis)
─ Benign changes with reactive atypia (e.g., cyst lining cells, Hashimoto's; Suspicious has more specific PTC-like changes)
Note
─ Molecular testing can be particularly useful in this subcategory

Suspicious for Medullary Thyroid Carcinoma

Features are suggestive of MTC, but may be limited by cellularity, preservation, or lack of definitive amyloid/immunostains
Cytology
Cells
─ Often dispersed, plasmacytoid, spindled, or polygonal
─ Monomorphic noncohesive population
─ May form loose clusters
─ Sparsely or moderately cellular
Cytoplasm
─ Granular (granules not discernible)
─ Eosinophilic to amphophilic
─ Eccentric nuclei common
Nuclei
─ "Salt-and-pepper" chromatin (may be smudged due to suboptimal preservation)
─ Eccentrically placed, round to oval or irregular shapes
─ Binucleation or multinucleation may be seen
─ Nucleoli usually inconspicuous
Background
─ May show amorphous material (fragments suspicious for amyloid but not definitive vs colloid)
─ Blood
Absent
─ Definitive amyloid (e.g., by Congo red)
─ Or confirmatory IHC (calcitonin) due to scant material or technical limitations
DDx
─ AUS (Suspicious for MTC has more specific features like plasmacytoid cells and granular chromatin)
─ Medullary thyroid carcinoma (Suspicious lacks definitive confirmation of amyloid or calcitonin positivity)
─ Hürthle cell neoplasm (can have granular cytoplasm and prominent nucleoli, but different nuclear chromatin; MTC is calcitonin +)
─ Lymphoma/Plasmacytoma (MTC cells are epithelial, not lymphoid/plasma cells; IHC helps)
Note
─ Calcitonin measurement in needle washout fluid can be helpful if material is available (or if inadequate material for IHC)

Suspicious for Lymphoma

Atypical lymphoid population is present (numerous atypical lymphoid cells OR sparsely cellular containing atypical lymphoid cells), but features are not definitive for lymphoma (e.g., due to cellularity, admixed reactive elements, or need for ancillary studies)
Cytology
Cells
─ Predominantly lymphoid
─ May show monomorphism or atypia
─ (Irregular nuclear contours, coarse chromatin, prominent nucleoli)
Cytoplasm
─ Scant
Nuclei
─ Show features concerning for lymphoma but not diagnostic
Background
─ May show lymphoglandular bodies
─ Follicular cells are typically scant or absent
Absent
─ Definitive features of Hashimoto's thyroiditis (polymorphous lymphoid population, Hürthle cells)
─ Definitive IHC/flow cytometry for lymphoma
DDx
─ Hashimoto thyroiditis (Suspicious for Lymphoma has a more monomorphic or atypical lymphoid population)
─ Lymphoma (Suspicious lacks definitive diagnostic features or confirmation by ancillary studies)
─ AUS (if lymphoid atypia is very focal or mild)
Note
─ Flow cytometry and/or cell block for IHC are crucial if lymphoma is suspected

Suspicious for Malignancy, Not Otherwise Specified (NOS)

Features are suspicious for malignancy but do not fit well into the specific subcategories above, or suggest a rare malignancy where definitive features are lacking
Cytology
Cells
─ Show significant atypia concerning for malignancy
Cytoplasm
─ Features do not clearly point to PTC, MTC, or lymphoma
Nuclei
─ Features do not clearly point to PTC, MTC, or lymphoma
Background
─ May be necrotic or inflammatory
Absent
─ Clear differentiation towards a specific common thyroid malignancy
Note
─ This subcategory is used infrequently
─ May include cases suspicious for metastatic malignancy or rare primary thyroid cancers

Malignant

Cytologic features are diagnostic of malignancy; specific type of malignancy is usually identifiable
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Papillary Thyroid Carcinoma (PTC)

Most common thyroid malignancy, characterized by a constellation of distinctive nuclear features
Clinical ─ Any age, peak 30-50 yrs, F>M; prognosis generally excellent, variable with subtype and stage; lymph node metastases common
Cytology
Cells
─ Follicular epithelial cells
─ Arranged in papillary structures (true fibrovascular cores)
─ Or monolayered sheets, 3D groups, microfollicles
─ Or syncytial-type clusters (cellular swirls, "onion-skin" or "cartwheel")
─ Variable cellularity
Cytoplasm
─ Usually moderate amount
─ Can be dense eosinophilic, squamoid, or clear
─ "Histiocytoid" in cystic PTC
─ Oncocytic or squamous metaplasia can occur
─ Hobnail features (e.g. around psammoma bodies)
Nuclei
─ Show characteristic features (one or more, ideally multiple):
─ Enlargement and overlapping, often molded
─ Irregular contours (longitudinal nuclear grooves, notches, cerebriform shapes)
─ Thick nuclear membranes
─ Chromatin clearing/pallor ("Orphan Annie eye" appearance, powdery)
─ Intranuclear cytoplasmic pseudoinclusions (INPIs)
─ Micronucleoli, often marginal (or macro, central or marginal)
Background
─ May show psammoma bodies (laminated calcifications)
─ "Ropy" or "bubble gum" colloid (thick, viscous)
─ Multinucleated giant cells
─ Hemosiderin-laden macrophages (in cystic variants)
─ Variable lymphocytes (esp with underlying thyroiditis)
─ Necrotic debris is extremely rare
Absent
─ Features of anaplastic carcinoma or medullary carcinoma (unless mixed tumor)
Ancillary studies
─ IHC (+): Thyroglobulin, TTF-1, PAX8 (generally useful for confirming thyroid origin if unusual presentation)
─ Molecular: BRAF V600E mutation common; RAS mutations; RET/PTC rearrangements
DDx
─ Benign follicular nodule with reactive atypia or papillary hyperplasia (PTC has more convincing and widespread nuclear features)
─ AUS/Suspicious for PTC (PTC shows definitive and sufficient features)
─ NIFTP (Non-invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features) (cytologically indistinguishable from infiltrative FVPTC; NIFTP lacks invasion on histology, has specific molecular profile)
─ Hyalinizing trabecular tumor (can have grooves/INPIs; distinct architecture, MIB1 membranous staining)
Note
─ Many variants exist (follicular, tall cell, columnar cell, cribriform-morular, diffuse sclerosing, Warthin-like, oncocytic, hobnail, solid) with varying cytologic features and prognoses
─ NIFTP is a non-malignant (or very low risk) entity on histology, but its FNA may be called AUS, SFN, Suspicious for PTC, or even PTC based on extent of nuclear features
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NIFTP (Non-Invasive Follicular Thyroid Neoplasm with Papillary-like Nuclear Features)

A non-invasive encapsulated or well-demarcated follicular-patterned neoplasm with PTC-like nuclear features (≥ some degree of atypia present, unlike benign nodule); reclassified from non-invasive encapsulated FVPTC to reflect its indolent behavior
Clinical ─ Indolent behavior, extremely low risk of adverse outcome if strict diagnostic criteria are met on histology
Cytology
Cells
─ Follicular cells
─ Predominantly in a microfollicular, trabecular, or solid pattern
─ May show some macrofollicles
Cytoplasm
─ Moderate, eosinophilic
Nuclei
─ Show features of PTC (score 2-3 on nuclear scoring system):
─ Enlargement, irregular shape/contours
─ Grooves, chromatin clearing
─ INPIs are rare or absent
Background
─ Scant or absent colloid
Absent
─ Psammoma bodies and true papillae (unlike PTC)
─ Invasion (histologic feature)
─ High-grade features
Ancillary studies
─ Molecular: Predominantly RAS mutations or RAS-like alterations (e.g., PPARG, THADA fusions); BRAF V600E and TERT promoter mutations are typically absent
DDx
─ Follicular adenoma (NIFTP has PTC-like nuclear features)
─ Infiltrative Follicular Variant PTC (cytologically indistinguishable; FVPTC shows invasion on histology)
─ Benign follicular nodule with atypia (NIFTP has more consistent and developed PTC nuclear features)
Note
─ Cytologic diagnosis of NIFTP is not possible
─ FNA may be classified as AUS, SFN, Suspicious for PTC, or Malignant (PTC) depending on the extent and quality of nuclear features
─ The goal of identifying potential NIFTP on FNA is to guide conservative management if appropriate, but definitive diagnosis is histologic
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Papillary Thyroid Carcinoma – Encapsulated Follicular Variant (EFVPTC) and NIFTP (for historical context/comparison)

Cytology (for EFVPTC that would now be NIFTP or invasive EFVPTC)
Cells
─ Follicular, low to moderate cellularity
─ Follicular architectural pattern
Nuclei
─ PTC features subtle, partial, and focally displayed:
─ Enlargement, elongation
─ Chromatin clearing, thick nuclear membranes
Absent
─ INPIs and nuclear grooves (may be rare)
─ True papillae, psammoma bodies
─ Sheet-predominant pattern
Molecular (for NIFTP/encapsulated forms)
─ RAS, or RAS-like (PPARG & THADA) (like follicular neoplasms)
─ RET & RET-like (BRAF V600E) absent (unlike classic PTC)

Papillary Thyroid Carcinoma – Follicular Variant (FVPTC) with Infiltrative Growth

PTC variant characterized by an almost exclusively follicular growth pattern with typical PTC nuclear features and infiltrative growth
Clinical ─ Behavior similar to classical PTC if infiltrative; frequent LN mets, risk recurrence
Cytology
Cells
─ Follicular cells
─ Predominantly in microfollicles, trabeculae, or solid sheets
Cytoplasm
─ Moderate, eosinophilic
Nuclei
─ Show characteristic PTC features:
─ Enlargement, irregular contours, grooves
─ Chromatin clearing, +/- INPIs
Background
─ Scant or absent colloid
─ Psammoma bodies are rare
Absent
─ True papillae (rare or absent)
─ Significant component of classic papillary architecture
Ancillary studies
─ Molecular: BRAF V600E (“BRAF-like PTCs”) more common in infiltrative FVPTC
DDx
─ Follicular neoplasm (FVPTC has PTC nuclear features)
─ NIFTP (cytologically indistinguishable; NIFTP is non-invasive on histology)
─ Benign follicular nodule with reactive atypia (FVPTC has more diffuse and convincing PTC nuclear features)
Note
─ Infiltrative FVPTC is considered malignant
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Papillary Thyroid Carcinoma – Macrofollicular Variant

Very rare PTC variant
Cytology
Cells
─ Follicular, in monolayered sheets or variably sized follicles
Nuclei
─ Subtle and patchy PTC nuclear features
Absent
─ Psammoma bodies and papillary structures

Papillary Thyroid Carcinoma – Cystic Variant

PTC variant presenting as a cyst or with prominent cystic change
Clinical ─ May be misdiagnosed as benign cyst if atypical cells are sparse
Cytology
Cells
─ Follicular, typically in small groups with irregular borders
─ Often "histiocytoid" (hypervacuolated cytoplasm)
Nuclei
─ Definite PTC-like features
─ Chromatin may be less fine (more smudged or degenerated)
Background
─ Cyst fluid with hemosiderin-laden macrophages
Absent
─ Extensive solid component (in pure cystic form)
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Papillary Thyroid Carcinoma – Oncocytic Variant

PTC variant composed predominantly (>75%) of oncocytic cells with characteristic PTC nuclear features
Clinical ─ Similar behavior to classic PTC of similar stage; may be associated with Hashimoto's thyroiditis
Cytology
Cells
─ Predominantly oncocytic (>75%)
─ Arranged in papillae, sheets, or clusters
Cytoplasm
─ Abundant, granular, eosinophilic
Nuclei
─ Definitive PTC features superimposed on oncocytic cells:
─ Enlargement, irregular contours, grooves
─ Chromatin clearing, +/- INPIs
Background
─ May show lymphocytes if Hashimoto's is present (or few lymphocytes)
─ Psammoma bodies can occur
Absent
─ Features of Hürthle cell neoplasm (which lacks PTC nuclei)
Ancillary studies
─ Molecular: BRAF V600E and RAS mutations can occur
DDx
─ Hürthle cell neoplasm (lacks PTC nuclear features)
─ Hashimoto's thyroiditis with extensive oncocytic metaplasia and atypia (PTC nuclear features are definitive in Oncocytic PTC)
─ Follicular nodular disease with oncocytic change (lacks PTC nuclear features)
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Papillary Thyroid Carcinoma – Warthin-Like Variant

PTC variant resembling Warthin tumor of salivary gland, with papillary architecture and oncocytic cells in a prominent lymphoid stroma
Clinical ─ Often associated with Hashimoto's thyroiditis; good prognosis
Cytology
Cells
─ Oncocytic cells
─ Lining papillary fronds with fibrovascular cores
─ Cores are permeated by lymphocytes
Nuclei
─ Convincingly PTC nuclear features
Background
─ Prominent lymphoplasmacytic infiltrate
Absent
─ Features of simple Hashimoto's with reactive atypia (PTC nuclei are key)

Papillary Thyroid Carcinoma – Tall Cell Variant

Aggressive variant of PTC defined by >30% tall cells (height at least 2-3 times width)
Clinical ─ Often older patients, M>F; associated with more aggressive behavior, extrathyroidal extension, metastases, and poorer prognosis
Cytology
Cells
─ Follicular, often in papillary structures or clusters
─ >30% of cells are tall (height 2-3x width)
Cytoplasm
─ Abundant, eosinophilic, granular
Nuclei
─ Classic PTC features, often more prominent
─ Larger and longer; chromatin more granular
─ Nucleoli can be prominent and central
Background
─ May show features of classic PTC; necrosis can be seen
─ Fewer psammoma bodies; more INPIs (often multiple within a nucleus)
Absent
─ Features of other aggressive variants like hobnail (unless mixed)
Ancillary studies
─ Molecular: BRAF V600E in vast majority; TERT promoter mutations also common, portending worse prognosis
DDx
─ Classic PTC (Tall Cell Variant has predominant tall cell morphology)
─ Oncocytic PTC (oncocytes are polygonal with abundant granular cytoplasm, central nuclei; tall cells are columnar)
─ Columnar Cell Variant PTC (columnar cells with pseudostratification and sub/supranuclear vacuoles; different nuclear features)
Note
─ Even 10% tall cells may indicate more aggressive behavior

Papillary Thyroid Carcinoma – Columnar Cell Variant

Rare aggressive PTC variant with cells resembling secretory endometrium or colonic adenoma
Clinical ─ Aggressive in older patients
Cytology
Cells
─ Arranged in papillae, flat sheets, or clusters
─ Pseudostratified columnar shape
Cytoplasm
─ Shows supranuclear & subnuclear vacuoles
Nuclei
─ PTC changes present, but are less prominent
─ Generally few INPI or nuclear grooves
─ Chromatin more hyperchromatic
Background
─ Generally lacks colloid or cystic changes
Ancillary studies
─ IHC (+): PAX8, most also CDX2 (key for DDx with metastatic colorectal)
─ Molecular: BRAF V600E, TERT promoter mutations
DDx
─ Tall Cell Variant PTC (different cell shape and cytoplasmic features)
─ Metastatic adenocarcinoma (colonic) (CDX2+, but thyroglobulin/PAX8-)

Papillary Thyroid Carcinoma – Solid / Trabecular Variant

Rare PTC variant with >50% solid, trabecular, nested, or insular growth and <50% follicles, papillae, colloid
Clinical ─ May have more aggressive behavior than classic PTC
Cytology
Cells
─ Follicular, in syncytial 3D fragments
─ Or microfollicles, trabeculae
Nuclei
─ Show PTC features
Absent
─ True papillae with fibrovascular cores
Ancillary studies
─ Molecular: RET, NTRK, TERT promoter mutations

Papillary Thyroid Carcinoma – Diffuse Sclerosing Variant

Rare PTC variant characterized by diffuse involvement of one or both thyroid lobes, prominent fibrosis, squamous metaplasia, and numerous psammoma bodies
Clinical ─ More common in children and young adults, esp with history of nuclear fallout; extensive LVI, frequent LN mets; prognosis vs. conventional: similar disease-free survival but similar mortality
Cytology
Cells
─ Follicular, in 3D ball-like clusters
─ Intermingling with inflammatory cells
─ Moderate-high cellularity
Cytoplasm
─ Dense with distinct cell borders
─ Unilocular cytoplasmic vacuoles common
─ Squamous metaplastic changes common
Nuclei
─ Show PTC changes; chromatin less pale
─ Fewer INPIs and nuclear grooves
Background
─ Numerous lymphocytes and psammoma bodies
─ Colloid scant or absent
Ancillary studies
─ Molecular: NCOA4::RET fusion common in setting of radiation/fallout

Papillary Thyroid Carcinoma – Hobnail Variant

Rare aggressive PTC variant characterized by cells with apical nuclei creating a "hobnail" or "comet-like" appearance
Clinical ─ Aggressive PTC subtype
Cytology
Cells
─ Show loss of polarity & cohesiveness
─ Micropapillary or papillary clusters
Cytoplasm
─ Tapered, comet- or teat drop-like
Nuclei
─ Eccentric (apically placed)
─ Typical PTC features present
─ Soap bubble-like INPIs
Ancillary studies
─ Molecular: BRAF V600E in vast majority

Medullary Thyroid Carcinoma (MTC)

Neuroendocrine malignancy derived from parafollicular C-cells; accounts for 3-5% of thyroid cancers
Clinical ─ Can be sporadic (75-80%) or hereditary (20-25%, associated with MEN2A, MEN2B, or familial MTC syndromes due to germline RET mutations); may present with neck mass, dysphagia, or symptoms of hormone production (calcitonin, CEA, rarely others); prognosis variable
Cytology
Cells
─ Variable: plasmacytoid, spindled, polygonal, or small/round
─ Often dispersed (non-cohesive)
─ Also in loose clusters, syncytia, or trabeculae
─ Moderate to high cellularity
Cytoplasm
─ Moderate to abundant, granular
─ (Fine eosinophilic or amphophilic granules)
─ Sometimes clear or oncocytic
─ May have vacuoles, melanin, and lumina
─ Eccentric nuclei common in plasmacytoid cells
Nuclei
─ Round, oval, or irregular shapes, eccentric
─ "Salt-and-pepper" (stippled, coarsely granular, neuroendocrine) chromatin characteristic
─ Binucleation/multinucleation common
─ Nucleoli mostly inconspicuous but can be prominent
─ INPIs uncommon
Background
─ Often shows amyloid deposits:
─ Amorphous, waxy, pink-purple on Diff-Quik
─ Orange-red with Congo red (apple-green birefringence)
─ Resembling thick colloid
─ Blood; scant or absent colloid
Absent
─ Definitive PTC nuclear features (no nuclear grooves)
─ Follicular differentiation (unless mixed tumor)
Ancillary studies
─ IHC (+): Calcitonin (highly specific and sensitive), CEA, Chromogranin A, Synaptophysin (these are key for diagnosis)
─ IHC (+, often): TTF-1
─ IHC (-): PAX8, Thyroglobulin
─ Molecular: Somatic RET mutations common in sporadic MTC; germline RET testing for hereditary cases (critical for patient/family management)
DDx
─ Hürthle cell neoplasm (granular cytoplasm, but different chromatin, prominent nucleoli, Calcitonin (-))
─ Lymphoma/Plasmacytoma (dispersed cells, but lymphoid/plasma cell features, specific markers (+), Calcitonin (-))
─ Anaplastic carcinoma, spindle cell type (more pleomorphism, mitoses, necrosis; Calcitonin (-))
─ Parathyroid lesions (can have salt & pepper chromatin; PTH (+), Calcitonin (-))
─ Hyalinizing trabecular tumor (trabecular pattern, but different nuclear features, amyloid (-), Calcitonin (-))
Note
─ Amyloid may not always be obvious on FNA smears; cell block for Congo red stain is helpful
─ Needle washout for calcitonin measurement is highly sensitive
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High-Grade Follicular Cell-Derived Non-Anaplastic Thyroid Carcinoma

This category includes poorly differentiated thyroid carcinoma (PDTC) and differentiated high-grade thyroid carcinoma (DHGTC), which are aggressive follicular cell-derived tumors that are not anaplastic. PDTC is defined by solid/trabecular/insular growth (cellular nests with fibrovascular border), necrosis, and/or increased mitoses (≥3 mits in 10 HPF), lacking well-formed papillae or follicles of PTC/FTC and not meeting criteria for anaplastic carcinoma. DHGTC encompasses PTC or FTC with high-grade features (necrosis or high mitoses).
Clinical ─ Aggressive behavior, intermediate prognosis between well-differentiated and anaplastic carcinomas; older patients; often presents with large mass, local invasion, or metastases
Cytology
Cells
─ Follicular epithelial, uniform malignant population
─ Often in solid sheets, insular clusters, or trabeculae
─ Can show discohesion; high cellularity
─ Microfollicles often present
Cytoplasm
─ Scant to moderate, eosinophilic or amphophilic
─ Sometimes plasmacytoid
Nuclei
─ Significant atypia: variable atypia, high N:C
─ Pleomorphism, hyperchromasia, irregular contours
─ Coarse chromatin; nucleoli may be prominent
─ Increased mitotic activity (often atypical)
─ Convoluted nuclei may be a feature of PDTC
Background
─ Often shows necrosis (tumor cell necrosis), apoptosis
─ Scant or absent (minimal) colloid
─ Inflammation may be present
Absent
─ Definitive features of classic PTC (though PTC nuclei not prominent throughout)
─ (Some PTC-like nuclear features may be focally present, or it may arise from PTC)
─ Extreme pleomorphism of anaplastic carcinoma (frank anaplasia)
Ancillary studies
─ IHC (+): Thyroglobulin (may be focal/weak), TTF-1, PAX8 (confirms thyroid origin)
─ Molecular: Can harbor BRAF, RAS, TERT promoter mutations, TP53 mutations (more common in higher-grade tumors)
DDx
─ Anaplastic thyroid carcinoma (PDTC/DHGTC shows less extreme pleomorphism and often retains some follicular differentiation markers)
─ Medullary thyroid carcinoma (PDTC/DHGTC is Calcitonin (-))
─ Metastatic carcinoma (clinical history and IHC panel crucial)
─ Follicular neoplasm with atypia (PDTC/DHGTC has more overt malignant features like necrosis, high mitoses, significant atypia)
Note
─ Cytologic diagnosis can be challenging; features like insular growth, necrosis, and high mitotic rate are key clues
─ DHGTC is a newer concept where high-grade features are found in a tumor otherwise classifiable as PTC or FTC

Anaplastic Thyroid Carcinoma (ATC)

Highly aggressive, undifferentiated malignancy of thyroid follicular epithelium; one of the most lethal human cancers
Clinical ─ Rapidly enlarging neck mass, often in elderly patients (>50 yo, female); symptoms from neck compression/invasion (dyspnea, dysphagia, hoarseness); lymphadenopathy and distant metastases common at presentation (mc lungs); history of long-standing goiter and euthyroid; dismal prognosis
Cytology
Cells
─ Highly pleomorphic:
─ Spindled (sarcomatoid)
─ Giant cells (osteoclast-like or bizarre multinucleated)
─ Epithelioid, small, or mixed
─ Often discohesive or in loose, irregular clusters
─ (Isolated &/or variably sized groups)
─ Variable cellularity (at least moderate)
─ Can be paucicellular if extensive necrosis/inflammation
Cytoplasm
─ Variable: scant to abundant
─ Eosinophilic, amphophilic, or clear
─ May be infiltrated by neutrophils
─ Phagocytosis of inflammatory cells or other tumor cells may be seen
Nuclei
─ Extreme pleomorphism, enlarged, irregular
─ Marked hyperchromasia
─ Irregular chromatin clumping with parachromatin clearing
─ Bizarre shapes, multiple irregular nucleoli
─ Frequent and atypical mitotic figures
Background
─ Typically necrotic, inflammatory
─ ("Abscess-like" with neutrophil predominant inflammation)
─ Hemorrhagic; tumor diathesis prominent
Absent
─ Differentiated features of PTC, MTC, or lymphoma
─ (Though ATC can arise from pre-existing differentiated thyroid cancer)
Ancillary studies
─ IHC (+): PAX8 (in ~50-80%, can be focal), Pan-Keratin (often, but can be lost)
─ IHC (-): Thyroglobulin (usually negative or very focal), TTF-1 (usually negative)
─ Molecular: Complex karyotypes; TP53 mutations very common; CTNNB1, RAS, BRAF V600E, TERT promoter mutations also frequent
DDx
─ Poorly differentiated/High-grade non-anaplastic carcinoma (ATC has more extreme pleomorphism, often loss of thyroid-specific markers)
─ Medullary carcinoma with anaplastic transformation (Calcitonin may be focally positive in MTC component)
─ Sarcoma (primary or metastatic) (IHC panel crucial; PAX8/keratin favors ATC)
─ Metastatic undifferentiated carcinoma or melanoma (clinical history, IHC panel)
─ Riedel thyroiditis (paucicellular, bland spindle cells, dense fibrosis, no overt malignant features)
Note
─ A small focus of ATC in a background of differentiated thyroid cancer is sufficient for diagnosis
─ Squamoid, rhabdoid, or osteoclastic features can be present
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Lymphoma (Primary Thyroid Lymphoma)

Malignant proliferation of lymphoid cells primarily involving the thyroid gland; most are non-Hodgkin lymphomas, typically B-cell type (esp MALT lymphoma and DLBCL)
Clinical ─ Often arises in background of Hashimoto thyroiditis; typically older women; rapidly enlarging neck mass, compressive symptoms; prognosis variable depending on type and stage
Cytology
Cells
─ Predominantly lymphoid
─ Often monotonous population of atypical lymphocytes
─ Discohesive; marked cellularity, non-cohesive
Cytoplasm
─ Scant, basophilic
Nuclei
─ Features dependent on lymphoma type:
─ MALT lymphoma: Small to medium-sized lymphocytes (2x mature lymphocyte)
─ Irregular nuclear contours, condensed/vesicular chromatin
─ Inconspicuous/small nucleoli (centrocyte-like cells)
─ Lymphoepithelial lesions (lymphocytes infiltrating follicular epithelium) may be seen
─ DLBCL: Large lymphocytes
─ Vesicular/coarse chromatin, prominent nucleoli
─ Irregular nuclear membranes; mitoses common
─ Abundant basophilic cytoplasm (for DLBCL)
Background
─ Lymphoglandular bodies (cytoplasmic fragments)
─ Follicular cells are scant or absent (or oncocytes)
─ Or show features of Hashimoto's
─ Necrosis may be present in high-grade lymphomas
─ Plasma cells absent
Absent
─ Features of Hashimoto's alone (polymorphous infiltrate, Hürthle cells, germinal centers without overt atypia)
Ancillary studies
─ IHC (on cell block) / Flow Cytometry: Crucial for diagnosis and classification; B-cell markers (CD20, CD79a), T-cell markers (CD3, CD5), Kappa/Lambda light chain restriction (for B-cell clonality), Ki-67 proliferation index
─ Molecular: Clonality studies (e.g., IgH gene rearrangement) can confirm B-cell lymphoma
DDx
─ Hashimoto thyroiditis (polymorphous lymphoid infiltrate, Hürthle cells, reactive germinal centers; lymphoma is more monotonous or overtly atypical)
─ Anaplastic carcinoma, small cell type (ATC is keratin+, PAX8+; lymphoma is CD45+)
─ Medullary carcinoma, small cell variant (MTC is Calcitonin+, neuroendocrine markers+)
─ Metastatic small cell carcinoma (clinical history, IHC)
Note
─ FNA is often diagnostic, especially with ancillary studies; however, excisional biopsy may be needed for definitive subtyping in some cases

Bethesda ROM & Management

Brief summary of Risk of Malignancy (ROM) and general management for each Bethesda category (as per 3rd Edition, 2023)
─ I. Non-Diagnostic/Unsatisfactory: ROM not well defined (varies with reason for ND); Repeat FNA with US-guidance
─ II. Benign: ROM <5% (typically 0-3%); Clinical and/or US follow-up
─ III. AUS: ROM ~10-40% (highly variable, can be sub-stratified); Repeat FNA, molecular testing, or diagnostic lobectomy
─ IV. Follicular Neoplasm (SFN/HCN): ROM ~15-45% (FN <25-40%); Diagnostic lobectomy or molecular testing to guide surgery
─ V. Suspicious for Malignancy: ROM ~45-85% (highly variable, <60-75%); (Near) total thyroidectomy or diagnostic lobectomy based on suspicion/moleculars
─ VI. Malignant: ROM ~95-99% (100%); (Near) total thyroidectomy (or lobectomy for low-risk PTC)
Note ─ ROMs are approximate and can vary by institution and specific features within categories; molecular testing increasingly refines risk. Management is individualized.
Media ─ placeholder ─ ─ Benign video; Atypia video; Follicular video

Other Tumors

Rare primary or secondary tumors of the thyroid

Cribriform-Morular Thyroid Carcinoma

Rare indolent tumor, often associated with Familial Adenomatous Polyposis (FAP) or Gardner syndrome; can also be sporadic
Clinical ─ Young females predominantly; excellent prognosis
Cytology
Cells
─ Epithelial cells
─ Arranged in cribriform, papillary-like formations, or solid patterns
─ Characteristic morules (squamoid whorls/eddy formation without keratinization)
─ Background spindle cells, background histiocytes
Cytoplasm
─ Moderate, eosinophilic
Nuclei
─ Elongated, pseudostratified appearance in papillary/cribriform areas
─ Hyperchromatic, vesicular chromatin
─ Nuclear grooves and pseudoinclusions common (few other PTC-like features)
─ Morular cells have bland, pale nuclei
Background
─ May show spindle cells
─ Colloid usually scant or absent
─ Psammoma bodies and multinucleate giant cells rare/absent
Absent
─ Typical thick colloid or extensive lymphocytic infiltrate
Ancillary studies
─ IHC (+): Nuclear/cytoplasmic β-catenin (strong and diffuse, key feature), TTF-1, PAX8 (often weak/focal), ER/PR (often)
─ IHC (-): Thyroglobulin (often negative or focal), Calcitonin
─ Molecular: APC gene mutations (in FAP-associated); CTNNB1 (Wnt/β-catenin) mutations (in sporadic)
DDx
─ Papillary thyroid carcinoma (CMTC has morules, β-catenin nuclear staining, often thyroglobulin negative)
─ Hyalinizing trabecular tumor (different architecture, MIB1 membranous staining)
Note
─ Morules are a key diagnostic feature

Hyalinizing Trabecular Tumor (HTT)

Rare follicular cell-derived neoplasm of uncertain malignant potential (vast majority women), though most behave indolently; some consider it a variant of PTC
Clinical ─ Predominantly adult females; usually presents as solitary nodule; excellent prognosis
Cytology
Cells
─ Follicular, round or spindle shaped
─ Arranged in trabeculae, nests, or clusters
─ Radiating from hyaline core
─ Embedded in hyaline stromal material (may be scant on FNA)
Cytoplasm
─ Moderate, eosinophilic or amphophilic
─ Paranuclear yellow bodies (intracytoplasmic hyaline globules) may be seen (rare on FNA)
Nuclei
─ Elongated or oval shape, fine chromatin
─ Numerous nuclear grooves, and intranuclear pseudoinclusions (PTC-like features common)
Background
─ May show hyaline material (pink, amorphous on Diff-Quik)
─ Colloid scant or absent; occasional PBs
Absent
─ True papillae or extensive microfollicular pattern
─ Papillary, sheet-like fragments
Ancillary studies
─ IHC (+): MIB1/Ki-67 shows characteristic membranous staining pattern (very specific)
─ Molecular: GLIS rearrangements (GLIS1 or GLIS3) are characteristic and specific; BRAF/RAS mutations absent
DDx
─ Papillary thyroid carcinoma (HTT has trabecular architecture, hyaline stroma, MIB1 membranous staining, GLIS rearrangement)
─ Medullary thyroid carcinoma (HTT is calcitonin negative, amyloid negative)
─ Follicular neoplasm (HTT has PTC-like nuclear features and characteristic IHC/molecular)
Note
─ Cytologic diagnosis can be challenging due to overlap with PTC; MIB1 staining on cell block can be very helpful

Main Document

Cervix

Table of Contents

Non-Diagnostic, Unsatisfactory

NILM

Squamous cells

Superficial cells

Intermediate cells (IMCs)

Parabasal cells

Glandular cells

Endocervical glandular cells

Endometrial glandular cells (exfoliated)

Directly sampled endometrial cells (stromal & glandular)

Benign Reactions

Repair (Typical Repair)

Metaplastic Squamous cells

Tubal metaplasia

Atrophy

Pregnancy related changes

Arias Stella reaction

Intrauterine device (IUD) effect

Keratinization

Parakeratosis

Hyperkeratosis

Lymphocytic (Follicular) Cervicitis

Radiation

Glandular cells status post hysterectomy

Contaminants and miscellaneous

Trichomonas vaginalis

Candida

Bacterial vaginosis (BV) / Gardnerella vaginalis

Actinomyces

Herpes Simplex Virus (HSV)

Cytomegalovirus (CMV)

Atypical Squamous Cells of Undetermined Significance (ASC-US)

Atypical Squamous Cells, cannot exclude HSIL (ASC-H)

Low-Grade Squamous Intraepithelial Lesion (LSIL)

High-Grade Squamous Intraepithelial Lesion (HSIL)

Atypical Glandular Cells (AGC)

Atypical Endocervical Cells, Not Otherwise Specified

Atypical Endocervical Cells, Favor Neoplastic

Atypical Endometrial Cells

Squamous cell carcinoma (SCC)

Non-keratinizing SCC (including basaloid)

Keratinizing SCC

Adenocarcinoma

Endocervical Adenocarcinoma in situ (AIS)

Endocervical Adenocarcinoma

Endometrial Adenocarcinoma

Other Malignant Neoplasms

Small cell carcinoma

Mucinous Carcinoma, Gastric Type (includes Minimal Deviation Adenocarcinoma/Adenoma Malignum)

Non-Diagnostic, Unsatisfactory

This section discusses adequacy criteria.

The assessment of specimen adequacy is a critical component of the cervical cytology report, ensuring that the sample is suitable for evaluation of epithelial abnormalities

Clinical

─ Unsatisfactory specimens generally require a repeat Pap test in 2-4 months; hrHPV triage is not recommended for unsatisfactory cytology

─ Specimens negative for intraepithelial lesion or malignancy (NILM) but lacking an endocervical/transformation zone (EC/TZ) component have specific follow-up guidelines based on age and HPV status

Cell patterns

─ Satisfactory: Estimated minimum of 8,000-12,000 well-visualized squamous cells (conventional smears) or 5,000 (liquid-based preparations, LBP); presence or absence of EC/TZ component (at least 10 well-preserved endocervical or squamous metaplastic cells) should be reported

Background

─ Unsatisfactory if >75% of epithelial cells obscured by blood, inflammation, debris, air-drying, or other artifacts; lubricants with carbomers/Carbopol polymers can adversely affect LBP adequacy

Absent

─ Unsatisfactory if insufficient squamous epithelial cells below the minimum estimated numbers (unless atypia present or other specific exceptions for low cellularity are met)

Note

─ Any specimen with ASC-US or a more significant epithelial abnormality is, by definition, satisfactory for evaluation

─ Laboratories should not rigidly apply the 5,000 cell threshold for LBP in vaginal and post-therapy specimens

─ Reporting of EC/TZ component is a quality indicator, though its absence in a negative Pap test does not necessarily mandate early repeat screening

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NILM

(Negative for Intraepithelial Lesion or Malignancy, i.e., Benign)

This category is used for specimens that show no evidence of intraepithelial neoplasia or malignancy, but may include non-neoplastic findings such as normal cellular elements, reactive changes, or organisms

Clinical

─ Management is typically routine age-appropriate screening, unless specific non-neoplastic findings (e,g,, certain organisms, endometrial cells in older women) or clinical history warrant other follow-up

Cell patterns

─ Predominantly mature superficial and intermediate squamous cells; endocervical cells and/or squamous metaplastic cells (EC/TZ component) may be present; endometrial cells may be seen (reported if woman ≥45 years)

Background

─ Generally clean or may show findings consistent with specific non-neoplastic conditions (e,g,, inflammation, cytolysis, atrophy)

Absent

─ Evidence of SIL, AGC, or carcinoma

Note

─ The Bethesda System provides detailed criteria for various normal cellular elements, non-neoplastic cellular variations, reactive changes, and organisms that fall under the NILM category

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Squamous cells

Normal squamous epithelial cells found in cervical cytology specimens, varying in maturity based on hormonal status and location

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Superficial cells

The most mature squamous cells, typically seen in the proliferative phase of the menstrual cycle or with estrogenic stimulation

Cell patterns

─ Large polygonal cells, often isolated or in loose sheets

Cytoplasm

─ Abundant, thin, eosinophilic (pink) or cyanophilic (blue-green); may contain keratohyaline granules

Nuclei

─ Small (~5-6 μm diameter), pyknotic (dense, condensed chromatin), centrally located

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Intermediate cells (IMCs)

Squamous cells from the mid-zone of the epithelium, predominant during the secretory phase, pregnancy, or with progestational agents

Cell patterns

─ Large polygonal cells, often with folded cytoplasm ("boat cells" esp in pregnancy); may show cytolysis (bare nuclei) in the presence of Döderlein bacilli (Lactobacillus)

Cytoplasm

─ Abundant, thin, typically cyanophilic (blue-green); often rich in glycogen (appearing clear or vacuolated)

Nuclei

─ Larger than superficial cell nuclei (~8-10 μm diameter), vesicular (open chromatin), finely granular chromatin, often with a longitudinal groove

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Parabasal cells

Immature squamous cells typically seen in atrophic smears (postmenopausal, postpartum) or from deeper layers if sampled vigorously

Cell patterns

─ Smaller, round to oval cells with a higher N:C ratio than mature squamous cells; often in sheets or isolated

Cytoplasm

─ Denser, thicker, often cyanophilic

Nuclei

─ Larger than intermediate cell nuclei (~10-12 μm diameter), round to oval, finely granular chromatin, smooth nuclear membranes; nucleoli generally inconspicuous

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Glandular cells

Normal glandular epithelial cells found in cervical cytology specimens, originating from the endocervix or endometrium

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Endocervical glandular cells

Cells lining the endocervical canal, characterized by their mucin-producing nature

Cell patterns

─ Arranged in sheets with a "honeycomb" appearance (en face view) or in strips with a "picket fence" arrangement (side view); single cells may also be present; cells larger than intermediate squamous cells, polarized

Cytoplasm

─ Abundant, finely granular or vacuolated (mucin-rich), basophilic or amphophilic; cilia may be present (tubal metaplasia often involves ciliated endocervical-type cells)

Nuclei

─ Round to oval, often basally located in columnar cells, uniform size (similar to or slightly larger than an intermediate cell nucleus); finely granular, evenly distributed chromatin; nucleoli usually small and inconspicuous but can be prominent if reactive

Absent

─ Significant nuclear atypia, hyperchromasia, or architectural disarray beyond reactive changes

Note

─ Reactive endocervical cells can show nuclear enlargement, prominent nucleoli, and some crowding but maintain overall cohesion and polarity

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Endometrial glandular cells (exfoliated)

Cells shed from the endometrium, typically seen during menses (days 1-5) and the proliferative phase (up to day 12); their presence in women ≥45 years old is reported

Clinical

─ Presence in women ≥45 years old requires clinical correlation to exclude endometrial pathology, though most are benign/physiologic

Cell patterns

─ Small, tightly cohesive, three-dimensional clusters ("exodus balls" often seen days 6-10, with central stromal cells rimmed by glandular cells); isolated cells less common and harder to identify; cells smaller than endocervical cells

Cytoplasm

─ Scant, often dense, basophilic, may be vacuolated; cell borders indistinct within clusters

Nuclei

─ Small (similar to or smaller than an intermediate cell nucleus), round to oval, often hyperchromatic and degenerated with smudgy chromatin; nucleoli usually inconspicuous but may be visible in LBP; apoptotic bodies (karyorrhexis) common within clusters

Background

─ Often bloody with inflammatory cells during menses; cleaner background in LBP

Absent

─ Significant nuclear atypia beyond degenerative changes

Note

─ Distinguishing exfoliated endometrial cells from small HSIL cells or AIS can be challenging; endometrial cells are generally smaller, more tightly clustered, and show degenerative nuclear changes rather than true dysplastic features

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Directly sampled endometrial cells (stromal & glandular)

Endometrial tissue fragments obtained by vigorous sampling, esp with endocervical brush or after cervical procedures (LEEP, cone); not considered clinically significant like exfoliated endometrial cells in older women

Cell patterns

─ Larger, more cellular, and often more organized fragments than exfoliated cells; may show glandular-stromal complexes with branching glands and dense stromal components; capillaries may be visible

Cytoplasm

─ Glandular cells: Columnar, scant to moderate; Stromal cells: Spindled, scant

Nuclei

─ Glandular cells: Round to oval, may be crowded and overlapping, variably hyperchromatic, smooth contours, inconspicuous nucleoli; mitoses may be seen in proliferative phase

─ Stromal cells: Oval to spindled, finely granular chromatin, inconspicuous nucleoli

Absent

─ Significant atypia beyond proliferative changes

Note

─ Can mimic atypical glandular cells or even HSIL due to hyperchromatic crowded groups (HCGs); recognition of biphasic (glandular and stromal) pattern and bland cytology helps in correct interpretation

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Benign Reactions

Non-neoplastic cellular responses to various stimuli such as inflammation, trauma, or hormonal changes

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Repair (Typical Repair)

Reactive cellular changes in squamous or glandular epithelium due to tissue injury and subsequent healing processes

Clinical

─ Often associated with inflammation, infection, trauma (e,g,, biopsy, IUD), or radiation

Cell patterns

─ Cohesive, flat sheets of cells, often with a streaming or "school of fish" appearance; cell borders usually well-defined; minimal nuclear overlap in a single plane

Cytoplasm

─ Abundant, often delicate or attenuated; may be eosinophilic or basophilic (polychromatic); vacuolization or perinuclear halos can occur; intracytoplasmic neutrophils common

Nuclei

─ Enlarged (often 1,5-2x normal intermediate or endocervical nucleus), round to oval, smooth nuclear contours; finely granular, evenly distributed chromatin, sometimes vesicular or hypochromatic; prominent, often multiple, smooth, round nucleoli are characteristic; mitoses may be present but are typical

Background

─ Often inflammatory; may contain fibrin or blood

Absent

─ Significant hyperchromasia, coarse or irregular chromatin, marked nuclear membrane irregularities, or significant loss of cohesion (features of "atypical repair" which would be classified as ASC or AGC)

Note

─ Key features are cellular cohesion, smooth nuclear outlines, fine chromatin, and prominent regular nucleoli

─ "Atypical repair" showing more significant atypia should be classified as ASC or AGC

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Metaplastic Squamous cells

Immature squamous cells replacing glandular epithelium, a common physiologic response in the transformation zone

Cell patterns

─ Cohesive sheets or isolated cells; cells vary from small, round/oval (immature) to larger, more polygonal (mature metaplasia); "spider cells" (cells with cytoplasmic projections) more common in conventional smears

Cytoplasm

─ Dense, often cyanophilic in immature cells; becomes more abundant and thinner with maturation

Nuclei

─ Round to oval, often slightly larger than an intermediate cell nucleus (mean area ~50 μm²); finely granular, evenly distributed chromatin; smooth nuclear membranes; nucleoli usually inconspicuous but can be prominent if reactive

Absent

─ Significant nuclear atypia (hyperchromasia, irregular contours, coarse chromatin) beyond mild reactive changes

Note

─ Immature squamous metaplasia with high N:C ratios can be a mimic of HSIL; however, metaplastic cells typically have smoother nuclear contours and finer chromatin

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Tubal metaplasia

Replacement of endocervical epithelium by ciliated and non-ciliated (peg) cells resembling fallopian tube epithelium; a benign finding

Cell patterns

─ Strips or small clusters of columnar cells, often with pseudostratification; cilia and/or terminal bars are diagnostic

Cytoplasm

─ Eosinophilic to amphophilic; apical cilia on ciliated cells; peg cells have scant cytoplasm

Nuclei

─ Round to oval, may be enlarged and mildly hyperchromatic; chromatin finely granular and evenly distributed; nucleoli usually inconspicuous; nuclear crowding and overlap can be prominent

Absent

─ Significant pleomorphism, coarse chromatin, or mitotic activity beyond that seen in reactive endocervical cells

Note

─ Can mimic Atypical Glandular Cells (AGC) or Adenocarcinoma in situ (AIS) due to nuclear crowding, enlargement, and hyperchromasia; identification of cilia/terminal bars is key to benign diagnosis

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Atrophy

Epithelial thinning due to decreased estrogen stimulation, typically seen in postmenopausal women or other low-estrogen states

Clinical

─ Smears may be sparsely cellular; inflammation (atrophic vaginitis) common

Cell patterns

─ Predominantly parabasal cells, often in flat monolayer sheets with preserved polarity or as dispersed single cells; intermediate cells may be present; superficial cells usually absent

Cytoplasm

─ Parabasal cells: Denser, cyanophilic

Nuclei

─ Parabasal cells: Round to oval, slightly larger than intermediate cell nuclei, finely granular chromatin, smooth nuclear membranes; mild hyperchromasia and nuclear enlargement may be seen; autolytic changes (stripped nuclei) can occur

Background

─ May be clean or show granular debris ("blue blobs" - inspissated mucus or degenerated cells), inflammation (atrophic vaginitis); pseudoparakeratosis (degenerated orangeophilic parabasal cells)

Absent

─ True dysplastic changes (though atrophic atypia can mimic SIL)

Note

─ Atrophic changes with atypia (ASC-US or ASC-H) should be reported if nuclear abnormalities exceed those of simple atrophy

─ Estrogen cream application can mature the epithelium and resolve atrophic atypia

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Pregnancy related changes

Navicular cells

Glycogen-rich intermediate squamous cells with a "boat-shaped" appearance, characteristic of pregnancy or progestational effect

Cell patterns

─ Intermediate squamous cells, often with folded cytoplasm imparting a boat shape; may form dense clusters

Cytoplasm

─ Abundant, basophilic to clear (due to glycogen)

Nuclei

─ Vesicular, finely granular chromatin, often eccentric

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Decidua

Hormonally stimulated endocervical or endometrial stromal cells seen during pregnancy or postpartum

Cell patterns

─ Single cells or small loose clusters; large cells

Cytoplasm

─ Abundant, granular or finely vacuolated, may have cytoplasmic processes

Nuclei

─ Large (35–50 μm² area), may be lobulated or multinucleated; finely granular, evenly distributed chromatin, normochromatic to hyperchromatic; smooth nuclear membranes; nucleoli usually prominent and basophilic

Note

─ Can be misinterpreted as LSIL or HSIL if not aware of pregnancy status; smooth nuclear contours and fine chromatin favor decidual cells

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Cytotrophoblast

Placental cells rarely seen in late pregnancy or postpartum

Cell patterns

─ Typically single cells, occasionally small clusters; small cells

Cytoplasm

─ Scant, may have prominent vacuoles

Nuclei

─ Enlarged, high N:C ratio, hyperchromasia; evenly distributed chromatin

Background

─ Often highly inflamed and sometimes bloody

Note

─ Can resemble reactive squamous cells or HSIL/ASC-H; clinical history is key

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Syncytiotrophoblast

Multinucleated placental cells rarely seen in late pregnancy or postpartum

Cell patterns

─ Large, multinucleated cells (up to 50+ nuclei)

Cytoplasm

─ Granular, often tapering at one end

Nuclei

─ Normochromatic with even chromatin distribution, often with irregular nuclear contours

Note

─ Differentiate from herpes (lacks ground-glass inclusions) and other multinucleated cells by history and nuclear features

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Arias Stella reaction

Benign glandular cell changes (endocervical or endometrial) associated with pregnancy or hormonal stimulation

Cell patterns

─ Glandular cells, singly or in clusters

Cytoplasm

─ Variable quantity, may be vacuolated

Nuclei

─ Large, hyperchromatic with contour irregularities (grooves, pseudoinclusions); granular to smudgy chromatin; multiple prominent nucleoli; N:C ratio variable, often high

Background

─ Usually inflammatory, often with leukophagocytosis

Note

─ Can mimic AGC or adenocarcinoma; history of pregnancy or hormonal stimulation is critical

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Intrauterine device (IUD) effect

Reactive glandular cell changes (endocervical or endometrial) associated with IUD use

Cell patterns

─ Glandular cells singly or in small clusters (usually 5–15 cells); signet-ring appearance due to large cytoplasmic vacuoles displacing nucleus; occasional single cells with high N:C

Cytoplasm

─ Variable amount, frequently large vacuoles

Nuclei

─ May be enlarged; degenerative changes ("wrinkled" chromatin, nuclear "cracking") may be present; nucleoli may be prominent

Background

─ Often clean; Actinomyces-like organisms may be present (up to 25% of cases)

Absent

─ True features of malignancy (though can mimic adenocarcinoma or HSIL)

Note

─ Changes may persist for months after IUD removal; if atypia is concerning, recommend repeat Pap after IUD removal or further investigation

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Keratinization

Parakeratosis

Miniature superficial squamous cells with dense orangeophilic/eosinophilic cytoplasm and small pyknotic nuclei; a reactive change

Cell patterns

─ Miniature superficial squamous cells, isolated, in sheets, or whorls; round, oval, polygonal, or spindle-shaped

Cytoplasm

─ Dense, orangeophilic or eosinophilic

Nuclei

─ Small (~10 μm² area), dense (pyknotic)

Absent

─ Significant nuclear atypia (if present, consider ASC or SIL)

Note

─ "Atypical parakeratosis" (with nuclear atypia) is classified as ASC or SIL

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Hyperkeratosis

Anucleate mature polygonal squamous cells, often associated with keratohyaline granules; a reactive change

Cell patterns

─ Anucleate mature polygonal squamous cells, isolated or in plaques

Cytoplasm

─ Mature, polygonal; may show keratohyaline granules

Nuclei

─ Absent; "ghost nuclei" (empty spaces) may be noted

Absent

─ Nuclear atypia in accompanying nucleated cells (if atypia present, consider ASC or SIL)

Note

─ Extensive hyperkeratosis may obscure underlying lesions; vulvar contamination can also introduce anucleate squames

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Lymphocytic (Follicular) Cervicitis

Chronic cervicitis with formation of lymphoid follicles in the subepithelium

Cell patterns

─ Polymorphous population of lymphocytes (small lymphocytes, centrocytes, centroblasts, immunoblasts, plasma cells); tingible body macrophages often present; germinal center fragments may be seen

Background

─ Lymphoglandular bodies

Note

─ In LBP, lymphocytes may be more dispersed or in loose aggregates; in CP, often in clusters or mucus strands

─ DDx includes HSIL (small cell type) and lymphoma; polymorphous population and presence of tingible body macrophages favor benign

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Radiation

Cellular changes due to ionizing radiation, which can persist long-term

Cell patterns

─ Marked cytomegaly with proportional karyomegaly (N:C ratio often preserved); bizarre cell shapes; multinucleation common

Cytoplasm

─ Often abundant, vacuolated, or polychromatic (two-toned); intracytoplasmic neutrophils

Nuclei

─ Variably sized, some very large; mild hyperchromasia; degenerative changes (pallor, wrinkling, smudging of chromatin, vacuolization); nucleoli may be prominent (esp with repair)

Absent

─ True malignant features (though can mimic SIL or carcinoma)

Note

─ Acute changes resolve within ~6 months; chronic changes can persist indefinitely

─ Distinguish from recurrent malignancy (more hyperchromasia, irregular chromatin, higher N:C in malignancy)

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Glandular cells status post hysterectomy

Benign endocervical-like glandular cells in vaginal smears after hysterectomy

Cell patterns

─ Benign-appearing endocervical-type glandular cells; goblet cell or mucinous metaplasia may be seen

Note

─ Origin may be vaginal adenosis, metaplasia, or prolapsed fallopian tube (if supracervical hysterectomy, from cervical stump)

─ If not atypical, no clinical significance; reporting is optional

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Contaminants and miscellaneous

Extraneous elements or non-cervical/vaginal cells sometimes encountered in cervical cytology specimens

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Spermatozoa

Male germ cells, may be seen in cervical cytology specimens

Cell patterns

─ Characteristic oval head with a dense, dark nucleus and a pale anterior acrosomal cap; long, delicate flagellum (tail) often visible

Note

─ Presence is usually of no clinical significance in routine screening; may be noted if relevant to clinical history (e,g,, postcoital sample, fertility assessment, forensic cases)

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Pollen grains

Airborne plant microspores, common environmental contaminants

Cell patterns

─ Variably sized and shaped (round, oval, triangular), often with a thick, refractile, and sometimes sculptured cell wall; may appear yellow, brown, or refractile; internal contents may or may not be visible

Note

─ Can be mistaken for fungal spores or parasite ova if morphology is not carefully assessed; usually larger and more complex than true pathogens

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Lubricant

Material used during speculum insertion, which can contaminate the cytologic sample

Cell patterns

─ Appears as amorphous, stringy, granular, or gel-like material, often basophilic or purple; may obscure epithelial cells or cause cellular distortion

Background

─ Can cause a "streaky" or "clumpy" appearance, esp in LBP

Note

─ Certain lubricants (esp those containing carbomers or Carbopol polymers) can significantly impair LBP adequacy, leading to unsatisfactory specimens due to scant cellularity or obscuration

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Other contaminants

Includes various fibers (e,g,, cotton, synthetic), talc (from gloves, though less common now), starch granules, and other debris

Cell patterns

─ Fibers are elongated, refractile structures; talc appears as small, birefringent crystals under polarized light; starch granules are round to oval with a central hilum, often showing a "Maltese cross" pattern under polarized light

Note

─ Usually easily recognizable and of no clinical significance, but extensive contamination can obscure cellular detail

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Organisms

Microorganisms that can be identified in cervical cytology specimens; some are pathogenic, others represent normal flora or shifts in flora

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Trichomonas vaginalis

A flagellated protozoan parasite, a common cause of vaginitis

Clinical

─ May cause itching, burning, and a malodorous, frothy, yellow-green vaginal discharge; many infections are asymptomatic

─ Sexually transmitted infection

Cell patterns

─ Pear-shaped or oval organisms, typically 15-30 μm long (variable size); pale, eccentrically placed nucleus; reddish cytoplasmic granules (often inconspicuous); flagella usually not visible on Pap stain; often associated with a polymorphous inflammatory infiltrate and "cannonball" clusters of neutrophils on squamous cells ("Trich parties")

Background

─ Often inflammatory, with neutrophils; "leptothrix" (long, filamentous bacteria) frequently coexists; perinuclear halos in squamous cells ("pseudokoilocytosis") may be seen but lack the nuclear atypia of true koilocytes

Absent

─ True hyphae or budding yeast (to distinguish from Candida)

Note

─ Reporting is recommended by Bethesda; treatment of patient and partners is indicated

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Candida

Yeast and pseudohyphae, common cause of vulvovaginal candidiasis ("yeast infection")

Clinical

─ May cause itching, burning, and a thick, white, "cottage cheese-like" vaginal discharge; many colonizations are asymptomatic

Cell patterns

─ Budding yeast forms (3-7 μm diameter) and/or pseudohyphae (chains of elongated budding yeast cells); true hyphae may also be present; typically stain pink/eosinophilic or basophilic; "shish kebab" arrangement (squamous cells skewered by pseudohyphae) may be seen

Background

─ Variable inflammation; squamous cells may show reactive changes

Absent

─ True septate hyphae with acute angle branching (seen in Aspergillus, a rare contaminant/pathogen)

Note

─ Reporting is recommended by Bethesda; treatment usually for symptomatic women

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Bacterial vaginosis (BV) / Gardnerella vaginalis

A polymicrobial clinical syndrome resulting from replacement of normal Lactobacillus-predominant vaginal flora with high concentrations of anaerobic bacteria (e,g,, Gardnerella vaginalis, Prevotella spp, Atopobium vaginae, Mobiluncus spp)

Clinical

─ May cause a thin, homogeneous, malodorous (fishy) vaginal discharge and elevated vaginal pH (>4,5); many cases are asymptomatic

─ Associated with adverse obstetric outcomes (e,g,, preterm labor) and increased risk of acquiring STIs

Cell patterns

─ "Clue cells" (squamous cells coated by a lawn of coccobacilli, obscuring cell borders); marked decrease or absence of normal Lactobacilli; predominance of small, pleomorphic coccobacillary forms

Background

─ Often "filmy" or granular due to numerous bacteria; neutrophils usually sparse or absent (unless co-infection)

Absent

─ Predominance of Lactobacilli; significant numbers of Trichomonas or Candida (though co-infections can occur)

Note

─ Reporting is recommended by Bethesda; diagnosis of BV is clinical (Amsel criteria or Nugent score on Gram stain), but cytology can suggest the diagnosis

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Actinomyces

Anaerobic or microaerophilic Gram-positive filamentous bacteria

Clinical

─ Strongly associated with intrauterine device (IUD) use, esp long-term; most women are asymptomatic carriers

─ Rarely, can cause pelvic actinomycosis (an invasive infection)

Cell patterns

─ Dense, tangled clumps of filamentous bacteria ("cotton balls" or "dust bunnies"), often with a radiating periphery; individual filaments are thin and branching; may be associated with neutrophils

Background

─ Often clean, but can be inflammatory if associated with IUD effect or infection

Absent

─ True fungal hyphae or yeast forms

Note

─ Reporting is recommended by Bethesda; clinical significance in asymptomatic IUD users is controversial, treatment usually not indicated unless symptomatic or pelvic infection suspected

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Herpes Simplex Virus (HSV)

Viral cytopathic effect due to HSV-1 or HSV-2 infection

Clinical

─ Primary infection may cause painful genital vesicles, ulcers, fever, and lymphadenopathy; recurrent episodes are usually milder

─ Viral shedding can occur in the absence of visible lesions

Cell patterns

─ The "3 Ms": Multinucleation, Nuclear molding (nuclei indenting each other), and Margination of chromatin (condensed along nuclear membrane); "ground glass" appearance of nuclei (homogeneous, pale, opaque chromatin); +/- distinct eosinophilic intranuclear inclusions (Cowdry type A)

Background

─ May be inflammatory

Absent

─ Cilia (to distinguish from multinucleated endocervical cells); prominent nucleoli (unless reactive changes superimposed)

Note

─ Reporting is recommended by Bethesda; antiviral treatment can manage outbreaks but does not eradicate latent virus

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Cytomegalovirus (CMV)

Viral cytopathic effect, rare in cervical cytology, usually seen in immunocompromised individuals

Clinical

─ Most infections are asymptomatic in immunocompetent hosts; can cause significant disease in immunocompromised patients or congenital infection

Cell patterns

─ Marked cytomegaly and karyomegaly of single cells (often glandular or stromal); characteristic large, basophilic, intranuclear "owl's eye" inclusion surrounded by a clear halo; smaller granular basophilic cytoplasmic inclusions may also be present

Background

─ Variable

Absent

─ Multinucleation and nuclear molding typical of HSV

Note

─ Reporting is recommended by Bethesda; clinical significance depends on host immune status

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Atypical Squamous Cells of Undetermined Significance (ASC-US)

Cytologic changes suggestive of a Squamous Intraepithelial Lesion (SIL) that are qualitatively or quantitatively insufficient for a definitive interpretation; encompasses atypia that is more marked than reactive changes but falls short of LSIL

Clinical

─ Most common epithelial abnormality reported

─ Risk of underlying HSIL (CIN 2/3) on biopsy is ~10-20%; risk of invasive cancer is low (~0,1-0,2%)

─ Management typically involves HPV testing (reflex or co-testing); if HPV positive, colposcopy is indicated; if HPV negative, co-testing in 3 years is recommended for women ≥25 years

Cell patterns

─ Usually mature superficial or intermediate-type squamous cells, singly or in sheets; atypical parakeratosis or cells with incomplete koilocytic features may be present

Cytoplasm

─ Generally abundant, similar to normal mature squamous cells; may show perinuclear halos or vacuolization suggestive but not diagnostic of koilocytosis; dense orangeophilic cytoplasm in atypical parakeratotic cells

Nuclei

─ Enlarged, approximately 2,5 to 3 times the area of a normal intermediate cell nucleus (or twice the size of a normal squamous metaplastic cell nucleus)

─ N:C ratio slightly increased but generally <50%

─ Mild hyperchromasia; chromatin evenly distributed or slightly granular; nuclear contours may be minimally irregular

─ Binucleation or multinucleation may be present

Background

─ Variable; may be clean or inflammatory

Absent

─ Definitive features of LSIL (esp unequivocal koilocytes with diagnostic nuclear atypia) or HSIL

Note

─ ASC-US is an interpretation of the entire specimen, not just individual cells

─ Criteria for ASC-US may differ subtly among labs due to variations in stains and preparation techniques

─ Common patterns include: Atypical parakeratosis (APK), atypical repair, and atypia in postmenopausal women/atrophy

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Atypical Squamous Cells, cannot exclude HSIL (ASC-H)

Cytologic changes suggestive of HSIL that are qualitatively or quantitatively insufficient for a definitive interpretation; reserved for a minority of ASC cases (<10% of all ASC)

Clinical

─ Higher risk of underlying HSIL (CIN 2/3) on biopsy than ASC-US (~25-50%, can be higher); risk of invasive cancer also higher than ASC-US but still low

─ Management is colposcopy, regardless of HPV status (reflex HPV testing not recommended for ASC-H)

Cell patterns

─ Usually immature squamous metaplastic-type cells, often in small groups (<10-15 cells) or as single cells; may present as "crowded sheet pattern" or atypical repair concerning for HSIL/cancer; cells often smaller than typical ASC-US cells

Cytoplasm

─ Often scant and dense (metaplastic type); N:C ratio is significantly increased, often approximating that of HSIL

Nuclei

─ Enlarged (may be only 1,5-2,5 times larger than normal metaplastic nucleus but N:C ratio is high)

─ Hyperchromasia, often with irregular chromatin distribution or coarse granularity

─ Nuclear contours frequently irregular, with notching or indentations

─ Nucleoli generally absent

Background

─ Variable; may be clean or inflammatory

Absent

─ Unequivocal features of HSIL (esp in terms of quantity of abnormal cells or classic HSIL morphology)

DDx

─ HSIL (esp small cell HSIL or poorly preserved HSIL)

─ Immature squamous metaplasia with reactive changes (can have high N:C ratio but nuclei usually smoother and chromatin finer)

─ Atrophy with degeneration (can show hyperchromasia and some nuclear irregularity, but N:C ratio usually not as high as HSIL)

─ Reactive endocervical cells or tubal metaplasia (beware of "endocervical cells seen on end" mimicking small atypical squamous cells)

─ AIS (if glandular features are subtle and cells appear in crowded groups)

Note

─ ASC-H is intended for cases where HSIL is a significant concern, but the changes are not definitive

─ Common patterns include "atypical immature metaplasia" (small cells with high N:C ratios) and "crowded sheet pattern"

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Low-Grade Squamous Intraepithelial Lesion (LSIL)

Encompasses human papillomavirus (HPV) cytopathic effect (koilocytosis) and what was formerly termed mild dysplasia or Cervical Intraepithelial Neoplasia grade 1 (CIN 1)

Clinical

─ Caused by HPV infection (both low-risk and high-risk types)

─ Most LSILs represent transient HPV infections and regress spontaneously, esp in young women (<25 years)

─ Risk of underlying HSIL (CIN 2/3) on biopsy is ~15-30%

─ Management for women ≥25 years is colposcopy; for women 21-24 years, repeat cytology at 12 months is preferred; HPV co-testing with LSIL cytology is generally not recommended for triage due to high rates of HPV positivity

Cell patterns

─ Cells usually mature superficial or intermediate type, occurring singly, in sheets, or loose clusters; koilocytes (cells with perinuclear cavitation and nuclear atypia) are pathognomonic but not required for diagnosis

Cytoplasm

─ Abundant, mature (superficial or intermediate type); koilocytotic change includes a sharply defined, broad perinuclear clear zone (halo) with a peripheral rim of condensed, often densely stained cytoplasm

Nuclei

─ Enlarged (at least 3 times the area of a normal intermediate nucleus, or ~100-175 μm² area); N:C ratio is low but slightly increased

─ Hyperchromasia is common, chromatin may be smudgy, coarsely granular, or opaque

─ Nuclear contours often irregular (wrinkled, "raisinoid") but can be smooth

─ Binucleation or multinucleation common

─ Nucleoli generally absent or inconspicuous

Background

─ Variable; may be clean or inflammatory

Absent

─ Features of HSIL (esp significantly increased N:C ratio in immature-appearing cells, marked hyperchromasia with very coarse chromatin)

DDx

─ Reactive changes with "pseudokoilocytosis" (e,g,, Trichomonas infection, glycogen effect): Perinuclear halos are usually smaller, less well-defined, and lack significant nuclear atypia (esp hyperchromasia and irregular contours)

─ ASC-US: Nuclear atypia or koilocytic features are suggestive but quantitatively or qualitatively insufficient for LSIL

─ HSIL: Cells usually smaller with higher N:C ratio and more severe nuclear atypia

Note

─ Koilocytosis is the hallmark of HPV infection; to be diagnostic, koilocytes must exhibit both the characteristic cytoplasmic halo and nuclear atypia (enlargement, hyperchromasia, irregular contours)

─ Non-koilocytic LSIL is diagnosed based on nuclear atypia in mature squamous cells that exceeds ASC-US criteria but falls short of HSIL

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High-Grade Squamous Intraepithelial Lesion (HSIL)

Encompasses what was formerly termed moderate and severe dysplasia, carcinoma in situ (CIS), CIN 2, and CIN 3; represents a true precancerous lesion

Clinical

─ Caused by persistent infection with high-risk HPV types (esp HPV 16 and 18)

─ Higher risk of progression to invasive squamous cell carcinoma if untreated compared to LSIL

─ Risk of underlying invasive cancer on biopsy is ~1-2% for cytologic HSIL

─ Management is colposcopy for all non-pregnant women; see-and-treat (excisional procedure) is an acceptable option for women ≥25 years if HSIL is present and colposcopy adequate

Cell patterns

─ Cells usually smaller than LSIL cells, often immature (metaplastic or parabasal-like); occur singly, in sheets, or syncytial-like aggregates (hyperchromatic crowded groups, HCGs)

Cytoplasm

─ Scant and often dense (immature/metaplastic type), but can be delicate or occasionally keratinized; N:C ratio is markedly increased (generally >50-60%)

Nuclei

─ Enlarged (may be similar in size to LSIL nuclei, but N:C ratio is much higher; or nuclei may be smaller than LSIL but still disproportionately large for cell size)

─ Hyperchromasia is usually marked; chromatin often coarsely granular and irregularly distributed, but can be finely granular or even hypochromatic

─ Nuclear contours are very irregular, with notching, indentations, and angulation

─ Nucleoli generally absent or inconspicuous

─ Mitotic figures may be seen, esp in tissue fragments

Background

─ Variable; may be clean, inflammatory, or show tumor diathesis if associated with invasive cancer or extensive necrosis

Absent

─ Features of LSIL (esp koilocytosis with only mild nuclear atypia and low N:C ratio)

─ Unequivocal features of invasive squamous cell carcinoma (e,g,, macronucleoli, definite tumor diathesis, bizarre cell shapes in keratinizing SCC)

DDx

─ LSIL: Larger cells, lower N:C ratio, less severe nuclear atypia

─ ASC-H: Atypical features suggestive of HSIL but quantitatively or qualitatively insufficient

─ Atrophy with reactive atypia or degeneration: Can mimic HSIL due to high N:C ratio and hyperchromasia; atrophic cells typically have smoother nuclear contours, finer chromatin (unless degenerated), and lack the marked irregularity of HSIL

─ Immature squamous metaplasia (reactive): Can have high N:C ratio, but nuclei are usually round/oval with smooth contours and fine, evenly distributed chromatin

─ Endometrial cells or directly sampled lower uterine segment: Can form HCGs; endometrial cells are smaller, often degenerated, and may have associated stroma; directly sampled LUS has biphasic pattern

─ AIS/AGC: Glandular features (columnar shape, feathering, rosettes, true glandular lumina) distinguish from HSIL, though HSIL involving glands can be a pitfall

Note

─ HSIL is a spectrum; "small cell HSIL" (CIN3-like) has very high N:C ratios and scant cytoplasm; "large cell HSIL" (CIN2-like) has slightly more cytoplasm but still high N:C and significant nuclear atypia

─ Keratinizing HSIL shows dense, orangeophilic cytoplasm with marked nuclear atypia; may mimic invasive keratinizing SCC

─ p16 immunohistochemistry is often diffusely positive (block positivity) in HSIL and can be helpful in biopsy correlation but is not typically used on cytology specimens for primary diagnosis

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Atypical Glandular Cells (AGC)

Glandular cells (endocervical, endometrial, or not otherwise specified) that display nuclear atypia exceeding obvious reactive or reparative changes but lacking unequivocal features of endocervical adenocarcinoma in situ (AIS), invasive adenocarcinoma, or endometrial carcinoma

Clinical

─ AGC is an uncommon interpretation (0,1-0,5% of Pap tests)

─ Associated with a higher risk of significant underlying pathology (SIL, esp HSIL; AIS; adenocarcinoma; endometrial carcinoma) compared to ASC-US

─ Risk of CIN 2/3+ is ~10-40%; risk of AIS or invasive adenocarcinoma is ~3-10%; risk of endometrial cancer (esp with atypical endometrial cells) varies by age and symptoms

─ Management for all AGC categories (except atypical endometrial cells) is colposcopy with endocervical sampling; endometrial sampling is also indicated for women ≥35 years or those <35 with risk factors for endometrial cancer (e,g,, abnormal bleeding, obesity, anovulation)

─ For atypical endometrial cells, endometrial and endocervical sampling is indicated; colposcopy may be deferred if endometrial pathology is identified

─ HPV testing may be useful in risk stratification for some AGC categories

Note

─ The Bethesda System recommends qualifying AGC as to endocervical or endometrial origin if possible; if not, "AGC, Not Otherwise Specified (NOS)" is used

─ "Atypical Endocervical Cells, NOS" and "Atypical Glandular Cells, NOS" may be further qualified as "favor neoplastic" if features are more worrisome

─ "Atypical Endometrial Cells" are not further qualified as "favor neoplastic"

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Atypical Endocervical Cells, Not Otherwise Specified

Endocervical-type cells displaying nuclear atypia that exceeds obvious reactive or reparative changes but lacks unequivocal features of endocervical adenocarcinoma in situ (AIS) or invasive adenocarcinoma

Cell patterns

─ Sheets or strips of endocervical-type cells with some cell crowding, nuclear overlap, and/or pseudostratification; cells usually maintain some polarity

Cytoplasm

─ May be fairly abundant, but N:C ratio is increased compared to normal endocervical cells; cell borders often discernible

Nuclei

─ Enlarged (up to 3-5 times normal endocervical nucleus area); some variation in nuclear size and shape; mild hyperchromasia; mild chromatin irregularity; occasional small nucleoli; mitotic figures rare

Background

─ Variable, may be clean or inflammatory

Absent

─ Unequivocal features of AIS or invasive adenocarcinoma (e,g,, numerous complex groups with feathering/rosettes, marked hyperchromasia, coarse chromatin, prominent irregular nucleoli, frequent mitoses/apoptosis)

DDx

─ Reactive endocervical cells (less crowding, smoother nuclei, finer chromatin, prominent but regular nucleoli)

─ Tubal metaplasia (cilia or terminal bars, often more nuclear elongation and finer chromatin)

─ Directly sampled lower uterine segment (LUS) or endometrium (often larger fragments, biphasic pattern with stroma, less atypia)

─ AIS or adenocarcinoma (more severe atypia, more complex architecture, more discohesion)

─ HSIL involving glands (squamous features in some cells, more irregular chromatin, lack of true glandular architecture)

Note

─ This is a heterogeneous category; includes changes that may be reactive mimics or true glandular neoplasia

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Atypical Endocervical Cells, Favor Neoplastic

Endocervical-type cells with cytologic and architectural features quantitatively or qualitatively borderline for, but highly suspicious of, endocervical adenocarcinoma in situ (AIS) or invasive adenocarcinoma

Cell patterns

─ Sheets, strips, or three-dimensional clusters with nuclear crowding, overlap, and pseudostratification; rare cell groups with rosettes or feathering may be present

Cytoplasm

─ Scant to moderate; N:C ratio increased

Nuclei

─ Enlarged, often elongated or irregular; hyperchromasia common; chromatin may be coarse and irregularly distributed; nucleoli may be prominent; occasional mitoses or apoptotic bodies

Background

─ Variable, may be clean or contain some debris

Absent

─ Unequivocal features of AIS or invasive adenocarcinoma (often a quantitative issue – too few abnormal groups)

DDx

─ AIS or invasive adenocarcinoma (distinction may be based on quantity of abnormal cells or subtlety of features)

─ Reactive endocervical changes or tubal metaplasia (less atypia, cilia in tubal metaplasia)

─ HSIL involving glands (squamous features, different chromatin pattern)

Note

─ This category carries a higher risk of significant glandular neoplasia than AGC-EC, NOS

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Atypical Endometrial Cells

Cells interpreted as endometrial in origin that display nuclear atypia beyond that of benign exfoliated or directly sampled endometrial cells, but lacking unequivocal features of endometrial adenocarcinoma

Clinical

─ Esp concerning in postmenopausal women or those with abnormal bleeding

─ Management involves endometrial and endocervical sampling

Cell patterns

─ Small groups (usually 5-10 cells) or isolated cells; cells often small and degenerated

Cytoplasm

─ Scant, often basophilic, may be vacuolated; cell borders poorly defined

Nuclei

─ Slightly enlarged compared to normal endometrial cells (may be only 1,5-2x intermediate nucleus); mild to moderate hyperchromasia; chromatin heterogeneity; nucleoli may be small but visible (esp in LBP)

Background

─ Variable; may be bloody or contain inflammatory cells or diathesis (esp if adenocarcinoma present)

Absent

─ Unequivocal features of endometrial adenocarcinoma (e,g,, numerous atypical cells, prominent nucleoli, definite tumor diathesis)

DDx

─ Benign exfoliated endometrial cells (smaller nuclei, more degenerative changes, less atypia)

─ Directly sampled LUS/endometrium (larger fragments, biphasic pattern, less atypia)

─ Endometrial adenocarcinoma (more severe atypia, often more cellular, tumor diathesis)

─ HSIL (can mimic small endometrial cells but usually has coarser chromatin, more irregular nuclear contours, and lacks glandular features)

─ IUD effect (vacuolated cytoplasm, but nuclear changes usually degenerative or reactive)

Note

─ This category is not further qualified as "favor neoplastic" due to difficulty in reliable subclassification

─ Watery diathesis (granular proteinaceous background) is suggestive of endometrial adenocarcinoma

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Squamous cell carcinoma (SCC)

An invasive malignancy composed of squamous cells, which may be keratinizing or non-keratinizing; most arise from pre-existing HSIL

Clinical

─ Symptoms may include abnormal vaginal bleeding (esp postcoital), vaginal discharge, or pain; many early SCCs are asymptomatic and detected by screening

─ Most SCCs are associated with high-risk HPV infection (esp HPV 16 and 18)

─ Management depends on stage and includes surgery, radiation, and/or chemotherapy

Cell patterns

─ Variable, depending on differentiation; may present as single cells, cohesive sheets, or syncytial-like aggregates; bizarre cell shapes (tadpole, spindle cells) common in keratinizing type

Cytoplasm

─ Keratinizing SCC: Often dense, orangeophilic or eosinophilic ("hard" cytoplasm); bizarre shapes

─ Non-keratinizing SCC: Usually less abundant, cyanophilic or amphophilic, more delicate; cell borders less distinct

Nuclei

─ Markedly atypical: Enlarged, pleomorphic, hyperchromatic (often with irregular, coarsely clumped chromatin); irregular nuclear contours are prominent; macronucleoli common in non-keratinizing SCC, often pyknotic/obscured in keratinizing SCC

Background

─ Often contains tumor diathesis (granular proteinaceous debris, old blood, necrotic tumor cells); acute inflammation common

Absent

─ Features of a benign process or lower-grade SIL (unless co-existing)

Ancillary studies

─ p16 IHC is usually diffusely positive (block-like staining) but is not specific for invasion

─ HPV testing is usually positive for high-risk types

DDx

─ HSIL (esp keratinizing HSIL): Lacks definite tumor diathesis and often has less pleomorphism and less prominent macronucleoli than invasive SCC

─ Repair/Reactive changes: Can show prominent nucleoli and some atypia but lack the marked hyperchromasia, coarse chromatin, and significant pleomorphism of SCC; diathesis absent

─ Adenocarcinoma: Glandular features (acini, columnar shape, mucin) distinguish from SCC, though poorly differentiated tumors can overlap

─ Metastatic SCC from other sites: Clinical history is key

Note

─ Cytologic grading (well, moderately, poorly differentiated) is not typically reported in Pap tests as it has limited clinical utility and reproducibility

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Non-keratinizing SCC (including basaloid)

A subtype of squamous cell carcinoma characterized by malignant cells lacking significant keratinization

Cell patterns

─ Syncytial aggregates and single cells; cells often smaller than keratinizing SCC, may have a basaloid appearance with scant cytoplasm

Cytoplasm

─ Scant to moderate, typically cyanophilic or amphophilic, indistinct cell borders

Nuclei

─ Hyperchromatic, with coarsely clumped, irregularly distributed chromatin; parachromatin clearing often evident; nuclear contours irregular; prominent nucleoli and/or macronucleoli are characteristic

Background

─ Tumor diathesis (necrotic debris, old blood) often prominent

Absent

─ Significant cytoplasmic keratinization (orangeophilia, dense "hard" cytoplasm, bizarre cell shapes like tadpoles or fiber cells)

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Keratinizing SCC

A subtype of squamous cell carcinoma characterized by malignant cells showing obvious keratinization

Cell patterns

─ Predominantly isolated single cells, less commonly in aggregates; marked variation in cell size and shape, with caudate (tadpole) and spindle cells often present

Cytoplasm

─ Dense, "hard", orangeophilic or eosinophilic; keratin pearls may be seen (rarely in smears)

Nuclei

─ Often pyknotic and hyperchromatic, obscuring chromatin detail; when discernible, chromatin is coarse and irregularly distributed; nuclear contours markedly irregular; macronucleoli less common or obscured by pyknosis compared to non-keratinizing SCC

Background

─ Tumor diathesis may be present but can be less prominent or different in quality (more keratin debris) than in non-keratinizing SCC

Absent

─ Predominance of non-keratinized malignant cells

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Adenocarcinoma

Malignant glandular neoplasms of the female genital tract that can be detected by cervical cytology include endocervical adenocarcinoma, endometrial adenocarcinoma, and, rarely, extrauterine adenocarcinomas (e,g,, ovarian, fallopian tube, metastatic from other sites)

Clinical

─ Endocervical adenocarcinoma is increasing in incidence, esp in younger women; most are HPV-related (esp HPV 18 and 16)

─ Endometrial adenocarcinoma is primarily a disease of postmenopausal women; symptoms include abnormal bleeding

─ Cytologic detection of adenocarcinoma can be challenging due to overlapping features with benign reactive changes, AGC, and AIS

Cell patterns

─ Variable, depending on origin and differentiation; generally show crowded groups, sheets, strips, rosettes, or papillae; single malignant cells often present

Cytoplasm

─ Variable, from scant to abundant; may be mucinous, clear, or eosinophilic; N:C ratio usually high

Nuclei

─ Enlarged, pleomorphic, often with irregular contours; hyperchromasia with coarse, irregularly distributed chromatin; macronucleoli common

Background

─ May be clean or show tumor diathesis (granular debris, old blood, necrosis), esp with invasive lesions

Absent

─ Definitive squamous differentiation (unless adenosquamous carcinoma)

Ancillary studies

─ IHC for p16 (diffuse block-like staining often seen in HPV-related endocervical adenocarcinomas/AIS), CEA, ER/PR, vimentin, and HPV DNA testing can aid in distinguishing endocervical from endometrial origin and from benign mimics

DDx

─ Reactive endocervical/endometrial cells; tubal metaplasia; Arias-Stella reaction; IUD effect; AIS; HSIL involving glands; metastatic adenocarcinoma

Note

─ The Bethesda System specifies reporting adenocarcinoma by probable site of origin if possible (endocervical, endometrial, extrauterine) or as Adenocarcinoma, Not Otherwise Specified (NOS)

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Endocervical Adenocarcinoma in situ (AIS)

A precursor to invasive endocervical adenocarcinoma, characterized by replacement of normal endocervical epithelium by cytologically malignant glandular cells confined to the native glandular structures

Clinical

─ Often associated with concurrent HSIL or invasive SCC; most are HPV-related (esp HPV 18)

─ Management typically involves excisional procedure (cone biopsy) to exclude invasion and assess margins

Cell patterns

─ Crowded sheets, strips, rosettes, or "feathered" groups of columnar cells; loss of normal honeycomb architecture; pseudostratification common; single atypical cells rare

Cytoplasm

─ Scant to moderate, often basophilic or amphophilic; mucin depletion common in usual type; N:C ratio increased

Nuclei

─ Enlarged (2-3x normal endocervical nucleus), oval to elongated, often cigar-shaped; hyperchromatic with coarsely granular, evenly distributed chromatin; nuclear membrane irregularities usually subtle; nucleoli generally inconspicuous or small; mitotic figures and apoptotic bodies common

Background

─ Usually clean; tumor diathesis absent (by definition, as it's an in situ lesion)

Absent

─ Stromal invasion; features of squamous differentiation (unless co-existing SIL)

Ancillary studies

─ IHC: (+) p16 (strong, diffuse block-like staining), CEA (cytoplasmic); (-) ER, PR, vimentin (usually)

─ HPV DNA testing usually positive for high-risk types (esp HPV 18)

DDx

─ Reactive endocervical cells/repair (less crowding, smoother nuclei, prominent but regular nucleoli, no feathering/rosettes usually)

─ Tubal metaplasia (cilia, less nuclear atypia, more nuclear elongation)

─ Directly sampled LUS/endometrium (biphasic pattern, less atypia)

─ Invasive endocervical adenocarcinoma (may show macronucleoli, tumor diathesis, more discohesion)

─ HSIL involving glands (squamous features, more irregular chromatin, p16+ but different cell morphology)

Note

─ Cytologic features can overlap with invasive endocervical adenocarcinoma; biopsy is required for definitive diagnosis and to rule out invasion

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Endocervical Adenocarcinoma

An invasive malignancy arising from the endocervical glands

Clinical

─ May present with abnormal bleeding, discharge, or be detected by screening

─ Most are HPV-related (esp HPV 18 and 16); non-HPV related types (e,g,, gastric-type, clear cell) exist and have different morphology/prognosis

Cell patterns

─ Highly cellular; crowded sheets, strips, clusters, papillae, or acinar structures; single malignant cells common; loss of polarity and architectural disarray

Cytoplasm

─ Variable: mucinous (usual type, intestinal, signet-ring), clear (clear cell type), eosinophilic (serous type); N:C ratio usually markedly increased

Nuclei

─ Enlarged, pleomorphic, irregular contours; hyperchromatic with coarse, irregularly distributed chromatin; parachromatin clearing often seen; macronucleoli common and often irregular; mitotic figures, including atypical forms, may be frequent

Background

─ Tumor diathesis (necrotic debris, old blood) often present, esp in LBP as "clinging diathesis"

Absent

─ Ovarian-type stroma; definitive squamous differentiation (unless adenosquamous)

Ancillary studies

─ IHC: (+) p16 (diffuse, HPV-related), CEA, CK7; (-) ER, PR, vimentin (usual endocervical type); specific markers for variants (e,g,, HNF1B for clear cell; Napsin A, MUC6 for gastric type)

─ HPV DNA testing usually positive for high-risk types (usual type)

DDx

─ AIS (lacks diathesis, often less pleomorphism and fewer macronucleoli)

─ Endometrial adenocarcinoma (often ER/PR/vimentin (+), p16 patchy, CEA (-); different morphology)

─ Metastatic adenocarcinoma (clinical history, IHC panel)

─ HSIL involving glands (squamous features, different chromatin)

─ Reactive changes/repair (less atypia, no diathesis)

Note

─ Subtyping (e,g,, usual endocervical, mucinous gastric-type, endometrioid, clear cell, serous, mesonephric) is important for prognosis and management but often requires histology

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Endometrial Adenocarcinoma

An invasive malignancy arising from the endometrial glands, occasionally detected in cervical cytology

Clinical

─ Most common gynecologic malignancy in developed countries; typically affects postmenopausal women; presents with abnormal uterine bleeding

─ Risk factors include unopposed estrogen, obesity, nulliparity, tamoxifen use, Lynch syndrome

Cell patterns

─ Often scant cellularity unless high-grade or advanced stage; small, tight clusters (esp low-grade endometrioid) or more dispersed, pleomorphic cells (high-grade, serous); single cells may be present

Cytoplasm

─ Scant to moderate, often basophilic or vacuolated; intracytoplasmic neutrophils ("bag of polys") can be seen, esp in endometrioid type

Nuclei

─ Enlarged (may be only slightly in low-grade), round to irregular; hyperchromasia variable; chromatin fine to coarse; nucleoli often prominent, esp in serous and clear cell types; macronucleoli in high-grade tumors

Background

─ Often bloody, esp if symptomatic bleeding; tumor diathesis (watery, granular debris) may be present, esp with higher-grade/stage tumors

Absent

─ Definitive endocervical features (e,g,, tall columnar cells in picket-fence or honeycomb, abundant apical mucin unless mucinous endometrial variant)

Ancillary studies

─ IHC: (+) ER, PR, vimentin (esp endometrioid type); (-) CEA, p16 (or patchy); specific markers for subtypes (e,g,, p53 aberrant in serous, Napsin A in clear cell)

DDx

─ Benign endometrial cells (smaller, more degenerated, less atypia)

─ Atypical endometrial cells (lesser degree of atypia)

─ Endocervical adenocarcinoma (often ER/PR/vimentin (-), p16 diffuse block (+), CEA (+))

─ Metastatic adenocarcinoma (clinical history, IHC)

─ HSIL (can mimic small cell endometrial carcinoma; look for squamous features)

Note

─ Cytologic detection of endometrial cancer is insensitive, esp for low-grade tumors; Pap test is not a screening tool for endometrial cancer

─ "Atypical endometrial cells" should be reported if criteria for adenocarcinoma are not met

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Other Malignant Neoplasms

This category includes rare malignant tumors of the cervix and uterus, such as small cell neuroendocrine carcinoma and certain uncommon adenocarcinoma variants

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Small cell carcinoma

A high-grade neuroendocrine carcinoma, often associated with HPV 16/18, characterized by small, relatively uniform cells with scant cytoplasm and neuroendocrine features

Clinical

─ Rare, aggressive tumor, often presenting at an advanced stage with early metastases

─ May be associated with paraneoplastic syndromes (e,g,, Cushing syndrome due to ACTH production)

─ Prognosis is generally poor

Cell patterns

─ Small, relatively uniform cells, often in loosely cohesive clusters, sheets, or singly dispersed; nuclear molding and crush artifact common

Cytoplasm

─ Very scant, often imperceptible, cyanophilic

Nuclei

─ Round to oval or angulated, hyperchromatic; chromatin finely granular or stippled ("salt-and-pepper"), sometimes smudged; nucleoli inconspicuous or absent

Background

─ Often shows extensive necrosis, apoptotic bodies, and mitotic figures; crush artifact prominent

Absent

─ Features of squamous or typical glandular differentiation (unless a mixed tumor)

Ancillary studies

─ IHC: (+) Neuroendocrine markers (Synaptophysin, Chromogranin A, CD56, INSM1); (+) Cytokeratins (often dot-like perinuclear)

─ IHC: (+) p16 (usually diffuse block-like if HPV-related)

─ HPV DNA testing often positive for high-risk types

DDx

─ HSIL (small cell type): HSIL cells usually larger, more pleomorphic, coarser chromatin, no prominent molding or neuroendocrine marker positivity

─ Endometrial cells (exfoliated or stromal): Smaller, more degenerated, lack prominent molding and neuroendocrine markers

─ Lymphoma: Dispersed pattern, lymphoglandular bodies, (+) lymphoid markers, (-) cytokeratin

─ Embryonal rhabdomyosarcoma: Rare in adults, strap cells, (+) desmin/myogenin

Note

─ Cytologic features are identical to small cell carcinoma of other sites (e,g,, lung)

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Mucinous Carcinoma, Gastric Type (includes Minimal Deviation Adenocarcinoma/Adenoma Malignum)

A rare subtype of endocervical adenocarcinoma characterized by gastric-type mucinous differentiation, often with bland cytologic features in its well-differentiated form (Minimal Deviation Adenocarcinoma/Adenoma Malignum); typically not HPV-related

Clinical

─ May present with profuse watery or mucoid vaginal discharge

─ Minimal Deviation Adenocarcinoma (MDA) is an extremely well-differentiated variant that can be very difficult to diagnose due to its bland cytology, often requiring deep cone biopsy

─ Associated with Peutz-Jeghers syndrome in some cases

─ Prognosis variable; MDA can be aggressive despite bland morphology

Cell patterns

─ Abundant glandular cells, often in large sheets, clusters, or isolated; cells columnar with copious apical mucin or goblet cell morphology

Cytoplasm

─ Abundant, pale, foamy, or clear (gastric-type mucin); may have a yellowish tinge; goblet cells common

Nuclei

─ Minimal Deviation Adenocarcinoma: Often bland, round to oval, basally located, with smooth contours and inconspicuous nucleoli, mimicking benign endocervical cells; subtle atypia (enlargement, mild hyperchromasia, slight irregularity) may be the only clue

─ Higher-grade gastric-type mucinous carcinoma: More overt nuclear atypia, pleomorphism, hyperchromasia, and prominent nucleoli

Background

─ Abundant extracellular mucin, often thick and viscous

Absent

─ Features of usual HPV-related endocervical adenocarcinoma (e,g,, feathering, rosettes, prominent apoptosis, p16 block positivity)

─ Definitive squamous differentiation

Ancillary studies

─ IHC: (+) CK7, CEA, MUC6, HIK1083 (gastric mucin markers)

─ IHC: (-) p16 (or only patchy/focal), ER, PR, vimentin

─ HPV DNA testing usually negative

DDx

─ Benign endocervical cells/reactive changes (esp in MDA: look for subtle atypia, architectural disarray, abnormal configuration of cell groups, clinical context)

─ Usual-type endocervical adenocarcinoma/AIS (HPV-related, p16 block (+), different mucin profile)

─ Metastatic mucinous adenocarcinoma (esp from GI tract or pancreas: clinical history, IHC for site-specific markers like CDX2, CK20)

Note

─ Diagnosis of MDA on cytology is extremely challenging and often missed; high index of suspicion needed in cases with abundant bland mucinous epithelium, esp with watery discharge

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Main Document

Respiratory

Table of Contents

Introduction ─ General Concepts

Cells

Collection methods

I ─ Non-diagnostic

II ─ Benign / Negative (for Malignancy)

Granular cell tumor

Pulmonary Hamartoma

Schwannoma

PEComa (Perivascular epithelioid cell tumor)

Meningioma

Sclerosing Pneumocytoma

Solitary tracheobronchial papilloma

III ─ Atypical

IV ─ Suspicious for Malignancy (SM)

V ─ Malignant

Adenocarcinoma

Squamous cell carcinoma

Squamous Cell Carcinoma ─ Well Differentiated

Squamous cell carcinoma ─ moderate to poorly differentiated

Neuroendocrine tumor / Carcinoid

Small cell lung carcinoma (SCLC)

Large cell neuroendocrine carcinoma (LCNEC)

Lymphoproliferative diseases

Lymphomas

Pulmonary Langerhans cell histiocytosis (PLCH)

Erdheim-Chester disease (ECD)

Mesothelioma

Paraganglioma

Thymoma

Salivary gland-type carcinomas (primary in lung/mediastinum)

Adenosquamous carcinoma

Pleomorphic carcinoma

Pulmonary blastoma

Carcinosarcoma

NUT carcinoma

Thoracic SMARCA4-deficient undifferentiated tumour

Spindle cell tumors

Primary germ cell tumours of the mediastinum

Primary angiosarcoma of the lung

Introduction ─ General Concepts

Cells

Respiratory columnar cells Cytolo ─ Cilia with terminal bar are characteristic features

Note ─ Uncommon in spontaneously exfoliated material like sputum; when present, may originate from the nasal cavity or nasopharynx if not accompanied by alveolar macrophages; typically seen in direct sampling methods like brushings or FNA of airways

Goblet cells Cytolo ─ Wider than ciliated cells; basal nucleus with distended supranuclear cytoplasm containing mucinous vacuoles

Note ─ Less predominant than ciliated columnar cells in normal airways; goblet cell hyperplasia can be seen in conditions like asthma and chronic bronchitis

Basal or reserve cells Note ─ Small, undifferentiated cells located adjacent to the basement membrane, considered precursors to ciliated and goblet cells; typically seen in tight clusters and may mimic small cell carcinoma, but nuclei are more regular and chromatin is not as finely stippled

IHC (+) ─ p40 (useful as an internal control in cell blocks, highlighting benign basal/reserve cells and squamous differentiation)

Macrophages (Alveolar Macrophages) Note ─ Presence in sputum or BAL confirms origin from alveoli (lower respiratory tract); cytoplasm often contains phagocytosed material (e g , dust, hemosiderin)

Squamous cells Note ─ In lower respiratory tract (LRT) specimens, usually represent contamination from upper airway/oral cavity, or reactive/metaplastic change (squamous metaplasia) in response to injury; atypical squamous metaplasia can be a diagnostic challenge

Collection methods

Various techniques are employed to obtain cytologic samples from the respiratory system, each with specific indications and expected cellular yield

Sputum Clin ─ Used to diagnose LRT infections (e g , Pneumocystis, fungi, bacteria), diffuse interstitial lung diseases (ILD), and centrally located malignancies

Adequacy ─ Need sufficient volume for at least 2 smears (or equivalent for liquid-based preparation); need abundant alveolar macrophages to confirm deep cough sample; scant macrophages may indicate saliva/upper airway contamination

Bronchoalveolar lavage (BAL) Adequacy ─ > 10 alveolar MΦ per 2mm² (approximately >20 alveolar macrophages per 10 HPF, or >2% alveolar macrophages on differential cell count in non-immunocompromised, non-smokers) is often cited; presence of bronchial epithelial cells is variable; primarily samples alveolar spaces

Bronchial brushing and bronchial washing Adequacy ─ Large number of ciliated columnar epithelial cells, goblet cells, and macrophages are expected if sampling airways; atypical or malignant cells if a lesion is targeted; cellularity depends on direct contact with the lesion

FNA (Fine-Needle Aspiration) Adequacy ─ Presence of cytomorphologic features that explain or correlate with the clinical and imaging findings (e g , lesional cells from a nodule, or specific benign elements like cartilage from a hamartoma); for suspected lymph node sampling (e g , EBUS-TBNA), adequate lymphocytes should be present if a lymph node is the target and no other pathology is identified

I ─ Non-diagnostic

This category indicates that the specimen is unsatisfactory for a diagnostic interpretation, or that the findings do not explain the targeted lesion

aka ─ Insufficient / Inadequate for diagnosis; Unsatisfactory for evaluation

Criteria ─

─ Preparation artifact (e g , extensive air drying, crush, poor staining) precludes evaluation

─ Excess blood or mucus obscures cellular material

─ Presence of only normal cellular elements (e g , bronchial cells, cartilage from airway wall) or alveolar macrophages when a discrete mass/lesion is targeted by FNA (unless the lesion is known to be composed of such elements, e g , an inflammatory lesion rich in macrophages)

─ Sputum samples need abundant alveolar macrophages to be considered diagnostic for LRT processes (absence suggests saliva or upper airway material)

─ Acellular specimen or insufficient cellular material to assess the target lesion

─ EBUS-TBNA of a lymph node target showing only benign bronchial cells/cartilage or blood, without adequate lymphoid material

Clin ─ Provides no useful diagnostic information about the targeted lesion; repeat sampling is often necessary, potentially with a different modality or improved technique

Note ─

─ Any degree of cellular atypia generally precludes this category, unless the atypia is clearly an artifact or contaminant unrelated to the patient's lesion

─ The entire specimen should be processed and examined before designating it as nondiagnostic

─ Correlation with clinical and imaging findings is essential; a "benign" sample from a highly suspicious mass might be considered nondiagnostic if it doesn't explain the mass

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II ─ Benign / Negative (for Malignancy)

This category is used when an adequate cytologic sample is representative of the clinical/radiologic finding and contains no evidence of malignancy or significant atypia beyond reactive changes

Criteria ─

─ Normal cellular elements are present in the absence of a discrete mass, or, if a mass is present, the benign findings adequately explain the nature of the mass (e g , granulomas in a patient with sarcoidosis presenting as a nodule)

─ Presence of cilia on columnar cells is a strong indicator of benignity for those cells

─ Orderly cell groups, low N:C ratios (in epithelial cells other than basal cells), smooth nuclear contours, fine chromatin, and inconspicuous nucleoli (even if reactive)

─ For sputum or BAL, adequate alveolar macrophages should be present

─ For targeted lesions (FNA, brush), the benign material should be consistent with the nature of the lesion (e g , specific infectious organisms, features of organizing pneumonia, amyloid)

Note ─

─ Sample adequacy criteria must be met (i e , adequate material to evaluate/define a lesion)

─ This category includes specific benign diagnoses when possible (e g , infection, specific inflammatory conditions)

─ Risk of malignancy (ROM) is not zero due to potential sampling error or interpretative challenges, especially with sputum; clinical and imaging follow-up is often still warranted if suspicion for malignancy persists

Inflammatoryxs ─

─ Acute inflammation (e g , bacterial pneumonia, abscess); neutrophils predominate

─ Granulomatous inflammation (e g , sarcoidosis, mycobacterial or fungal infection); epithelioid histiocytes, giant cells, lymphocytes; necrosis may be present depending on etiology (caseous for TB, often absent in sarcoidosis)

─ Pulmonary infarction (hemosiderin-laden macrophages, reactive pneumocytes, possibly ischemic atypia)

─ Nodular amyloidosis (amorphous, acellular, waxy material, apple-green birefringence with Congo red under polarized light)

─ Viral pneumonia / cytopathic effects (e g , herpes: multinucleation, molding, margination, ground-glass nuclei; CMV: large cells with basophilic intranuclear and smaller cytoplasmic inclusions)

Neoplasm ─

(Note: According to Papanicolaou Society of Cytopathology guidelines, specific benign neoplasms are generally categorized under "Neoplastic - Benign" (Category IVa) rather than Category II) ─ Pulmonary hamartoma (cartilage, fibromyxoid stroma, benign epithelium, fat)

─ Sclerosing pneumocytoma (dual population of surface cuboidal cells and stromal round cells)

─ Granular cell tumor (large cells with abundant granular eosinophilic cytoplasm, small bland nuclei)

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Granular cell tumor

An uncommon benign neoplasm, thought to be of Schwann cell origin, that can occur in the tracheobronchial tree and is composed of cells with abundant eosinophilic granular cytoplasm

Clinical ─

─ Typically occurs in middle-aged adults (4th-6th decades), but wide age range

─ Slight female predominance noted in some series, others report male predominance or no sex predilection for pulmonary lesions

─ Most are endobronchial, arising in larger airways; less commonly parenchymal

─ Patients may be asymptomatic (incidental finding on bronchoscopy or imaging) or present with cough, wheezing, dyspnea, or hemoptysis if the tumor causes obstruction

─ Often solitary, but multiple tumors can occur, especially in the tracheobronchial tree

Cytology ─

─ Cells are polygonal to round, arranged in cohesive clusters, syncytial groups, or as isolated cells; cellularity can be variable

─ Cytoplasm is abundant, dense, and strikingly granular (eosinophilic with Pap stain, can be basophilic with Giemsa); granules are fine and uniform; cell borders are often distinct but can be ill-defined in groups

─ Nuclei show small, round to oval features, often eccentrically placed; chromatin is finely granular and evenly distributed; nucleoli are typically inconspicuous or absent

─ Background shows may contain numerous stripped, bare nuclei and granular cytoplasmic debris due to cell fragility; generally clean otherwise, without significant inflammation or necrosis unless ulcerated

─ Absence of significant nuclear pleomorphism, hyperchromasia, mitotic activity, or true necrosis (in benign cases); absence of keratinization or glandular features

Ancillary studies ─

─ IHC (+): S100 protein (strong and diffuse); SOX10; CD68 (lysosomal marker, highlights granules); PAS-D (Periodic Acid-Schiff with diastase, highlights granules); Inhibin-alpha; TFE3

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2); Neuroendocrine markers (Chromogranin, Synaptophysin); TTF-1; Desmin; Myogenin

DDx ─

─ Macrophages/Histiocytes (less cohesive, more variable nuclei, cytoplasm often foamy or vacuolated rather than densely granular, CD68 positive but S100/SOX10 negative)

─ Oncocytic cells / Oncocytoma (more prominent nucleoli, mitochondria-rich cytoplasm but not as distinctly granular, S100 negative or focal)

─ Carcinoid tumor (neuroendocrine nuclear features – salt-and-pepper chromatin, more cohesive, synaptophysin/chromogranin positive)

─ Squamous metaplasia (more cohesive sheets, denser cytoplasm, intercellular bridges if visible, keratin positive)

─ Alveolar soft part sarcoma (rare in lung, different clinical context, prominent nucleoli, characteristic crystals, TFE3 positive but S100 negative)

─ Rhabdomyoma (extremely rare in lung, may show cross-striations, desmin/myogenin positive)

─ Melanoma (can have granular cells, but usually more pleomorphism, prominent nucleoli, other melanoma markers like Melan-A/HMB45 positive)

Prognosis ─

─ Almost always benign; malignant granular cell tumors are exceedingly rare, especially in the lung

─ Local recurrence is possible if incompletely excised

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Pulmonary Hamartoma

A benign mesenchymal neoplasm of the lung, the most common benign lung tumor, typically discovered incidentally as a solitary, well-circumscribed peripheral "coin lesion"

Clinical ─

─ Most common benign lung tumor

─ Usually discovered incidentally in adults (peak 50-70 years) on imaging

─ Slight male predominance

─ Typically a solitary, peripheral, well-circumscribed, slow-growing "coin lesion" <4 cm

─ Less commonly central/endobronchial, which may cause obstructive symptoms

─ Multiple hamartomas are rare, may be associated with Carney triad (pulmonary hamartoma, GIST, paraganglioma) or Cowden syndrome

─ Radiologic "popcorn" calcification is characteristic but not always present

Cytology ─

─ Cells are a biphasic admixture of mesenchymal and epithelial elements; aspirates may be variably cellular depending on the components sampled

─ Cytoplasm is variable: epithelial cells (bronchial type) have scant to moderate, often pale or clear cytoplasm, may be ciliated; spindle cells (fibromyxoid stroma) have scant, wispy cytoplasm; chondrocytes have clear cytoplasm within lacunae; adipocytes have abundant clear cytoplasm

─ Nuclei show benign features: epithelial cells have round to oval, normochromatic nuclei with smooth contours and fine chromatin; spindle cells have bland, elongated nuclei; chondrocytes have small, dark, round nuclei; adipocyte nuclei are small and peripheral

─ Background shows often a key diagnostic feature: fragments of mature hyaline cartilage (chondroid material – dense, glassy, metachromatic with Giemsa, pale blue/green with Pap); fibromyxoid stroma (loose, fibrillary, metachromatic with Giemsa); mature adipose tissue fragments; benign respiratory epithelial cells in flat sheets or small clusters

─ Absence of significant atypia, mitoses, or necrosis; absence of a predominant atypical epithelial population (unlike adenocarcinoma); absence of atypical chondrocytes (unlike chondrosarcoma)

Ancillary studies ─

─ IHC (+): Mesenchymal spindle cells and chondrocytes are S100 positive; Epithelial cells are cytokeratin positive (e g , CK7, CAM5.2) and TTF-1 positive

─ Molecular ─ Clonal chromosomal abnormalities involving HMGA2 (12q14-15) or HMGA1 (6p21) genes are common, confirming neoplastic nature

DDx ─

─ Adenocarcinoma, well-differentiated (especially lepidic or acinar; shows more atypia, complex glandular architecture, lacks mesenchymal components)

─ Carcinoid tumor (neuroendocrine features, lacks mesenchymal components)

─ Metastatic chondrosarcoma (rare, more cellular and atypical cartilage, clinical history)

─ Reactive bronchial cells (may show some atypia but usually ciliated, no mesenchymal elements)

─ Sclerosing pneumocytoma (dual population of cuboidal surface cells and stromal round cells, lacks cartilage and fat)

─ Organizing pneumonia (fibroblastic proliferation, inflammatory cells, no true cartilage)

Prognosis ─

─ Benign with excellent prognosis; malignant transformation is exceptionally rare

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Schwannoma

A benign, encapsulated neoplasm arising from Schwann cells of nerve sheaths; primary pulmonary schwannomas are rare

Clinical ─

─ Primary pulmonary schwannomas are very rare (0.2% of all lung neoplasms)

─ Can occur at any age, no strong sex predilection for pulmonary lesions

─ Often asymptomatic and discovered incidentally as a solitary, well-circumscribed peripheral nodule or an endobronchial mass

─ Endobronchial lesions may cause cough, dyspnea, or obstruction

─ Multiple lesions may occur in the context of neurofibromatosis type 2 (NF2) or schwannomatosis

Cytology ─

─ Cells are predominantly spindle-shaped, arranged in cohesive fascicles, Verocay bodies (palisading nuclei around acellular eosinophilic zones – Antoni A areas), or loosely textured myxoid areas (Antoni B areas); cellularity varies with Antoni A areas being more cellular

─ Cytoplasm is typically scant to moderate, pale, fibrillary, with indistinct cell borders; may appear wavy or "shredded"

─ Nuclei show elongated, slender, wavy, buckled, or comma-shaped features with pointed ends; chromatin is finely granular and evenly distributed; nucleoli are usually inconspicuous; "ancient change" may show focal pleomorphism and hyperchromasia but lacks mitoses

─ Background shows often contains fragments of collagenous or myxoid stroma; generally clean unless cystic degeneration (common in larger lesions) or hemorrhage has occurred; hemosiderin-laden macrophages may be seen with cystic change

─ Absence of significant mitotic activity (usually rare or absent), necrosis (unless ancient/cystic change), or epithelial features (glands, keratinization)

Ancillary studies ─

─ IHC (+): S100 protein (strong and diffuse, nuclear and cytoplasmic); SOX10; Vimentin; CD57 (Leu-7); GFAP (variable, often focal)

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2); EMA; Desmin; SMA; CD34; TTF-1

DDx ─

─ Neurofibroma (less cellular, more haphazard arrangement, wavy cells in a collagenous stroma, admixed mast cells, S100 often less diffuse and includes staining of admixed axons)

─ Leiomyoma (blunt-ended "cigar-shaped" nuclei, more abundant eosinophilic cytoplasm, desmin/SMA positive)

─ Solitary fibrous tumor (patternless architecture, "hemangiopericytoma-like" vessels, CD34/STAT6 positive)

─ Spindle cell (sarcomatoid) carcinoma (more atypia, pleomorphism, mitoses, cytokeratin positive)

─ Malignant peripheral nerve sheath tumor (MPNST) (more cellularity, atypia, mitoses, necrosis, often associated with NF1, S100 often lost or focal)

─ Fibroblastic/myofibroblastic lesions (variable S100, other markers depending on entity)

Prognosis ─

─ Benign; recurrence after complete excision is rare; malignant transformation is exceptionally rare (usually in NF1 setting or large, deep-seated tumors)

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PEComa (Perivascular epithelioid cell tumor)

A rare mesenchymal neoplasm family characterized by distinctive perivascular epithelioid cells, co-expressing melanocytic and smooth muscle markers; pulmonary PEComas include the benign clear cell "sugar" tumor and lymphangioleiomyomatosis (LAM)

Clinical ─

─ Primary pulmonary PEComas (including "sugar tumor") are very rare

─ "Sugar tumors" typically occur in middle-aged adults (median 50s), slight female predilection; usually asymptomatic, incidental peripheral solitary nodules

─ LAM almost exclusively affects women of reproductive age, often associated with tuberous sclerosis complex (TSC); presents with cystic lung disease, dyspnea, pneumothorax

─ Other PEComas can occur at various sites and may rarely metastasize to the lung

Cytology ─

(Features primarily describe benign clear cell "sugar" tumor, as LAM is usually diagnosed on biopsy/imaging) ─ Cells are polygonal to epithelioid, sometimes spindle-shaped, arranged in loose clusters, sheets, or as single cells; often intimately associated with thin-walled blood vessels; aspirates can be variably cellular, sometimes paucicellular

─ Cytoplasm is typically abundant, clear to pale eosinophilic, and finely granular or vacuolated due to glycogen (PAS positive, diastase sensitive); cell borders are usually distinct

─ Nuclei show round to oval features with smooth contours; finely granular, evenly distributed chromatin; nucleoli are generally small and inconspicuous in benign lesions; minimal pleomorphism

─ Background shows usually clean; may have delicate capillary network; necrosis is absent in benign lesions

─ Absence of significant nuclear atypia, hyperchromasia, high mitotic activity, or necrosis (in benign "sugar tumor"); absence of overt keratinization or glandular formation

Ancillary studies ─

─ IHC (+): HMB-45 (often strong and diffuse); Melan-A (MART-1); Smooth muscle actin (SMA); Desmin (variable); Calponin; MiTF (microphthalmia-associated transcription factor)

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2 usually); S100 protein (usually); TTF-1; Neuroendocrine markers (Chromogranin, Synaptophysin); CD34

─ Molecular ─ Mutations in TSC1 or TSC2 genes are characteristic of LAM and a subset of other PEComas, leading to mTOR pathway activation; TFE3 gene fusions (e g , with SFPQ) reported in a distinct subset of PEComas not associated with TSC

DDx ─

─ Metastatic clear cell renal cell carcinoma (RCCAM positive, PAX8 positive, CD10 positive, cytokeratin positive)

─ Metastatic adrenal cortical carcinoma (inhibin, calretinin, Melan-A positive, but different morphology and clinical context, SF-1 positive)

─ Metastatic melanoma (S100, SOX10 positive, HMB-45/Melan-A also positive but usually more pleomorphism and prominent nucleoli if not amelanotic)

─ Granular cell tumor (S100 positive, more distinctly granular cytoplasm, HMB-45/Melan-A negative)

─ Clear cell variant of adenocarcinoma (TTF-1, Napsin A, cytokeratin positive)

─ Oncocytoma/oncocytic carcinoid (oncocytic features, neuroendocrine markers for carcinoid, HMB-45/Melan-A negative)

─ Alveolar soft part sarcoma (ASPSCR1-TFE3 fusion, different morphology, characteristic crystals, HMB-45/Melan-A negative)

─ Paraganglioma (neuroendocrine markers positive, S100 positive sustentacular cells, HMB-45/Melan-A negative)

Prognosis ─

─ Benign clear cell "sugar" tumors of the lung are indolent with excellent prognosis after resection

─ LAM is a progressive disease, though mTOR inhibitors (e g , sirolimus) can stabilize lung function

─ Other PEComas have a variable prognosis; malignant PEComas are defined by features like large size, infiltrative growth, high nuclear grade, necrosis, and mitotic activity, and can metastasize

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Meningioma

Benign neoplasms arising from arachnoidal (meningothelial) cells; primary intrapulmonary or mediastinal meningiomas are exceedingly rare, but can occur, and are more common in middle-aged women

Clinical ─

─ Primary pulmonary or mediastinal meningiomas are rare; usually asymptomatic and found incidentally as solitary, well-circumscribed peripheral nodules

─ More common in middle-aged to elderly women (F>M)

─ Metastatic meningioma to the lung is more common than primary pulmonary meningioma, though CSF spread of CNS meningiomas is itself uncommon

─ May be associated with a history of meningioma elsewhere if metastatic

Cytology ─

─ Cells are spindle to epithelioid, arranged in cohesive, syncytial-type clusters, sheets, nests, and characteristic whorls; cellularity can be moderate

─ Cytoplasm is usually pale, eosinophilic, and ill-defined with wispy cell borders; amount is moderate

─ Nuclei show bland, oval to elongated features with smooth contours and finely granular chromatin; small, indistinct nucleoli; intranuclear cytoplasmic pseudoinclusions are characteristic and can be numerous; nuclear grooves may be seen

─ Background shows psammoma bodies (laminated calcifications) may be present, especially in the psammomatous variant; generally clean unless hemorrhage or cystic change has occurred

─ Absence of significant atypia, frequent mitoses (in benign WHO Grade I cases); absence of prominent macronucleoli (unlike some carcinomas); absence of diffuse S100 positivity (unlike schwannoma); absence of TTF-1 (unlike lung adenocarcinoma)

Ancillary studies ─

─ IHC (+): EMA (epithelial membrane antigen, often strong), Vimentin, Progesterone receptor (PR, often strong in female patients), Somatostatin receptor 2A (SSTR2A)

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2 usually, though focal positivity can occur), S100 protein (usually negative or only focal/weak, helps distinguish from schwannoma), TTF-1, Napsin A, Chromogranin, Synaptophysin

─ Molecular ─ NF2 gene mutations are common in CNS meningiomas (especially fibrous, transitional, psammomatous types); data for primary pulmonary meningiomas is limited but similar alterations can be seen

DDx ─

─ Solitary fibrous tumor (STAT6 positive, CD34 positive, EMA negative)

─ Schwannoma (S100 and SOX10 strong/diffuse positive, EMA negative, lacks whorls and pseudoinclusions)

─ Carcinoid tumor (neuroendocrine markers positive, EMA negative or focal)

─ Low-grade adenocarcinoma of lung (TTF-1, Napsin A positive, more glandular features)

─ Metastatic carcinoma (e g , breast, renal; depends on primary, specific markers like GATA3, PAX8 helpful)

─ Reactive fibroblasts or mesothelial cells (especially if few cells and prominent spindle morphology or if pleural-based)

─ Well-differentiated papillary mesothelioma (if pleural based, calretinin, WT-1, D2-40 positive)

Prognosis ─

─ Primary pulmonary/mediastinal WHO Grade I meningiomas are typically benign and have an excellent prognosis after complete resection

─ WHO Grade II (atypical) and Grade III (anaplastic/malignant) meningiomas are much rarer in extraneuraxial sites but behave more aggressively

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Sclerosing Pneumocytoma

A rare, benign neoplasm of the lung, previously termed sclerosing hemangioma, thought to derive from primitive respiratory precursor cells related to type II pneumocytes; typically presents in middle-aged women as an incidental peripheral, well-circumscribed nodule

Clinical ─

─ More common in middle-aged women (4th-6th decades), particularly in Asian populations

─ Usually an asymptomatic, incidental finding as a solitary, peripheral, well-circumscribed nodule on imaging (often <3 cm)

─ Rarely, patients may present with cough or hemoptysis

─ Multiple lesions are very rare

─ Low 18F-fluorodeoxyglucose (FDG) avidity on PET scan is typical

Cytology ─

─ Cells are typically a dual population: (1) cuboidal to polygonal surface cells lining papillae or small spaces, and (2) round to oval, sometimes spindled, stromal-like cells forming solid sheets or clusters, often predominating; poorly formed papillary structures and hemorrhagic background are common

─ Cytoplasm is scant to moderate and pale or eosinophilic in stromal-like cells; surface cuboidal cells have features similar to type II pneumocytes with clear, vacuolated, or granular eosinophilic cytoplasm

─ Nuclei show monomorphic, round to ovoid features with smooth nuclear contours, finely granular chromatin, and inconspicuous or small nucleoli in both cell types; intranuclear pseudoinclusions may be seen in surface cells

─ Background shows often hemorrhagic; hemosiderin-laden macrophages may be present; sclerosis (fibrous stroma) is a key histologic feature but may be difficult to appreciate as distinct stromal fragments cytologically, though some hyalinized stromal fragments can be seen

─ Absence of significant atypia, high mitotic activity, or necrosis; the dual population with bland cytology is characteristic

Ancillary studies ─

─ IHC (+): Both surface cuboidal cells and stromal-like cells are positive for TTF-1 and EMA (EMA often dot-like or membranous in stromal cells); Surface cells are also positive for cytokeratins (e g , CK7, CAM5.2) and surfactant proteins (e g , Napsin A)

─ IHC (-): Both cell types are negative for neuroendocrine markers (Chromogranin, Synaptophysin, CD56) and S100 protein

DDx ─

─ Adenocarcinoma, well-differentiated (especially lepidic or papillary predominant; typically shows more nuclear atypia, architectural complexity, prominent nucleoli, and lacks the distinct bland round stromal cell component; stromal cells of pneumocytoma are CK negative or only focally positive unlike adenocarcinoma)

─ Carcinoid tumor (neuroendocrine features, "salt-and-pepper" chromatin, synaptophysin/chromogranin positive, lacks dual population)

─ Pulmonary hamartoma (contains cartilage and often fibromyxoid stroma, lacks the characteristic dual cell population of pneumocytoma)

─ Metastatic papillary thyroid carcinoma (papillary structures, psammoma bodies may be present, characteristic nuclear features of PTC, thyroglobulin and PAX8 positive)

─ Reactive pneumocyte hyperplasia (often in a background of inflammation or injury, lacks the distinct stromal cell component and organized papillary/solid architecture)

─ Meningioma (if prominent stromal cells; meningioma has whorls, pseudoinclusions, EMA/PR positive, TTF-1 negative)

Prognosis ─

─ Benign, with excellent prognosis after complete resection; recurrence or metastasis is exceptionally rare

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Solitary tracheobronchial papilloma

A rare, benign exophytic tumor of the larger airways (trachea and bronchi), characterized by fibrovascular cores lined by squamous, glandular (respiratory or mucinous), or mixed epithelium

Clinical ─

─ Rare, accounting for <1% of all lung tumors

─ Can occur at any age, but often in adults; squamous type more common in adults, glandular in children

─ Patients may be asymptomatic or present with cough, hemoptysis, wheezing, or postobstructive pneumonia due to airway obstruction

─ Usually solitary, but multiple squamous papillomas (papillomatosis) can occur, especially in children and young adults, strongly associated with HPV types 6 and 11 (often acquired from maternal genital infection)

─ Glandular papillomas are not typically HPV-related

Cytology ─

(Obtained via bronchial brushing, washing, or FNA of an endobronchial lesion) ─ Cells are dependent on the papilloma subtype:

─ Squamous papillomas: Show clusters and sheets of mature squamous cells, which may be keratinizing or non-keratinizing; koilocytic atypia (perinuclear halo, nuclear atypia) may be seen if HPV-related; anucleated squames and parakeratotic cells can be present

─ Glandular papillomas: Show clusters of benign-appearing columnar cells, which may be ciliated (respiratory type) or mucin-producing (goblet cell type); cells are often arranged in papillary fronds or tight clusters

─ Mixed papillomas: Show features of both squamous and glandular types

─ Cytoplasm is typically abundant and corresponds to cell type (e g , dense/keratinized in squamous, vacuolated/mucinous in glandular, ciliated in respiratory)

─ Nuclei show generally bland features consistent with benign squamous or glandular epithelium; mild reactive atypia (nuclear enlargement, slight hyperchromasia) may be present, especially if inflamed or previously manipulated; mitoses are usually few or absent

─ Background shows may be clean or contain inflammatory cells (neutrophils, lymphocytes) if secondarily infected or irritated; mucin may be present with glandular or mixed types; fragments of fibrovascular cores are rarely seen but are diagnostic if present

─ Absence of high-grade malignant features (significant pleomorphism, coarse chromatin, macronucleoli, atypical mitoses); absence of true stromal invasion if tissue fragments are evaluable

Ancillary studies ─

─ IHC (+): Squamous cells are pankeratin positive, p40/p63 positive; Glandular cells are pankeratin positive (e g , CK7); HPV (by ISH or PCR, especially for HPV 6/11 in squamous papillomas/papillomatosis)

─ Molecular ─ BRAF V600E mutations have been reported in some peripheral glandular papillomas/adenomas; EGFR and KRAS mutations also described in some benign papillary lesions of distal airways; significance in tracheobronchial papillomas less clear

DDx ─

─ Squamous cell carcinoma (if squamous papilloma with atypia; carcinoma shows more significant atypia, pleomorphism, irregular chromatin, often necrosis, and invasive pattern if tissue fragments present)

─ Adenocarcinoma (if glandular papilloma with atypia; carcinoma shows more complex glands, infiltrative features if evaluable, and greater atypia)

─ Adenosquamous carcinoma (if mixed papilloma with atypia; shows features of both SCC and adenocarcinoma)

─ Reactive epithelial changes/metaplasia (may show atypia but usually lack the distinct exophytic papillary architecture if sampled adequately; clinical context of irritation important)

─ Papillary carcinoma metastatic to lung (e g , thyroid, renal, breast; clinical history and site-specific IHC markers are key)

─ Mucoepidermoid carcinoma (if glandular/mucinous features; typically shows intermediate cells and often MAML2 rearrangement)

Prognosis ─

─ Benign; however, malignant transformation has been reported rarely, most commonly in squamous papillomas (especially in adults with long-standing papillomatosis) to squamous cell carcinoma

─ Recurrence can occur if incompletely excised, particularly for HPV-related multiple papillomas

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III ─ Atypical

This category is used for specimens that demonstrate cytomorphologic features (architectural or nuclear) that exceed those expected for benign and reactive conditions but fall short of those required for a "Suspicious for Malignancy," "Neoplastic," or "Malignant" diagnosis. The atypia may be due to qualitative or quantitative deficiencies.

Criteria ─

─ Cellular changes (nuclear enlargement, anisonucleosis, nuclear membrane irregularities, chromatin abnormalities, loss of polarity, nuclear overlapping) that are more pronounced than definite reactive/reparative changes but are not overtly malignant

─ Often involves scant cells showing these atypical features

─ May include cases where inflammatory or infective changes, or effects of radiation/chemotherapy, cause atypia that cannot be definitively distinguished from a neoplastic process without further information or follow-up

─ Background may be suggestive of neoplasm (e g , necrotic or keratinous debris, thick mucin, or apoptotic cells) but definitive malignant cells are lacking or poorly preserved

─ The Papanicolaou Society of Cytopathology (PSC) guidelines note that this category should be used when atypical epithelial cells are seen, but the quantity or quality is insufficient to determine if they represent reactive bronchial cells/pneumocytes versus malignant cells

─ Examples from PSC guidelines that might fall here:

─ Minor degrees of loss of polarity and/or nuclear overlapping

─ Focal nuclear membrane irregularities beyond subtle reactive undulations

─ Focal chromatin clearing without distinct clumping

─ Atypical squamous metaplasia in the setting of intubation or inflammation

─ Hypocellular specimens with focal marked atypia but insufficient for "Suspicious"

─ Technically limited specimens where atypia is present but poorly preserved

Note ─

─ This category acknowledges a degree of uncertainty

─ Clinical and radiologic correlation is critical (e g , history of infarct, ARDS, pneumonia, radiation/chemotherapy may explain some atypia)

─ The term "reactive atypia" should generally be avoided; if changes are confidently reactive, "Benign" (Category II) is preferred

─ Risk of Malignancy (ROM): Variable, reported as high as ~50-60% in some studies, but can be lower (~20-30%) depending on institutional thresholds and patient population; the PSC atlas notes a ROM of 54% in one report and 22% in their own institution's experience for this category

Management ─ (according to PSC guidelines)

─ Further work-up is usually indicated:

─ Repeat cytology or tissue biopsy

─ Additional imaging studies

─ Clinical follow-up

─ Ancillary histochemical or immunohistochemical stains are typically of limited value in further classifying these specimens based on the cytology alone; morphologic criteria and correlation are key

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IV ─ Suspicious for Malignancy (SM)

This category applies to samples showing features that are highly suggestive but not unequivocal for a malignant neoplasm. The uncertainty may be due to insufficient quantity of abnormal cells, suboptimal quality/preservation, or cytologic features that are very concerning but fall just short of a definitive malignant diagnosis.

Criteria ─

─ Features are highly suggestive of malignancy, but fall short of an unequivocal diagnosis either quantitatively (too few cells) or qualitatively (atypia not fully expressed or obscured)

─ This is a "bridging" category between "Atypical" and "Positive for Malignancy"

─ Includes specimens with cells showing significant architectural disarray (loss of polarity, crowding, overlapping, nuclear molding, cell-in-cell) and/or individual cell dysmorphology (changes in nuclear size/shape, chromatin pattern, nucleoli) that are strongly worrisome for malignancy

─ May include cases with extensive necrosis or poor preservation where rare intact cells exhibit highly suspicious features

─ PSC guidelines note that this category is used when cellular atypia is favored to be malignant but is not sufficient for an unequivocal diagnosis

─ Examples from PSC guidelines:

─ Low numbers of cells with features diagnostic of carcinoma

─ Poorly preserved cells that otherwise would be diagnostic of carcinoma

─ Well-differentiated neoplasms where cytologic features are subtle (e g , some well-differentiated adenocarcinomas or neuroendocrine tumors where definitive features are not fully apparent)

─ High-grade dysplasia (though this term is more often used in histology)

Note ─

─ Interobserver variability for this category is high, and its use should be limited (i e , not a "wastebasket" category)

─ The Papanicolaou Society of Cytopathology considers "Suspicious for Malignancy" and "Malignant" as distinct categories with different risks of malignancy

─ A diagnosis of "Suspicious for Malignancy" alone should generally not serve as the basis for definitive therapy without further confirmation or strong multidisciplinary consensus

─ Risk of Malignancy (ROM): High, often cited as >80% (e g , PSC atlas notes 82%)

Management ─ (according to PSC guidelines)

─ Further diagnostic intervention is usually required:

─ Repeat biopsy (cytology or histology)

─ Ancillary testing if sufficient material

─ Expert consultation

─ Presentation at a multidisciplinary tumor board for correlation of clinical, imaging, and cytopathologic findings (triple diagnosis approach)

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V ─ Malignant

This category is used when the cytologic features are conclusive for malignancy. The specimen should ideally contain an adequate number of well-preserved malignant cells to allow for definitive classification. Whenever possible, the tumor should be subtyped (e g , adenocarcinoma, squamous cell carcinoma, small cell carcinoma). If subtyping is not possible based on cytomorphology alone, the diagnosis may be "Non-small cell carcinoma, not otherwise specified (NOS)," with a recommendation for ancillary studies (immunohistochemistry, molecular testing) if material allows, to further classify the tumor.

General Criteria for Malignancy (Non-Small Cell Carcinoma) ─

─ Increased cellularity with dyscohesion or abnormal architectural arrangements (e g , disorganized clusters, single malignant cells)

─ Large cell size (often, but not always)

─ High nuclear-to-cytoplasmic (N:C) ratio

─ Nuclear pleomorphism (variation in size and shape)

─ Nuclear membrane irregularities (indentations, protrusions, sharp angles)

─ Hyperchromasia (darkly staining nuclei)

─ Irregular chromatin clumping (coarse, cleared areas)

─ Macronucleoli or irregular nucleoli (often multiple)

─ Mitotic figures, including atypical forms (may be present)

─ Necrotic background (tumor diathesis) or significant apoptosis (may be present)

Note ─

─ The Papanicolaou Society of Cytopathology (PSC) guidelines emphasize that a definitive diagnosis of malignancy should be made with a high degree of certainty (approaching 100% specificity)

─ The risk of malignancy (ROM) for this category is very high (typically >95-99%)

─ Ancillary studies (immunohistochemistry on cell blocks or direct smears, molecular testing) are often crucial for accurate subtyping, prognostication, and guiding targeted therapy, especially for adenocarcinoma and when distinguishing primary lung tumors from metastases

─ It is important to conserve material for potential molecular testing, especially in cases of adenocarcinoma or NSCLC-NOS

Management ─ (according to PSC guidelines)

─ Depends on the specific tumor type and stage

─ Typically involves multidisciplinary team discussion for staging and treatment planning (e g , surgery, chemotherapy, radiation therapy, targeted therapy, immunotherapy)

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Adenocarcinoma

A malignant epithelial tumor with glandular differentiation (e g , acini, papillae) or mucin production by tumor cells, representing the most common histologic subtype of lung cancer in many populations

Clinical ─

─ Most common type of lung cancer, particularly in never-smokers, women, and younger patients

─ Often arises peripherally, but can be central; may present as solitary nodule, multiple nodules, or consolidation

─ Symptoms are variable: cough, dyspnea, hemoptysis, chest pain, or incidental finding; may present with paraneoplastic syndromes or symptoms of metastatic disease

─ Precursor lesions include Atypical Adenomatous Hyperplasia (AAH) and Adenocarcinoma in Situ (AIS); Minimally Invasive Adenocarcinoma (MIA) is an early invasive form

─ Invasive adenocarcinomas are subtyped by predominant histologic pattern (lepidic, acinar, papillary, micropapillary, solid) and include variants like invasive mucinous adenocarcinoma

Cytology ─

─ Cells are arranged in flat sheets, three-dimensional clusters, acini, papillae with or without fibrovascular cores, or micropapillary tufts; single malignant cells are also common; cellularity is often moderate to high

─ Cytoplasm is usually moderate to abundant, delicate, translucent, finely vacuolated (foamy), or granular; distinct mucin vacuoles (intracytoplasmic or pushing nucleus aside like signet-ring cells) may be present; cell borders can be distinct or ill-defined

─ Nuclei show round to oval or irregular contours; typically enlarged with an increased N:C ratio; chromatin is often finely to moderately granular, sometimes vesicular with parachromatin clearing; nucleoli are usually prominent, often single and central, but can be multiple or irregular; intranuclear pseudoinclusions and nuclear grooves may be seen

─ Background shows may be clean, contain mucin (especially in mucinous subtypes), or show necrosis/inflammatory cells in higher-grade or cavitating tumors; psammoma bodies can be seen with papillary patterns

─ Absence of extensive keratinization (unlike squamous cell carcinoma); absence of prominent nuclear molding and finely stippled "salt & pepper" chromatin with scant cytoplasm (unlike small cell carcinoma); absence of a definite biphasic population with chondromyxoid stroma (unlike hamartoma)

Ancillary studies ─

─ IHC (+): TTF-1 (most, ~75-85%, except some mucinous/enteric variants), Napsin A (most, similar sensitivity to TTF-1, but more specific for lung primary vs thyroid), CK7, CEA

─ IHC (-): p40, p63 (or only focal/weak), CK5/6 (usually), CK20 (except some mucinous/enteric variants, which can be CK20+ and TTF-1-), CDX2 (except enteric variant), Thyroglobulin, GATA3, Calretinin, WT1

─ Molecular ─ EGFR mutations (common in never-smokers, Asian ethnicity, women, lepidic/papillary patterns), ALK rearrangements (younger patients, never-smokers, solid/signet ring/mucinous patterns), ROS1 rearrangements, KRAS mutations (common in smokers, mucinous patterns), BRAF mutations, MET exon 14 skipping alterations, RET fusions, ERBB2 (HER2) mutations, NTRK fusions, PIK3CA mutations

DDx ─

─ Reactive bronchial epithelial cells/Creola bodies (cilia often present, less atypia, tight clustering, smooth nuclear contours)

─ Reactive type II pneumocytes (often in background of diffuse lung injury, less architectural complexity, uniform atypia, prominent nucleoli but smooth nuclear membranes)

─ Squamous cell carcinoma, poorly differentiated (p40/p63, CK5/6 positive; TTF-1/Napsin A negative)

─ Large cell neuroendocrine carcinoma (neuroendocrine markers positive, often high mitotic rate, coarser chromatin)

─ Carcinoid tumor (organoid pattern, salt-and-pepper chromatin, neuroendocrine markers positive)

─ Mesothelial cells/Malignant mesothelioma (especially peripheral lesions; mesothelioma is calretinin, WT-1, D2-40 positive, TTF-1/Napsin A negative, MOC31/BerEP4 negative)

─ Metastatic adenocarcinoma (clinical history and site-specific IHC markers: e g , CDX2 for colorectal, PAX8 for gynecologic/renal, GATA3/ER for breast, PSA for prostate)

─ Sclerosing pneumocytoma (dual population of surface cuboidal and stromal round cells, lacks significant atypia)

Prognosis ─

─ Variable; depends on stage, histologic subtype/grade (e g , micropapillary and solid patterns generally have poorer prognosis), and molecular profile (presence of targetable mutations offers specific therapeutic options)

─ AIS and MIA have excellent prognosis with complete resection

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Squamous cell carcinoma

Squamous Cell Carcinoma ─ Well Differentiated

A malignant epithelial tumor showing keratinization and/or intercellular bridges, arising from squamous metaplasia or dysplasia of the bronchial epithelium, strongly associated with smoking

Clinical ─

─ Strongly associated with smoking history; more common in men but incidence increasing in women

─ Typically arises centrally in major bronchi, leading to cough, hemoptysis, or post-obstructive pneumonia; can also occur peripherally and may cavitate

─ Precursor lesions include squamous dysplasia and carcinoma in situ

Cytology ─

─ Cells are often numerous, appearing as single cells, in irregular sheets, or syncytial clusters; well-differentiated tumors show obvious keratinization

─ Cytoplasm is typically dense, refractile, glassy, and brightly eosinophilic or orangeophilic (Papanicolaou stain); may be cyanophilic; bizarre cell shapes (tadpole cells, spindle/fiber cells, keratin pearls) are characteristic

─ Nuclei show marked pleomorphism (variation in size and shape), often enlarged, hyperchromatic (sometimes pyknotic or smudged), with irregular, angulated, or convoluted nuclear contours; chromatin is coarsely granular or opaque

─ Background shows often contains keratinous debris (anucleate squames, "ghost cells"), acute inflammation, and/or necrotic tumor diathesis (granular proteinaceous material with cellular debris)

─ Absence of distinct glandular features (acini, papillae, intracellular mucin); absence of delicate "salt & pepper" chromatin and nuclear molding (unlike small cell carcinoma)

Ancillary studies ─

─ IHC (+): p40 (highly specific and sensitive), p63, CK5/6, High Molecular Weight Cytokeratins (e g , 34βE12)

─ IHC (-): TTF-1, Napsin A, CK7 (usually, though focal positivity can occur), Neuroendocrine markers (Chromogranin, Synaptophysin)

DDx ─

─ Reactive squamous metaplasia with atypia (less pleomorphism, more regular nuclear features, often associated with inflammation or injury, lacks tumor diathesis)

─ Keratinizing squamous dysplasia/carcinoma in situ (features may overlap, but invasive carcinoma often shows more dyscohesion, bizarre forms, and tumor diathesis)

─ Poorly differentiated non-small cell carcinoma (may require IHC to confirm squamous differentiation if keratinization is not obvious)

─ Adenocarcinoma with squamous metaplasia (adenocarcinoma component should be identifiable, TTF-1/Napsin A positive in glandular areas)

─ Metastatic squamous cell carcinoma (from head and neck, esophagus, cervix, etc; clinical history and p16 status may be helpful in some contexts)

─ Radiation atypia (history of radiation, bizarre but often degenerative nuclear changes, cytoplasmic vacuolization and polychromasia)

─ Fungal infections with pseudoepitheliomatous hyperplasia (identify fungal elements)

Prognosis ─

─ Depends on stage; generally, prognosis has been considered slightly better than adenocarcinoma stage for stage in some older studies, but this is debated and depends on many factors including specific molecular features if any

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Squamous cell carcinoma ─ moderate to poorly differentiated

A malignant epithelial tumor with limited or no obvious keratinization, but showing other features of squamous differentiation such as intercellular bridges (histologically) or dense cytoplasm and sharp cell borders, often with marked nuclear atypia

Clinical ─

─ Similar to well-differentiated SCC: strongly associated with smoking, typically central but can be peripheral, may cavitate

─ May represent a less differentiated component of a tumor that also has well-differentiated areas, or be poorly differentiated throughout

Cytology ─

─ Cells are arranged in syncytial-like sheets, three-dimensional clusters, or as single cells; cellularity is often high

─ Cytoplasm is variable, often dense and basophilic or amphophilic, but may be scant; overt keratinization (orangeophilia, bizarre shapes) is focal or absent; cell borders may be relatively well-defined

─ Nuclei show moderate to marked pleomorphism, hyperchromasia, and irregular nuclear contours; chromatin is often coarsely granular and irregularly distributed; nucleoli can be prominent, especially in less differentiated forms, and may be irregular

─ Background shows tumor diathesis (necrotic debris) is common; acute inflammation may be present

─ Absence of distinct glandular features (acini, papillae, intracellular mucin); absence of delicate "salt & pepper" chromatin and nuclear molding (unlike small cell carcinoma); if truly poorly differentiated, may lack obvious squamous features without IHC

Ancillary studies ─

─ IHC (+): p40 (highly specific and sensitive), p63, CK5/6, High Molecular Weight Cytokeratins

─ IHC (-): TTF-1, Napsin A, CK7 (usually, though focal positivity can occur), Neuroendocrine markers

DDx ─

─ Adenocarcinoma, poorly differentiated (may require IHC; adenocarcinoma is TTF-1/Napsin A positive, p40/p63 negative)

─ Large cell neuroendocrine carcinoma (neuroendocrine markers positive, often large cells with prominent nucleoli, high mitotic rate)

─ Small cell carcinoma (if cells are small and molding is present; SCLC has finer chromatin, scant cytoplasm, neuroendocrine markers positive)

─ Metastatic carcinoma, poorly differentiated (clinical history and IHC panel crucial)

─ Lymphoma, large cell (dispersed population, lymphoglandular bodies, lymphoid markers positive)

─ Melanoma (S100, SOX10, Melan-A/HMB45 positive, often prominent nucleoli, single cell pattern)

─ Reparative atypia/severe reactive changes (less pleomorphism, smoother nuclear contours generally, clinical context)

Prognosis ─

─ Depends on stage; poorly differentiated tumors generally have a worse prognosis than well-differentiated tumors of the same stage

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Neuroendocrine tumor / Carcinoid

A relatively low-grade malignant epithelial neoplasm with neuroendocrine differentiation, subdivided histologically into typical carcinoid (low-grade) and atypical carcinoid (intermediate-grade) based on mitotic activity and presence of necrosis

Clinical ─

─ Epid: Typically adults, mean age 50s, but can occur in adolescence; atypical carcinoids tend to occur in slightly older patients

─ Etio: Typical carcinoids are generally not related to smoking; atypical carcinoids have a stronger association with smoking

─ Site: Central (endobronchial or peribronchial, ~75-80%) > peripheral (intraparenchymal, ~20-25%); atypical carcinoids are more often peripheral

─ Clin: Often asymptomatic (incidental finding); symptomatic cases (especially central) may present with cough, hemoptysis, wheezing, post-obstructive pneumonia; hormonal or paraneoplastic syndromes (e g , carcinoid syndrome, Cushing syndrome) are uncommon with pulmonary carcinoids (<5%)

─ Imaging: Central lesions are often well-defined, round to oval endobronchial nodules or peribronchial masses; Peripheral lesions are typically sharply circumscribed solitary nodules

Cytology ─

─ Cells are relatively uniform, small to medium-sized, round, oval, or occasionally spindled (especially peripheral tumors); arranged in loose clusters, sheets, trabeculae, acinar structures, pseudorosettes, or as dispersed single cells; cellularity is often high

─ Cytoplasm is scant to moderate, finely granular (eosinophilic with H&E/Pap, grey-blue with Pap, magenta with Giemsa), often with eccentrically placed nuclei giving a plasmacytoid appearance; cell borders are usually indistinct

─ Nuclei show monotonous, round to oval features with smooth contours; chromatin is characteristically finely granular, stippled ("salt and pepper" appearance); nucleoli are typically small and inconspicuous (may be more prominent in atypical carcinoid)

─ Background shows often clean or may contain stripped bare nuclei; branching, thin fibrovascular strands with attached tumor cells may be seen; significant necrosis is absent in typical carcinoid but may be present (usually focal) in atypical carcinoid

─ Absence of extensive nuclear molding, significant crush artifact, or widespread necrosis (unlike small cell carcinoma or LCNEC); absence of high-grade pleomorphism

Ancillary studies ─

─ IHC (+): Synaptophysin (diffuse), Chromogranin A (often patchy), CD56 (NCAM), Cytokeratins (AE1/AE3, CAM5.2, often dot-like), TTF-1 (positive in ~50-70%, more often in peripheral tumors and atypical carcinoids)

─ IHC (-): p40, p63, Napsin A

─ Note: Ki-67 proliferation index is low in typical carcinoid (<2-5%), higher in atypical carcinoid (up to 20%, but generally <10% by cytology criteria if assessable); mitotic counts are difficult and often not validated for cytologic assessment (<2 mitoses/2mm² and no necrosis for typical; 2-10 mitoses/2mm² OR necrosis for atypical, histologically)

─ Mol: No specific molecular alterations are routinely tested for therapeutic targets in typical/atypical carcinoids

DDx ─

─ Small cell lung carcinoma (more atypia, nuclear molding, crush artifact, high Ki-67, often scantier cytoplasm, different IHC profile for TTF-1 which is usually strong, and p53 aberrant expression)

─ Large cell neuroendocrine carcinoma (larger cells, more pleomorphism, prominent nucleoli, higher mitotic rate, more necrosis)

─ Adenocarcinoma (glandular structures, mucin, TTF-1/Napsin A positive, neuroendocrine markers negative)

─ Lymphoma/Lymphoid hyperplasia (dispersed lymphoid cells, lymphoglandular bodies, CD45 positive, keratin/neuroendocrine markers negative)

─ Basal cell hyperplasia/Reserve cell hyperplasia (tight clusters, scant cytoplasm, but more regular nuclei and associated with bronchial epithelium, p40 positive)

─ Metastatic neuroendocrine tumor (clinical history, IHC profile may vary e g , CDX2 for GI, PAX8/Islet-1 for pancreatic)

Prognosis ─

─ Typical carcinoid: Excellent prognosis, >90% 5-year survival; metastases are rare (<15%, usually to regional lymph nodes)

─ Atypical carcinoid: Intermediate prognosis, ~40-70% 5-year survival; higher risk of lymph node and distant metastases (~20-50%)

─ Mitotic counts and necrosis are key prognostic factors but are best assessed on histology

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Small cell lung carcinoma (SCLC)

A high-grade neuroendocrine carcinoma composed of small cells with scant cytoplasm, finely granular chromatin, absent or inconspicuous nucleoli, and frequent mitoses and necrosis; strongly associated with smoking

Aka ─ Oat cell carcinoma

Clinical ─

─ Etio: Strongly associated with cigarette smoking (>95% of cases); rarely, may arise as a transformation mechanism of TKI resistance in EGFR-mutated NSCLC

─ Epid: Represents ~13-15% of all lung cancers; incidence has been declining in some regions with decreased smoking rates

─ Site: Typically central, arising from major bronchi, but can be peripheral

─ Clin: Aggressive clinical course, often presents with advanced (metastatic) disease at diagnosis; common sites of metastases include brain, liver, bone, adrenals, contralateral lung; frequently associated with paraneoplastic syndromes (e g , SIADH, Cushing syndrome due to ectopic ACTH, Lambert-Eaton myasthenic syndrome)

─ Imaging: Often large central mass with extensive mediastinal/hilar lymphadenopathy; peripheral nodules less common

Cytology ─

─ Cells are small (typically 2-3 times the size of a mature lymphocyte), round, oval, or spindled, often arranged in loose clusters, sheets, linear arrays (Indian filing), or as dispersed single cells; nuclear molding is a characteristic feature in cell groups; crush artifact is common

─ Cytoplasm is very scant, often appearing as a thin rim or inapparent, leading to high N:C ratios and "naked nuclei" appearance

─ Nuclei show round to oval or angulated/irregular contours; chromatin is finely granular ("salt and pepper") to coarsely granular or smudged/pyknotic, especially in degenerated cells or due to crush artifact; nucleoli are usually absent or inconspicuous, if present they are small

─ Background shows often necrotic (tumor diathesis), with granular debris and apoptotic bodies; Azzopardi phenomenon (basophilic staining of vascular walls by DNA from necrotic tumor cells) may be seen

─ Absence of prominent nucleoli, abundant cytoplasm, or distinct glandular/squamous differentiation (though combined SCLC with NSCLC components can occur)

Ancillary studies ─

─ IHC (+): Synaptophysin, Chromogranin A (often focal), CD56 (NCAM), TTF-1 (most cases, ~85-90%), Cytokeratins (AE1/AE3, CAM5.2, often with a perinuclear dot-like pattern); Ki-67 proliferation index is very high (typically >70-80%, often approaching 100%); aberrant p53 expression (overexpression or null pattern) is common

─ IHC (-): p40, p63 (or only rare scattered positive cells), Napsin A, CD45 (LCA), S100 protein

─ Mol: RB1 inactivation (mutation or deletion) and TP53 mutations are nearly ubiquitous; other alterations (e g , MYC family amplification, CREBBP/EP300 mutations) are common but not currently used for routine targeted therapy decisions

DDx ─

─ Carcinoid tumor (more cytoplasm, more organoid patterns, less atypia, no/rare molding, lower Ki-67)

─ Large cell neuroendocrine carcinoma (larger cells, more cytoplasm, often prominent nucleoli)

─ Basaloid squamous cell carcinoma (more cohesive, peripheral palisading, p40/p63 positive, neuroendocrine markers negative)

─ Lymphoma (dispersed cells, lymphoglandular bodies, CD45 positive, keratin/neuroendocrine markers negative)

─ Reserve cell hyperplasia (tight cohesive clusters, associated with bronchial cells, lacks necrosis and high Ki-67)

─ Crushed benign bronchial cells or lymphocytes (can mimic SCLC, but benign cells lack true atypia and high Ki-67; lymphocytes CD45+)

─ Metastatic small cell carcinoma (from other sites like prostate, bladder, cervix; clinical history and IHC for site-specific markers if applicable, e g , PSA for prostate)

Prognosis ─

─ Very poor prognosis, especially when presenting with extensive-stage disease (most common); highly sensitive to initial chemotherapy and radiation, but recurrence is common and rapid

─ Median survival for extensive stage is ~8-12 months, for limited stage ~15-20 months with treatment

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Large cell neuroendocrine carcinoma (LCNEC)

A high-grade neuroendocrine carcinoma morphologically resembling non-small cell carcinoma (large cells, moderate/abundant cytoplasm, often prominent nucleoli) but showing histologic evidence of neuroendocrine differentiation (organoid nesting, trabecular growth, rosettes, peripheral palisading) and immunohistochemical expression of neuroendocrine markers

Clinical ─

─ Rare, accounts for ~1-3% of all lung cancers

─ Strongly associated with smoking, predominantly affects older males

─ Site: More often peripheral than central, but can occur in either location; rarely involves main airways

─ Clin: Aggressive behavior, similar to or worse than SCLC; most are inoperable at diagnosis due to advanced stage or metastases

─ Imaging: Often presents as a large peripheral mass with irregular tumor borders and expansive growth; mediastinal lymphadenopathy is common

Cytology ─

─ Cells are large (typically >3 times the size of a lymphocyte), polygonal or pleomorphic, arranged in loose clusters, syncytial sheets, or as single cells; neuroendocrine architectural features like rosettes or peripheral palisading may be subtle or focal in cytology

─ Cytoplasm is moderate to abundant, often granular or eosinophilic; cell borders may be distinct or ill-defined

─ Nuclei show large, irregular, often vesicular features with coarse to clumped chromatin; nucleoli are frequently prominent and can be large; subtle nuclear molding may be present in some clusters

─ Background shows prominent necrosis (tumor diathesis) is common; mitotic figures, including atypical ones, are often readily identifiable

─ Absence of extensive keratinization or definitive glandular structures (though focal mucin or squamous differentiation can occur in combined forms); differs from SCLC by larger cell size, more cytoplasm, and prominent nucleoli

Ancillary studies ─

─ IHC (+): At least one neuroendocrine marker (Synaptophysin, Chromogranin A, CD56) must be positive (often diffuse Synaptophysin, Chromogranin may be focal); Cytokeratins (AE1/AE3, CAM5.2) are positive; TTF-1 is positive in ~40-60% of cases; Ki-67 proliferation index is high (typically >40-50%, often >70%)

─ IHC (-): p40, p63 (usually, unless combined with SCC), Napsin A (usually, unless combined with adenocarcinoma)

─ Mol: Molecular profile can be heterogeneous, some resembling SCLC (RB1/TP53 mutations), others more like NSCLC (KRAS, STK11/LKB1 mutations); no specific targetable alterations define LCNEC as a group for routine therapy

DDx ─

─ Poorly differentiated non-small cell carcinoma (adenocarcinoma or squamous cell carcinoma lacking neuroendocrine markers)

─ Small cell lung carcinoma (smaller cells, scant cytoplasm, finer chromatin, inconspicuous nucleoli, though some overlap exists, especially with "intermediate cell" SCLC)

─ Carcinoid tumor, atypical (less atypia, lower Ki-67, usually more organized architecture)

─ Metastatic large cell neuroendocrine carcinoma (from other sites like GI tract; clinical history and IHC for site-specific markers if applicable)

─ Melanoma (S100, SOX10, Melan-A/HMB45 positive, neuroendocrine markers negative)

─ Basaloid squamous cell carcinoma (p40/p63 positive, neuroendocrine markers negative)

Prognosis ─

─ Poor prognosis, similar to or worse than SCLC, with high rates of recurrence and metastasis even after resection of early-stage disease

─ Optimal treatment strategies are still evolving, often treated similarly to SCLC or high-grade NSCLC depending on institutional protocols and molecular features if known

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Lymphoproliferative diseases

A heterogeneous group of disorders characterized by the proliferation of lymphocytes or their precursors, ranging from benign reactive processes to malignant lymphomas. Pulmonary involvement can be primary or secondary. Cytologic evaluation, often in conjunction with flow cytometry and immunohistochemistry on cell block material, plays a crucial role in diagnosis and classification. Accurate diagnosis is essential as treatment and prognosis vary significantly among different lymphoproliferative disorders.

Lymphomas

Malignant neoplasms of lymphoid cells that can involve the lung either as a primary site (rare) or more commonly as secondary involvement from a systemic lymphoma. Primary pulmonary lymphomas are most often low-grade B-cell lymphomas of MALT type.

Clinical ─

─ Primary pulmonary lymphoma (PPL) is rare, <1% of all lymphomas and <0.5% of primary lung malignancies

─ Secondary lung involvement by lymphoma is more common (up to 50% of non-Hodgkin and Hodgkin lymphoma patients)

─ PPLs: Most common are extranodal marginal zone B-cell lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma), followed by diffuse large B-cell lymphoma (DLBCL); primary pulmonary Hodgkin lymphoma is very rare

─ Symptoms: Can be asymptomatic (incidental finding); or present with cough, dyspnea, chest pain, B symptoms (fever, night sweats, weight loss)

─ Imaging: Variable; solitary nodule, multiple nodules, consolidation, diffuse interstitial infiltrates, or pleural effusion

Cytology ─

─ Cells are predominantly a dispersed population of lymphoid cells; cellularity varies from low to high; architectural patterns (e g , follicular, diffuse) are difficult to assess on cytology alone, but cohesive clusters are generally absent (unlike carcinoma)

─ Cytoplasm is typically scant in small cell lymphomas, more abundant in large cell lymphomas; may be pale, basophilic, or clear; lymphoglandular bodies (cytoplasmic fragments) may be seen in the background, especially with Giemsa stain

─ Nuclei show features dependent on lymphoma type:

─ Low-grade (e g , MALT lymphoma, CLL/SLL): Small, round to slightly irregular nuclei, coarse or condensed chromatin, inconspicuous nucleoli

─ High-grade (e g , DLBCL, Burkitt): Large nuclei, vesicular or coarse chromatin, prominent nucleoli (DLBCL) or multiple small nucleoli (Burkitt), irregular nuclear contours

─ Hodgkin lymphoma: Presence of classic Reed-Sternberg cells and their mononuclear variants (Hodgkin cells) in a mixed inflammatory background containing eosinophils, plasma cells, histiocytes, and small lymphocytes

─ Background shows often a dispersed, monotonous population of lymphoid cells; necrosis and karyorrhectic debris can be prominent in high-grade lymphomas (e g , Burkitt, some DLBCL); reactive elements (macrophages, bronchial cells) may be admixed

─ Absence of true epithelial cohesion, keratinization, or glandular formation (unlike carcinoma); absence of "salt & pepper" chromatin with nuclear molding (unlike small cell carcinoma, though some lymphomas can show molding)

Ancillary studies ─

─ Note: Flow cytometry on fresh/unfixed sample is highly valuable for immunophenotyping and clonality assessment; Cell block for IHC is crucial if flow cytometry is not feasible or for specific markers not amenable to flow (e g , some nuclear transcription factors, EBV-EBER ISH)

─ IHC (+): CD45 (pan-leukocyte marker, confirms hematopoietic origin); B-cell lymphomas: CD20, CD19, PAX5, CD79a; T-cell lymphomas: CD3, CD2, CD5, CD7; Hodgkin lymphoma: CD30, CD15 (classic HL), PAX5 (weak in RS cells), CD20 (LP cells in NLPHL)

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2), TTF-1, Napsin A, neuroendocrine markers (in most lymphomas, helps exclude carcinoma)

─ Mol: Clonality studies (e g , IgH/IgK/IgL gene rearrangements for B-cell, TCR gene rearrangements for T-cell lymphomas by PCR) can confirm neoplasia if morphologically equivocal; specific translocations by FISH or RT-PCR (e g , t(14,18) for follicular lymphoma, t(11,14) for mantle cell, MYC rearrangements for Burkitt/high-grade B-cell lymphoma)

DDx ─

─ Reactive lymphoid hyperplasia/Lymphoid interstitial pneumonia (LIP) (polymorphous lymphoid population, no light chain restriction by flow/IHC, often germinal center fragments)

─ Small cell lung carcinoma (nuclear molding, crush artifact, "salt & pepper" chromatin, neuroendocrine markers positive, CD45 negative)

─ Poorly differentiated non-small cell carcinoma (may show dispersed cells but usually some cohesion, keratin or TTF-1/Napsin A positive, CD45 negative)

─ Carcinoid tumor (organoid clusters, "salt & pepper" chromatin, neuroendocrine markers positive, CD45 negative)

─ Thymoma (if mediastinal; mixture of epithelial cells and lymphocytes, epithelial cells CK positive)

─ Chronic inflammation with atypical lymphocytes (may require flow cytometry/molecular to exclude lymphoma)

Prognosis ─

─ Highly variable, depends on specific lymphoma subtype, grade, stage, and patient factors

─ Primary pulmonary MALT lymphoma often has an indolent course

─ High-grade lymphomas like DLBCL and secondary lung involvement by aggressive systemic lymphomas have a poorer prognosis

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Pulmonary Langerhans cell histiocytosis (PLCH)

A rare, smoking-related interstitial lung disease characterized by the proliferation and accumulation of Langerhans cells, typically forming nodular or cystic lesions predominantly in the upper lobes of young adult smokers

Clinical ─

─ Epid: Almost exclusively occurs in young adult cigarette smokers (90-100% of cases); mean age 20-40 years; slight male predominance or equal sex distribution

─ Site: Lungs are the primary site; can be isolated to lungs or part of multisystem LCH (rare in adults)

─ Clin: Variable presentation; up to 25% asymptomatic (incidental finding); symptomatic patients may have nonproductive cough, dyspnea, chest pain, fatigue, weight loss; spontaneous pneumothorax occurs in ~10-20%

─ Imaging: Characteristic findings include bilateral, often symmetric, nodules and cysts, predominantly in the mid and upper lung zones, with sparing of costophrenic angles; cysts may be bizarrely shaped

─ Note: Smoking cessation is key to management and may lead to regression or stabilization in many cases

Cytology ─

(Usually diagnosed by BAL or transbronchial/surgical biopsy; FNA is less common)

─ Cells are Langerhans cells, which are large histiocytoid cells, often seen singly or in loose aggregates, admixed with other inflammatory cells, especially eosinophils and pigmented macrophages

─ Cytoplasm is abundant, pale, eosinophilic, and often finely granular or vacuolated; cell borders may be distinct or ill-defined

─ Nuclei show characteristic features: oval, elongated, or indented ("coffee-bean" shaped) with delicate nuclear membranes, fine, pale chromatin, and often prominent longitudinal grooves; nucleoli are usually inconspicuous

─ Background shows a mixed inflammatory infiltrate including eosinophils (can be numerous), lymphocytes, neutrophils, and pigmented alveolar macrophages (smoker's macrophages); cellular debris may be present; Charcot-Leyden crystals may be seen if eosinophils are abundant

─ Absence of overt malignant features; absence of granulomas (unlike sarcoidosis or infection); absence of extensive fibrosis in early lesions (though end-stage PLCH is fibrotic)

Ancillary studies ─

─ IHC (+): Langerhans cells are positive for CD1a (membranous), S100 protein (nuclear and cytoplasmic), and Langerin (CD207, cytoplasmic, highly specific)

─ IHC (-): CD68 may be positive but is nonspecific (stains macrophages as well)

─ Note: Ultrastructurally, Birbeck granules (tennis-racket shaped cytoplasmic organelles) are pathognomonic but EM is rarely performed for diagnosis

─ Mol: BRAF V600E mutations are found in ~40-50% of PLCH cases, suggesting a neoplastic component for at least a subset

DDx ─

─ Eosinophilic pneumonia (numerous eosinophils but lacks the characteristic Langerhans cells)

─ Reactive histiocytic proliferations (macrophages lack the specific nuclear features and IHC profile of Langerhans cells)

─ Infections (especially fungal or mycobacterial if granulomatous features are present, but LCH is not typically granulomatous)

─ Other interstitial lung diseases with eosinophilia

─ Poorly differentiated carcinoma or melanoma (if Langerhans cells are pleomorphic; IHC is crucial)

─ Lymphoma (especially Hodgkin lymphoma if eosinophils are prominent; RS cells have different morphology and IHC)

Prognosis ─

─ Variable; smoking cessation is the most important factor and leads to improvement or stabilization in many patients

─ Some progress to irreversible pulmonary fibrosis and respiratory failure

─ Risk of developing other smoking-related malignancies is increased

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Erdheim-Chester disease (ECD)

A rare, non-Langerhans cell histiocytosis characterized by systemic infiltration of lipid-laden (foamy) histiocytes, often involving long bones, retroperitoneum, cardiovascular system, skin, and CNS; pulmonary involvement occurs in a subset of patients

Clinical ─

─ Epid: Rare; typically affects adults, median age ~50-60 years; slight male predominance

─ Site: Multisystemic; bone pain (especially long bones of legs) is the most common presenting symptom; pulmonary involvement in ~20-50% of cases, can be symptomatic (dyspnea, cough) or an incidental finding

─ Clin: Highly variable depending on organs involved; can range from indolent to aggressive and life-threatening

─ Imaging: Pulmonary involvement shows interstitial thickening (interlobular septa, peribronchovascular), ground-glass opacities, centrilobular nodules, pleural thickening/effusion; symmetric osteosclerosis of long bones is characteristic

─ Note: Associated with BRAF V600E mutation in >50% of cases; other MAPK pathway mutations (NRAS, MAP2K1) also described

Cytology ─

(Diagnosis is usually made by tissue biopsy of an involved site, e g , bone, retroperitoneum; lung cytology is less common)

─ Cells are predominantly foamy histiocytes (lipid-laden macrophages), often numerous and appearing in sheets or singly; admixed with lymphocytes, plasma cells, and sometimes eosinophils; Touton-type giant cells (multinucleated histiocytes with a peripheral wreath of nuclei and central eosinophilic cytoplasm surrounded by foamy cytoplasm) may be present but are not specific

─ Cytoplasm is abundant, pale, and distinctly foamy or multivacuolated due to lipid accumulation

─ Nuclei show small, round to oval, often bland features, typically eccentrically placed; chromatin is fine; nucleoli are inconspicuous

─ Background shows often clean, may have lipid droplets from ruptured foamy cells; fibrosis may be present in tissue fragments if sampled from a sclerotic lesion

─ Absence of the characteristic nuclear grooves of Langerhans cells; absence of Birbeck granules; absence of overt malignant features

Ancillary studies ─

─ IHC (+): Histiocytes are positive for CD68, CD163; Factor XIIIa may be positive

─ IHC (-): CD1a, S100 protein (usually negative or only weakly/focally positive in a subset, helps distinguish from LCH), Langerin (CD207)

─ Mol: BRAF V600E mutation is common (>50%) and can be detected by PCR or IHC (VE1 antibody); testing is important for potential targeted therapy (e g , BRAF inhibitors like vemurafenib, dabrafenib)

DDx ─

─ Pulmonary Langerhans cell histiocytosis (Langerhans cells have grooved nuclei, CD1a/S100/Langerin positive, BRAF V600E can be present in both)

─ Xanthogranulomatous inflammation / Endogenous lipoid pneumonia (often associated with obstruction or infection, foamy macrophages but usually more mixed inflammation, lacks systemic features of ECD)

─ Storage diseases (e g , Gaucher, Niemann-Pick; very rare, specific enzyme deficiencies, different clinical context)

─ Malakoplakia (histiocytes contain Michaelis-Gutmann bodies, associated with bacterial infection, especially E coli)

─ Rosai-Dorfman disease (sinus histiocytosis with massive lymphadenopathy; histiocytes show emperipolesis, S100 positive, CD1a negative, different clinical presentation)

─ Metastatic clear cell renal cell carcinoma (epithelial cells, CK positive, PAX8 positive, CD10 positive, lipid may be present but cells are malignant)

Prognosis ─

─ Variable; formerly considered to have a poor prognosis, but outcomes have improved with targeted therapies (BRAF/MEK inhibitors) for BRAF-mutated cases and interferon-alpha or other immunomodulatory agents

─ Involvement of cardiovascular or central nervous systems is associated with worse prognosis

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Mesothelioma

A malignant neoplasm arising from mesothelial cells lining the pleura, peritoneum, pericardium, or tunica vaginalis; pleural mesothelioma is the most common form and is strongly associated with asbestos exposure. Cytologic diagnosis is often made on pleural fluid, but FNA of pleural-based masses can also be performed.

Clinical ─

─ Epid: Strong association with asbestos exposure (all types, especially crocidolite); long latency period (typically 20-40+ years)

─ Site: Pleura (most common, ~80%), peritoneum (~15-20%), pericardium (rare), tunica vaginalis (rare)

─ Clin: Often presents with dyspnea, non-pleuritic chest pain, weight loss; unilateral pleural effusion is common; imaging shows pleural thickening, effusion, or discrete pleural masses

─ Note: Histologically classified into epithelioid (most common, ~60-70%), sarcomatoid (~10-20%), and biphasic (mixed, ~10-20%) types; epithelioid and biphasic types are more likely to exfoliate cells into effusions or be sampled by FNA

Cytology ─

(Features primarily describe epithelioid mesothelioma in pleural fluid or FNA of pleural mass)

─ Cells are large, epithelioid, appearing singly, in loose clusters, or in characteristic large, cohesive, three-dimensional, knobby, morula-like clusters (often >12 cells); papillary structures may be seen; "cell-embracing" (one cell wrapping around another) and intercellular "windows" (slit-like spaces between cells) are common features

─ Cytoplasm is typically abundant, dense, and often has a two-tone appearance (dense endoplasm, paler ectoplasm or "lacy skirt"); may be eosinophilic, basophilic, or amphophilic; vacuolization can occur but usually does not indent the nucleus significantly (unlike adenocarcinoma signet rings)

─ Nuclei show round to oval features, often centrally located (but can be eccentric), with smooth to slightly irregular nuclear membranes; chromatin is vesicular or finely granular; nucleoli are usually prominent and can be large, but typically single and round

─ Background shows may be hemorrhagic or contain inflammatory cells; hyaluronic acid may be present (stains with Alcian blue at pH 2.5, PAS-D negative, and can sometimes be seen as a "wispy" or "feathery" precipitate or within cytoplasmic vacuoles)

─ Absence of extensive keratinization (unlike squamous cell carcinoma); absence of overt, large, single intracytoplasmic mucin vacuoles indenting the nucleus (unlike adenocarcinoma); absence of delicate "salt & pepper" chromatin (unlike neuroendocrine tumors)

Ancillary studies ─

─ Note: A panel of IHC stains is essential to distinguish mesothelioma from adenocarcinoma, its main differential. At least two mesothelial markers and two carcinoma markers are recommended.

─ IHC (+): Calretinin (nuclear and cytoplasmic), WT1 (nuclear), CK5/6 (cytoplasmic), D2-40 (podoplanin, membranous), EMA (epithelial membrane antigen, often thick membranous "rind-like" staining); BAP1 loss (nuclear staining absent in tumor cells, present in internal controls like lymphocytes/stroma) is a strong indicator of malignancy in mesothelial proliferations

─ IHC (-): CEA (monoclonal), MOC31, Ber-EP4, TTF-1, Napsin A, Claudin-4 (these are typically positive in adenocarcinoma)

─ Mol: Homozygous deletion of CDKN2A (p16) by FISH can support malignancy in equivocal cases; BAP1 gene mutations are common

DDx ─

─ Adenocarcinoma, metastatic to pleura or primary lung adenocarcinoma involving pleura (key differential; adenocarcinomas are typically calretinin, WT1, CK5/6, D2-40 negative, and positive for CEA, MOC31, Ber-EP4, Claudin-4; TTF-1/Napsin A positive for lung primary)

─ Reactive mesothelial hyperplasia (cells are usually less atypical, form smaller, flatter sheets, lack BAP1 loss or p16 deletion; can be very difficult to distinguish from well-differentiated mesothelioma on morphology alone, especially in limited samples; clinical context and imaging crucial)

─ Squamous cell carcinoma, poorly differentiated (p40/p63 positive, mesothelial markers negative)

─ Synovial sarcoma, epithelioid variant (rare in pleura, TLE1 positive, specific translocations)

Prognosis ─

─ Generally poor, especially for sarcomatoid and biphasic types; median survival for epithelioid mesothelioma is ~12-18 months

─ BAP1 loss may be associated with a somewhat better prognosis in some studies, but this is complex

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Paraganglioma

Rare neuroendocrine neoplasms arising from paraganglia of the autonomic nervous system, which are derived from neural crest cells. Thoracic paragangliomas are uncommon, usually located in the posterior mediastinum (aortosympathetic chain) or middle mediastinum (aortopulmonary paraganglia, visceral-autonomic paraganglia related to heart/great vessels).

Clinical ─

─ Site: Most common in head and neck (e g , carotid body, jugulotympanic, vagal); intrathoracic paragangliomas are rare, typically posterior or middle mediastinum; primary intrapulmonary paragangliomas are exceptionally rare

─ Clin: Usually benign, slow-growing masses; may be asymptomatic or cause symptoms due to mass effect; a subset (~1-3% overall, higher for some specific sites like organ of Zuckerkandl) are functional, secreting catecholamines (epinephrine, norepinephrine), leading to hypertension, palpitations, headache, sweating (pheochromocytoma-like syndrome)

─ Epid: Can occur at any age, peak in 4th-5th decades; familial syndromes (e g , mutations in SDHx genes, VHL, RET, NF1) account for ~30-40% of cases

─ Imaging: Typically well-circumscribed, highly vascular masses on CT/MRI; may show intense uptake on 123I-MIBG or 68Ga-DOTATATE PET/CT

Cytology ─

(FNA is often performed for mediastinal or other accessible masses)

─ Cells are polygonal to epithelioid, sometimes spindled or plasmacytoid, arranged in cohesive clusters (characteristic "Zellballen" or nested/alveolar pattern with cells grouped by a rich capillary network), trabeculae, or as dispersed single cells; cellularity can be moderate to high

─ Cytoplasm is typically moderate to abundant, finely granular (eosinophilic or amphophilic), often with ill-defined cell borders; red cytoplasmic granules may be seen with Giemsa ("NE granules"); clear or oncocytic change can occur

─ Nuclei show round to oval features, often with "salt-and-pepper" (stippled) neuroendocrine chromatin; nucleoli may be inconspicuous or small and distinct; significant anisonucleosis (variation in nuclear size) can be present even in benign lesions and is not a reliable sign of malignancy; intranuclear pseudoinclusions may be seen

─ Background shows often hemorrhagic due to high vascularity; fragments of delicate fibrovascular stroma (capillary network) are characteristic, often entwining cell clusters; sustentacular cells (spindle-shaped, S100-positive supporting cells) may be present at the periphery of Zellballen, but are often difficult to identify on cytology alone

─ Absence of extensive necrosis, high mitotic activity (mitoses are rare in benign cases), or definitive features of carcinomatous differentiation (e g , keratin pearls, true glands)

Ancillary studies ─

─ IHC (+): Chief cells (tumor cells) are positive for neuroendocrine markers: Synaptophysin (diffuse), Chromogranin A (often strong), CD56 (NCAM); Neuron-specific enolase (NSE); Sustentacular cells are positive for S100 protein and SOX10 (highlighting the periphery of Zellballen if present and sampled)

─ IHC (-): Chief cells are negative for Cytokeratins (AE1/AE3, CAM5.2 usually, though focal/dot-like positivity can occur, especially in FNA material or with broad-spectrum keratins), TTF-1, Napsin A, PAX8 (except some bladder/renal paragangliomas)

─ Note: Ki-67 index is typically low (<3%) in benign paragangliomas; higher rates may suggest aggressive potential but are not definitive for malignancy

─ Mol: Germline testing for SDHx, VHL, RET, NF1 mutations is recommended, especially in younger patients, multiple tumors, or family history

DDx ─

─ Carcinoid tumor (pulmonary or metastatic; also neuroendocrine, but typically more cohesive, less vascular stroma, often TTF-1 positive if pulmonary primary, lacks prominent sustentacular cells)

─ Medullary thyroid carcinoma, metastatic (calcitonin positive, CEA positive, may have amyloid, different clinical context)

─ Melanoma, metastatic (S100, SOX10, Melan-A/HMB45 positive, neuroendocrine markers negative, often prominent nucleoli, can have plasmacytoid/spindled cells)

─ Alveolar soft part sarcoma (rare, ASPSCR1-TFE3 fusion, characteristic PAS-D positive crystals, different IHC profile)

─ Neuroendocrine carcinoma, high-grade (e g , SCLC, LCNEC; show much greater atypia, mitoses, necrosis, higher Ki-67)

─ Hyalinizing trabecular tumor of thyroid, metastatic (rare, distinct nuclear features, thyroglobulin positive)

─ Clear cell renal cell carcinoma, metastatic (if clear cell change in paraganglioma; RCC is PAX8, CD10, RCCma positive)

Prognosis ─

─ Most paragangliomas are benign and cured by complete surgical excision

─ Malignancy is defined by the presence of metastases (to sites where paraganglionic tissue is normally absent, e g , lymph nodes, liver, bone, lung); occurs in ~5-15% of cases overall, higher for extra-adrenal abdominal and some familial tumors

─ Histologic features (e g , size, necrosis, vascular invasion, mitotic activity, Ki-67) can suggest increased risk of malignancy, but no single criterion is definitive; SDHB mutations are associated with higher risk of malignancy and metastases

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Thymoma

A neoplasm of thymic epithelial cells, typically arising in the anterior mediastinum, characterized by a variable admixture of neoplastic epithelial cells and non-neoplastic lymphocytes (thymocytes). Cytologic diagnosis is usually by FNA of a mediastinal mass.

Clinical ─

─ Epid: Most common primary tumor of the anterior mediastinum in adults (20-30% of mediastinal tumors); rare in children

─ Site: Anterior mediastinum (90%); rarely ectopic in neck, posterior mediastinum, or lung (primary pulmonary thymoma)

─ Clin: ~30-50% are asymptomatic (incidental finding); symptomatic patients may have chest pain, cough, dyspnea, superior vena cava syndrome; strongly associated with paraneoplastic syndromes, especially myasthenia gravis (MG, in ~30-50% of thymoma patients), pure red cell aplasia, hypogammaglobulinemia (Good syndrome)

─ Imaging: Typically a well-circumscribed, lobulated anterior mediastinal mass; calcification or cystic change may be present

─ WHO Classification: Based on epithelial cell morphology and lymphocyte-to-epithelial cell ratio (Type A, AB, B1, B2, B3, and rare other types including thymic carcinoma); this classification has prognostic and clinical relevance but is difficult to apply precisely on cytology alone

Cytology ─

─ Cells are a characteristic dual population of (1) neoplastic epithelial cells and (2) non-neoplastic lymphocytes (immature T-lymphocytes/thymocytes); the proportion of these two components varies greatly depending on the thymoma subtype and sampling

─ Epithelial cells: Can be spindle-shaped (Type A, AB), polygonal/epithelioid with bland, oval nuclei and inconspicuous nucleoli (Type B1, B2), or more atypical with prominent nucleoli and irregular contours (Type B3, thymic carcinoma); often seen in cohesive clusters or singly

─ Lymphocytes: Typically small, mature-appearing lymphocytes with scant cytoplasm, round nuclei, and condensed chromatin; may be very numerous, obscuring epithelial cells (especially in Type B1 and B2, "lymphocyte-rich" thymomas) or sparse (Type A, B3, "lymphocyte-poor" thymomas)

─ Cytoplasm of epithelial cells: Scant to moderate, pale, eosinophilic, or clear; spindle cells have wispy cytoplasm; epithelioid cells have more defined cytoplasm

─ Nuclei of epithelial cells:

─ Type A: Bland, oval to spindle, fine chromatin, inconspicuous nucleoli

─ Type AB: Mix of Type A areas and lymphocyte-rich areas

─ Type B1: Resemble normal thymus; epithelial cells are scattered, bland, with vesicular nuclei and small nucleoli, among predominant lymphocytes

─ Type B2: Epithelial cells more numerous, in clusters, round to polygonal, vesicular nuclei, distinct nucleoli

─ Type B3: Epithelial cells predominate, sheets of polygonal cells with mild to moderate atypia, vesicular nuclei, prominent nucleoli

─ Thymic Carcinoma: Overtly malignant epithelial cells, marked pleomorphism, large irregular nuclei, coarse chromatin, macronucleoli (see separate entry if needed)

─ Background shows often clean or bloody; cystic fluid may be aspirated if cystic degeneration is present; Hassall's corpuscles are rare on cytology but specific if seen

─ Absence of significant keratinization (unlike SCC), true glandular formation (unlike adenocarcinoma), or Reed-Sternberg cells (unlike Hodgkin lymphoma)

Ancillary studies ─

─ IHC (+): Epithelial cells are positive for cytokeratins (AE1/AE3, CAM5.2, CK5/6, CK7, CK19), p63, p40; Lymphocytes (thymocytes) are T-cells, positive for TdT (terminal deoxynucleotidyl transferase), CD3, CD1a, CD4, CD8 (often double positive)

─ IHC (-): Epithelial cells are negative for CD45 (LCA), neuroendocrine markers (usually), S100 (usually), CD20; Lymphocytes are negative for B-cell markers (CD20, PAX5 usually)

─ Note: Flow cytometry can confirm the immature T-cell phenotype of the lymphocytes (TdT+, CD4+/CD8+ double positive, expression of CD1a)

DDx ─

─ Lymphoma (especially T-lymphoblastic lymphoma, Hodgkin lymphoma): T-LBL has blasts with fine chromatin, high N:C, TdT+, but usually more atypia and lacks the cohesive epithelial clusters of thymoma; Hodgkin lymphoma has Reed-Sternberg cells and a different inflammatory background, epithelial cells are absent

─ Germ cell tumor (e g , seminoma): Seminoma cells are large, dispersed, with clear cytoplasm, prominent nucleoli, tigroid background, PLAP/OCT3/4/SALL4 positive; epithelial cells of thymoma are CK positive

─ Metastatic carcinoma: Clinical history, IHC for site-specific markers (e g , TTF-1 for lung, GATA3 for breast)

─ Carcinoid tumor/Neuroendocrine carcinoma of thymus: Neuroendocrine markers positive, different morphology

─ Benign thymic hyperplasia/Normal thymus (especially in children/young adults): Similar dual population, but often more lobulated architecture histologically, less distinct mass on imaging; cytology may be indistinguishable from Type B1 thymoma

Prognosis ─

─ Generally good for encapsulated, non-invasive thymomas (WHO Types A, AB, B1, B2), especially after complete surgical resection

─ Type B3 thymoma has a higher risk of recurrence and aggressive behavior

─ Thymic carcinomas have a much poorer prognosis

─ Presence of myasthenia gravis does not significantly impact overall survival related to the thymoma itself

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Salivary gland-type carcinomas (primary in lung/mediastinum)

Rare primary tumors of the lung or mediastinum that histologically resemble salivary gland carcinomas, such as adenoid cystic carcinoma and mucoepidermoid carcinoma. They are thought to arise from submucosal bronchial glands or minor salivary gland rests.

Adenoid Cystic Carcinoma (ACC)

Clinical ─

─ Rare primary lung/tracheobronchial tumor (<0.2% of lung cancers); more common in trachea and main bronchi than peripherally

─ Clin: Often slow-growing but relentlessly infiltrative with a high propensity for perineural invasion; presents with cough, dyspnea, hemoptysis, or post-obstructive symptoms; long-term prognosis is poor despite slow growth, due to local recurrences and late distant metastases

Cytology ─

─ Cells are small, basaloid, with scant cytoplasm, hyperchromatic nuclei, and inconspicuous nucleoli; arranged in characteristic three-dimensional spherical or branching ("glove-finger") clusters surrounding acellular, hyaline, metachromatic (magenta on Giemsa, pale blue/gray on Pap) globules or cylinders of basement membrane material

─ Cytoplasm is scant, poorly defined

─ Nuclei show round to oval, relatively uniform features with dark, finely granular chromatin; molding may be seen; nucleoli are typically inconspicuous

─ Background shows often clean, but may have some of the hyaline globules dispersed; necrosis is uncommon unless high-grade transformation

─ Absence of significant pleomorphism, keratinization, or glandular mucin

Ancillary studies ─

─ IHC (+): CK7, CAM5.2, p63, p40 (basal/myoepithelial cells), c-Kit (CD117), SOX10; MYB overexpression is common due to MYB-NFIB fusion

─ IHC (-): TTF-1, Napsin A, neuroendocrine markers

─ Mol: MYB-NFIB fusion gene is characteristic

DDx ─

─ Small cell carcinoma (more atypia, molding, crush, neuroendocrine markers+, MYB-)

─ Basaloid squamous cell carcinoma (p40/p63+, but lacks hyaline globules and MYB fusion)

─ Carcinoid tumor (organoid, salt-and-pepper chromatin, neuroendocrine markers+)

─ Pleomorphic adenoma (if basaloid with stromal spheres; PA has fibrillary chondromyxoid stroma, more myoepithelial cell heterogeneity)

Prognosis ─

─ Indolent but relentless course; high rates of local recurrence and late distant metastases (lung, bone, brain); 5-year survival ~60-80%, but 10-15 year survival is much lower

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Adenosquamous carcinoma

A carcinoma showing components of both squamous cell carcinoma (SCC) and adenocarcinoma (ADC), with each component constituting at least 10% of the tumor. It is considered a subtype of non-small cell lung carcinoma (NSCLC).

Clinical ─

─ Rare, accounts for ~0.4-4% of NSCLCs

─ Strong association with smoking, similar to SCC and ADC

─ Site: Can be central or peripheral

─ Clin: Symptoms similar to other NSCLCs (cough, dyspnea, hemoptysis, chest pain)

─ Imaging: May appear as a cavitating mass (like SCC) or a peripheral nodule/mass (like ADC)

Cytology ─

─ Cells are a mixture of two distinct malignant populations:

1. Squamous component: Cells with features of SCC (keratinization, dense cytoplasm, intercellular bridges if visible, bizarre shapes, hyperchromatic, irregular nuclei)

2. Glandular component: Cells with features of ADC (gland formation, acini, papillae, mucin production, vesicular nuclei, prominent nucleoli)

─ Cytoplasm is dimorphic: dense/keratinized in SCC component, vacuolated/mucinous in ADC component

─ Nuclei show features of both SCC (hyperchromatic, irregular, pyknotic) and ADC (vesicular, prominent nucleoli)

─ Background shows may have features of both (keratinous debris and/or mucin, tumor diathesis)

─ Absence of a pure population of either SCC or ADC; both components must be clearly identifiable and malignant

Ancillary studies ─

─ Note: Diagnosis often relies on identifying both components morphologically. IHC can support if one component is poorly differentiated.

─ IHC (+): Squamous component: p40, p63, CK5/6; Glandular component: TTF-1, Napsin A, CK7, CEA, mucin stains

─ IHC (-): Markers for one component should be negative in the other (e g , TTF-1 negative in pure SCC component)

─ Mol: Molecular profile can be heterogeneous, sometimes showing alterations typical of ADC (e g , EGFR, KRAS) or less commonly SCC; some studies suggest EGFR mutations are more frequent than in pure SCC

DDx ─

─ Squamous cell carcinoma with reactive glandular changes or entrapped benign glands (glandular component is benign)

─ Adenocarcinoma with squamous metaplasia (squamous component is metaplastic, not overtly malignant; p40/p63 may be positive in metaplastic cells but atypia is less)

─ Mucoepidermoid carcinoma (another biphasic tumor with squamous and mucinous/glandular cells, but also has intermediate cells, often arises endobronchially, and may have MAML2 rearrangement)

─ Collision tumor (rare, two separate primary tumors colliding; components are usually more geographically distinct)

─ Poorly differentiated NSCLC-NOS (if both components are poorly differentiated and specific features are hard to discern without IHC)

Prognosis ─

─ Generally considered to have a poor prognosis, possibly worse than pure ADC or SCC of similar stage, but data are somewhat conflicting and depend on the proportion and grade of each component

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Pleomorphic carcinoma

A poorly differentiated non-small cell lung carcinoma (NSCLC), usually squamous cell carcinoma, adenocarcinoma, or large cell carcinoma, that contains at least 10% spindle cells and/or giant cells. It is a subtype of sarcomatoid carcinoma of the lung.

Clinical ─

─ Rare, accounts for ~0.1-0.4% of all lung malignancies

─ Strong association with heavy smoking; predominantly affects elderly males

─ Site: Typically peripheral, often large and cavitating

─ Clin: Aggressive clinical course, often presents with advanced stage, symptoms of chest pain, cough, dyspnea, hemoptysis; rapid growth and early metastasis are common

─ Imaging: Large peripheral mass, often with necrosis and cavitation; may invade chest wall or mediastinum

Cytology ─

─ Cells are highly pleomorphic, showing a mixture of (1) recognizable NSCLC component (squamous, adeno, or large cell) and (2) a component of malignant spindle cells and/or bizarre, multinucleated or mononuclear giant cells; the spindle/giant cell component must be at least 10%

─ Cytoplasm is variable: epithelial component may show keratinization or mucin; spindle cells have elongated, wispy cytoplasm; giant cells have abundant, often dense or vacuolated cytoplasm

─ Nuclei show extreme pleomorphism, anisonucleosis, hyperchromasia, irregular chromatin, and prominent, often irregular nucleoli in both epithelial and spindle/giant cell components; "monster" cells are common

─ Background shows often necrotic (tumor diathesis), hemorrhagic, and may contain inflammatory cells

─ Absence of a pure sarcoma (an epithelial component must be present or demonstrable by IHC); absence of features of small cell carcinoma or typical carcinoid

Ancillary studies ─

─ IHC (+): Epithelial component: Cytokeratins (AE1/AE3, CAM5.2), EMA; specific markers depending on NSCLC type (e g , p40 for squamous, TTF-1 for adeno); Spindle/giant cell component: Often positive for Vimentin, may co-express cytokeratins (confirming carcinomatous nature), but can be CK negative in some areas

─ IHC (-): Spindle/giant cell component is usually negative for specific sarcoma markers (e g , Desmin, Myogenin, S100, CD34, unless divergent differentiation occurs, which is rare) and lymphoid/melanoma markers

─ Mol: KRAS mutations are common; EGFR/ALK alterations are rare; PD-L1 expression can be high in some cases

DDx ─

─ Metastatic sarcoma with entrapped benign epithelium (sarcoma cells are CK negative, clinical history of sarcoma elsewhere)

─ Metastatic pleomorphic carcinoma from another site (e g , pancreas, thyroid anaplastic; clinical history and site-specific markers if any)

─ Melanoma, pleomorphic/spindle cell type (S100, SOX10, Melan-A/HMB45 positive)

─ True primary pulmonary sarcoma (very rare, lacks epithelial component, specific sarcoma markers may be positive)

─ Poorly differentiated NSCLC without overt spindle/giant cells (distinction can be subtle if spindle/giant cells are focal or poorly sampled)

─ Reactive atypia with bizarre stromal cells (e g , post-radiation/chemotherapy, organizing pneumonia; clinical context, less diffuse atypia in stromal cells)

Prognosis ─

─ Very poor prognosis, one of the most aggressive subtypes of lung cancer; median survival is often <1 year, even with treatment

─ High rates of local recurrence and distant metastasis

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Pulmonary blastoma

A rare, aggressive biphasic malignant tumor of the lung, composed of a primitive mesenchymal stroma and malignant epithelial (glandular) components resembling fetal lung. It is a subtype of sarcomatoid carcinoma.

Clinical ─

─ Rare, <0.5% of all lung malignancies

─ Classic (biphasic) pulmonary blastoma typically occurs in adults (mean age ~40 years), slight male predominance, often associated with smoking

─ Well-differentiated fetal adenocarcinoma (monophasic, purely epithelial variant) occurs in younger patients, not smoking-related, and has a better prognosis

─ Site: Usually a large, well-circumscribed peripheral mass; can be central

─ Clin: Symptoms include cough, hemoptysis, chest pain, dyspnea; can be asymptomatic

─ Imaging: Large, solitary, often lobulated mass; may show cystic change or necrosis

Cytology ─

─ Cells are a characteristic biphasic population:

1. Epithelial component: Cohesive clusters, tubules, or glands of cuboidal to columnar cells, often with subnuclear and supranuclear vacuoles containing glycogen (resembling fetal lung tubules or endometrioid glands); morules (solid nests of cells with bland, eosinophilic cytoplasm and indistinct borders) may be present

2. Mesenchymal (stromal) component: Dispersed or loosely clustered small, round to oval, primitive-appearing cells with scant cytoplasm and hyperchromatic nuclei, resembling the blastemal component of Wilms tumor or other embryonal sarcomas; spindle cell areas may also be present; chondroid or rhabdomyoblastic differentiation is rare

─ Cytoplasm of epithelial cells is moderate, clear to eosinophilic, often vacuolated; stromal cells have scant, poorly defined cytoplasm

─ Nuclei of epithelial cells are relatively uniform, round to oval, with fine chromatin and small nucleoli; stromal cells have hyperchromatic, molded nuclei with coarse chromatin

─ Background shows often bloody; necrosis may be present

─ Absence of mature cartilage or extensive fibromyxoid stroma (unlike hamartoma); absence of overt keratinization (unlike adenosquamous or SCC)

Ancillary studies ─

─ IHC (+): Epithelial component: Cytokeratins (AE1/AE3, CAM5.2, CK7), EMA, TTF-1 (often), CEA; Mesenchymal (stromal) component: Vimentin, occasionally Desmin, Myogenin (if rhabdomyoblastic), S100 (if chondroid); both components may show nuclear beta-catenin in well-differentiated fetal adenocarcinoma variant and some classic blastomas

─ IHC (-): Epithelial component: Usually negative for neuroendocrine markers; Stromal component: Usually negative for cytokeratins (though focal positivity reported)

─ Mol: DICER1 mutations are common in classic pulmonary blastomas, especially in children and young adults; CTNNB1 (beta-catenin) mutations are characteristic of the well-differentiated fetal adenocarcinoma variant

DDx ─

─ Carcinosarcoma (another biphasic tumor, but both epithelial and mesenchymal components are overtly malignant and usually high-grade, often resembling conventional NSCLC and a specific sarcoma type, unlike the primitive appearance in blastoma)

─ Adenocarcinoma with sarcomatoid change (pleomorphic carcinoma; epithelial component is usually typical NSCLC, spindle/giant cell component is high-grade, lacks primitive blastemal cells)

─ Synovial sarcoma, biphasic (rare in lung, epithelial component CK7/EMA positive, spindle cells TLE1 positive, characteristic SS18 gene rearrangement)

─ Small cell lung carcinoma (SCLC) with NSCLC component (SCLC component has typical neuroendocrine features, neuroendocrine markers positive)

─ Metastatic Wilms tumor or other embryonal sarcomas (clinical history crucial, site-specific markers if any)

─ Pulmonary hamartoma (if blastoma has chondroid stroma; hamartoma has mature cartilage and fibromyxoid stroma, lacks malignant epithelial and primitive stromal cells)

Prognosis ─

─ Poor for classic (biphasic) pulmonary blastoma, with 5-year survival ~15-50%; frequent recurrence and metastasis (brain, liver, bone)

─ Well-differentiated fetal adenocarcinoma (monophasic epithelial variant) has a significantly better prognosis

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Carcinosarcoma

A rare, aggressive malignant neoplasm composed of an intimate admixture of a carcinomatous component (non-small cell lung carcinoma, usually squamous cell, adenocarcinoma, or large cell) and a sarcomatous component (malignant spindle and/or giant cells, or showing specific heterologous mesenchymal differentiation like chondrosarcoma, osteosarcoma, or rhabdomyosarcoma). It is a subtype of sarcomatoid carcinoma of the lung.

Clinical ─

─ Rare, <1% of lung cancers

─ Strong association with smoking; predominantly affects elderly males (6th-7th decades)

─ Site: Usually large peripheral masses, but can be central/endobronchial

─ Clin: Aggressive clinical course, often presents with symptoms of cough, dyspnea, chest pain, hemoptysis, weight loss; frequently advanced stage at diagnosis

─ Imaging: Large, often heterogeneous mass, may show necrosis or cavitation; can invade chest wall

Cytology ─

─ Cells are a biphasic malignant population:

1. Carcinomatous component: Clusters or single cells with features of squamous cell carcinoma (keratinization, dense cytoplasm), adenocarcinoma (gland formation, mucin), or large cell undifferentiated carcinoma

2. Sarcomatous component: Malignant spindle cells (often in fascicles or storiform pattern) and/or bizarre, pleomorphic multinucleated or mononuclear giant cells; heterologous elements (e g , malignant cartilage, bone, or muscle cells) are less commonly seen on cytology but are diagnostic if present

─ Cytoplasm is dimorphic, corresponding to the epithelial and sarcomatous elements

─ Nuclei show high-grade malignant features in both components (pleomorphism, hyperchromasia, irregular chromatin, prominent/irregular nucleoli)

─ Background shows often necrotic (tumor diathesis), hemorrhagic

─ Absence of primitive blastemal cells (unlike pulmonary blastoma); the sarcomatous component is typically high-grade and overtly malignant, not just reactive stroma

Ancillary studies ─

─ IHC (+): Carcinomatous component: Cytokeratins (AE1/AE3, CAM5.2), EMA; specific markers depending on NSCLC type (p40 for squamous, TTF-1 for adeno); Sarcomatous component: Vimentin (usually strong); may co-express cytokeratins (supporting sarcomatoid carcinoma over true sarcoma); specific mesenchymal markers if heterologous differentiation is present (e g , Desmin/Myogenin for rhabdomyosarcoma, S100 for chondrosarcoma)

─ IHC (-): Sarcomatous component is usually negative for TTF-1, Napsin A (unless it's an adenocarcinoma with sarcomatoid change where spindle cells are derived from it)

─ Mol: KRAS mutations are relatively common; EGFR/ALK alterations are rare; TP53 mutations are frequent

DDx ─

─ Pleomorphic carcinoma (carcinosarcoma is a type of pleomorphic carcinoma where the sarcomatous component shows specific mesenchymal differentiation or is a fibrosarcoma/MFH-like sarcoma; if only undifferentiated spindle/giant cells, it's pleomorphic carcinoma NOS)

─ Spindle cell (sarcomatoid) squamous cell carcinoma or adenocarcinoma (if sarcomatoid component is dominant and epithelial component is subtle or not sampled)

─ Primary pulmonary sarcoma (very rare, lacks epithelial component; diagnosis of exclusion)

─ Metastatic sarcoma or carcinosarcoma (clinical history crucial)

─ Melanoma, pleomorphic/spindle cell (S100, SOX10, Melan-A/HMB45 positive)

─ Mesothelioma, sarcomatoid or biphasic (pleural-based, specific mesothelial IHC markers positive in epithelioid component if present)

Prognosis ─

─ Very poor, similar to or worse than pleomorphic carcinoma; highly aggressive with early metastasis and poor response to conventional chemotherapy/radiation

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NUT carcinoma

A rare, highly aggressive, poorly differentiated carcinoma defined by the presence of a nuclear protein in testis (NUTM1) gene rearrangement, most commonly a BRD4-NUTM1 fusion. It typically arises in midline structures of the head, neck, and thorax (mediastinum, lung).

Aka ─ NUT midline carcinoma (NMC)

Clinical ─

─ Epid: Rare; affects all ages, from infants to older adults, with a peak in adolescents and young adults; no clear sex predilection

─ Site: Midline structures; ~50% in head and neck (sinonasal, nasopharynx, salivary glands), ~50% in thorax (mediastinum, thymus, lung); rarely other sites

─ Clin: Extremely aggressive, rapidly growing tumors; often presents with locally advanced or metastatic disease; symptoms depend on location (e g , airway obstruction, pain, mass effect); median survival is typically <1 year despite aggressive multimodality therapy

─ Imaging: Often large, infiltrative masses with necrosis

Cytology ─

─ Cells are predominantly dispersed or in loose, poorly cohesive clusters or sheets; may show focal abrupt squamous differentiation (small foci of keratinization or intercellular bridges) in a subset of cases, but often appears undifferentiated

─ Cytoplasm is scant to moderate, often pale, basophilic, or clear/vacuolated, imparting a "fried egg" appearance in some cells

─ Nuclei show monomorphic, round to oval features with vesicular or finely granular chromatin; nucleoli can be prominent, often single and eosinophilic; mitotic figures are usually numerous, and apoptosis is common

─ Background shows often necrotic, with karyorrhectic debris; a neutrophilic infiltrate (rather than lymphocytic) is a characteristic, though not constant, finding

─ Absence of definitive glandular differentiation, overt keratinization in most of the tumor (if present, it's focal and abrupt), or neuroendocrine features

Ancillary studies ─

─ IHC (+): NUT (nuclear staining, using a specific monoclonal antibody to the NUTM1 protein, is pathognomonic, often showing a characteristic speckled pattern); Cytokeratins (AE1/AE3, CAM5.2, CK5/6, CK7 may be positive, often patchy); p63 and/or p40 (often positive, especially in cases with squamous differentiation); CD34 (focal); CEA (focal)

─ IHC (-): TTF-1 (usually, though a subset of pulmonary NUT carcinomas may stain), Napsin A, S100 protein, Melanoma markers, Lymphoid markers, Neuroendocrine markers (Chromogranin, Synaptophysin), CDX2, PAX8

─ Mol: Demonstration of NUTM1 gene rearrangement by FISH (break-apart probes) or RT-PCR (detecting specific fusions like BRD4-NUTM1, BRD3-NUTM1, or NSD3-NUTM1) is diagnostic

DDx ─

─ Poorly differentiated squamous cell carcinoma (especially basaloid or non-keratinizing types; SCC is typically NUT negative, though p63/p40 can be positive in both)

─ Small cell lung carcinoma (SCLC has finer "salt & pepper" chromatin, nuclear molding, scantier cytoplasm, neuroendocrine markers positive, NUT negative, TTF-1 often strongly positive)

─ Poorly differentiated adenocarcinoma (TTF-1/Napsin A positive, NUT negative)

─ Germ cell tumor (e g , seminoma, embryonal carcinoma; OCT3/4, SALL4, PLAP positive, specific morphology, NUT negative)

─ Lymphoma (especially large cell or lymphoblastic; CD45 positive, specific lymphoid markers, NUT negative)

─ Ewing sarcoma/PNET (CD99 positive, FLI1 positive, EWSR1 rearrangement, NUT negative, CK often negative)

─ Thymic carcinoma (can be poorly differentiated, but different IHC profile, NUT negative)

─ Metastatic poorly differentiated carcinoma from other sites

Prognosis ─

─ Extremely poor; highly aggressive with rapid progression and resistance to conventional chemotherapy; median survival is approximately 6-9 months

─ Some responses seen with investigational drugs targeting BET bromodomains (e g , birabresib, molibresib) or HDAC inhibitors

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Thoracic SMARCA4-deficient undifferentiated tumour

A recently recognized, highly aggressive, undifferentiated malignant neoplasm occurring primarily in the thorax (lung, pleura, mediastinum) of adult smokers, characterized by loss of SMARCA4 (BRG1) protein expression, a core subunit of the SWI/SNF chromatin remodeling complex.

Aka ─ SMARCA4-deficient thoracic sarcoma (older term, but now recognized as often having at least focal epithelial differentiation or being purely carcinomatous)

Clinical ─

─ Epid: Predominantly affects adult male smokers (median age ~50-60 years)

─ Site: Thorax – lung, pleura, mediastinum; can present as large masses

─ Clin: Very aggressive, often presents with advanced-stage disease, dyspnea, chest pain, cough; rapid clinical deterioration is common

─ Imaging: Large, often heterogeneous masses, may show necrosis; pleural effusion can be present

Cytology ─

─ Cells are typically undifferentiated, appearing in dyscohesive sheets, loose clusters, or as numerous single cells; may show a rhabdoid or plasmacytoid morphology

─ Cytoplasm is moderate to abundant, eosinophilic or amphophilic, sometimes with eccentric nuclei and paranuclear eosinophilic inclusions (rhabdoid feature)

─ Nuclei show large, round to oval, often vesicular features with irregular nuclear contours; chromatin is often open or coarsely clumped; nucleoli are usually prominent and can be large and irregular; mitotic figures are frequent

─ Background shows often necrotic and hemorrhagic

─ Absence of clear squamous, glandular, or neuroendocrine differentiation by morphology (though focal keratin positivity by IHC is common)

Ancillary studies ─

─ IHC (+): Cytokeratins (AE1/AE3, CAM5.2, often patchy or focal), Vimentin (often); CD34 (can be positive in a subset); Claudin-4 (often positive, helping distinguish from true sarcomas); SOX2 (often positive)

─ IHC (-): Complete loss of nuclear SMARCA4 (BRG1) expression is definitional (internal positive control in stromal/inflammatory cells is essential); SMARCB1 (INI1/SNF5) expression is usually intact (helps distinguish from SMARCB1-deficient tumors like epithelioid sarcoma or atypical teratoid/rhabdoid tumor); TTF-1, Napsin A, p40, S100 protein, Melanoma markers, Lymphoid markers, Neuroendocrine markers are typically negative; HepPar-1 has been reported as positive in a subset, potentially causing confusion with metastatic HCC if not correlated with other findings

─ Mol: Inactivating mutations or deletions of SMARCA4 gene

DDx ─

─ Poorly differentiated non-small cell carcinoma (adenocarcinoma, squamous cell carcinoma, large cell carcinoma; these are SMARCA4-proficient and usually express more definitive lineage markers like TTF-1 or p40)

─ Malignant mesothelioma, epithelioid or pleomorphic (mesothelial markers like calretinin, WT1 positive; SMARCA4 loss is rare but reported in mesothelioma)

─ Metastatic carcinoma, poorly differentiated (clinical history, IHC for site-specific markers)

─ Melanoma (S100, SOX10, Melan-A/HMB45 positive)

─ Lymphoma, large cell (CD45 positive, lymphoid markers)

─ True sarcoma (e g , epithelioid sarcoma which is SMARCB1-deficient, synovial sarcoma; specific sarcoma markers, CK may be focal but SMARCA4 intact)

─ NUT carcinoma (NUT IHC positive, SMARCA4 intact)

Prognosis ─

─ Extremely poor, highly aggressive with rapid progression and early metastasis; median survival is very short (typically around 4-7 months)

─ Poor response to conventional chemotherapy and radiotherapy; potential for targeted therapies based on SWI/SNF pathway vulnerabilities is under investigation

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Spindle cell tumors

This is a broad category encompassing various benign and malignant neoplasms characterized by a predominant proliferation of spindle-shaped cells. In the thoracic cavity (lung, pleura, mediastinum), this group includes tumors of mesenchymal origin as well as sarcomatoid carcinomas. Accurate diagnosis often requires immunohistochemistry and sometimes molecular studies to differentiate among entities with overlapping morphologic features.

Solitary Fibrous Tumor (SFT)

A mesenchymal neoplasm of fibroblastic/myofibroblastic type, most commonly arising from the pleura, but can occur in virtually any anatomical site, including intrapulmonary or mediastinal locations. Most SFTs are benign, but a subset can behave aggressively.

Clinical ─

─ Epid: Adults, wide age range (peak 5th-7th decades); no sex predilection

─ Site: Pleura (visceral > parietal, ~80% of thoracic SFTs); less commonly intrapulmonary, mediastinal, pericardial, or chest wall

─ Clin: Often asymptomatic and discovered incidentally; symptomatic cases may present with cough, chest pain, dyspnea; large tumors can cause compressive symptoms; paraneoplastic syndromes like hypoglycemia (Doege-Potter syndrome, due to IGF2 production) occur in <5%, usually with large tumors

─ Imaging: Typically a well-circumscribed, often lobulated, solitary mass that may be pedunculated (especially pleural SFTs); can be very large; avidly enhances with contrast

Cytology ─

(FNA or exfoliative cytology of pleural fluid if tumor cells shed, which is uncommon)

─ Cells are predominantly bland spindle cells, appearing singly, in loose clusters, or in more cohesive fascicular or storiform fragments; cellularity can be variable, often moderate

─ Cytoplasm is scant to moderate, pale, wispy, with ill-defined cell borders; may appear bipolar

─ Nuclei show oval to elongated, often tapered features with smooth contours; chromatin is typically fine and evenly distributed; nucleoli are usually inconspicuous; mild nuclear atypia may be seen, but significant pleomorphism is uncommon in conventional SFT

─ Background shows often contains characteristic dense, ropey, or hyalinized collagenous stromal fragments (may appear as "ropy collagen" or "amianthoid fibers"); a prominent feature is the presence of numerous, often branching, thin-walled "staghorn" or hemangiopericytoma-like blood vessels, which may be seen within or intimately associated with cell clusters; myxoid change can be present in some tumors

─ Absence of overt epithelial features (glands, keratinization), significant necrosis, or high mitotic activity (in most benign/conventional SFTs)

Ancillary studies ─

─ IHC (+): STAT6 (nuclear, highly sensitive and specific, reflects NAB2-STAT6 gene fusion), CD34 (strong and diffuse in most cases, but can be lost in malignant SFTs), CD99, BCL2; Vimentin

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2), S100 protein, Desmin, SMA (smooth muscle actin), EMA, Calretinin, WT1, TTF-1

─ Mol: NAB2-STAT6 gene fusion resulting from intrachromosomal inversion inv(12)(q13q13) is the defining molecular alteration, present in virtually all SFTs

DDx ─

─ Schwannoma (S100, SOX10 positive; STAT6, CD34 negative; Antoni A/B areas, Verocay bodies)

─ Leiomyoma/Leiomyosarcoma (Desmin, SMA positive; STAT6 negative)

─ Synovial sarcoma, monophasic spindle cell (TLE1 positive; CK7, EMA may be focally positive; specific SS18 gene rearrangement; STAT6 negative)

─ Spindle cell (sarcomatoid) carcinoma (Cytokeratins positive, often p40/p63 if squamous component; STAT6 negative)

─ Sarcomatoid mesothelioma (pleural-based, may have history of asbestos exposure; mesothelial markers like calretinin/WT1 may be positive in epithelioid areas if biphasic, but often lost in pure sarcomatoid; BAP1 loss common; STAT6 negative)

─ Fibrosarcoma (diagnosis of exclusion, STAT6 negative, CD34 often negative)

─ Malignant peripheral nerve sheath tumor (MPNST) (often associated with NF1, S100 variable/lost, SOX10 often positive, more atypia/mitoses, STAT6 negative)

─ Desmoplastic mesothelioma (pleural, bland spindle cells in dense collagen, but infiltrative, mesothelial markers may be focally positive)

Prognosis ─

─ Most SFTs are benign and cured by complete surgical excision

─ ~10-20% are malignant or behave aggressively (malignant SFT), defined by features such as large size (>10-15 cm), high cellularity, nuclear pleomorphism, >4 mitoses/10 HPF, tumor necrosis, and infiltrative margins

─ Risk stratification models (e g , Demicco score for soft tissue SFTs) can help predict metastatic risk

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Primary germ cell tumours of the mediastinum

A heterogeneous group of neoplasms derived from primordial germ cells that aberrantly migrate to or arise in the mediastinum, most commonly the anterior compartment. They parallel gonadal germ cell tumors in histology and include teratoma (mature, immature), seminoma, embryonal carcinoma, yolk sac tumor, choriocarcinoma, and mixed forms.

Clinical ─

─ Rare, account for ~10-15% of anterior mediastinal masses in adults, but are the most common anterior mediastinal tumor in children and young adults

─ Epid: Predominantly affect adolescents and young adults (peak 20-35 years); strong male predominance for malignant GCTs (seminoma, non-seminomatous GCTs); teratomas occur equally in males and females

─ Site: Anterior mediastinum is the most common extragonadal site

─ Clin: Symptoms vary; ~50% asymptomatic (incidental finding); can cause chest pain, cough, dyspnea, superior vena cava syndrome; non-seminomatous GCTs may produce serum tumor markers (AFP for yolk sac tumor, hCG for choriocarcinoma/some embryonal carcinomas); seminomas may produce hCG (syncytiotrophoblasts) or LDH

─ Association: Mediastinal non-seminomatous GCTs in males are frequently associated with Klinefelter syndrome (47,XXY) and hematologic malignancies (especially acute myeloid leukemia)

Cytology ─

(FNA of anterior mediastinal mass; features depend on specific GCT type)

─ Teratoma, Mature:

─ Cells are a mixture of elements from all three germ layers: mature squamous cells (keratinizing, anucleate), glandular cells (respiratory, intestinal, mucinous), fragments of cartilage, adipose tissue, neural tissue (glial cells, ganglion cells), mesenchymal spindle cells

─ Cytoplasm varies with cell type

─ Nuclei are generally bland, consistent with mature differentiated tissues

─ Background shows often contains keratinous debris, mucin, amorphous material; inflammatory cells may be present if ruptured

─ Absence of immature elements or overtly malignant cells (in pure mature teratoma)

─ Seminoma:

─ Cells are large, polygonal, predominantly dispersed or in loose, poorly cohesive clusters; a "tigroid" background (periodic acid-Schiff [PAS]-positive, glycogen-rich cytoplasmic fragments) is characteristic but not always prominent on cytology

─ Cytoplasm is moderate to abundant, clear or pale eosinophilic, fragile, often with distinct cell borders

─ Nuclei show large, round to oval, centrally located features with vesicular chromatin and one or more prominent, often angular or "squared-off," macronucleoli

─ Background shows often contains a variable number of mature lymphocytes (T-cells) intimately admixed with tumor cells; granulomatous inflammation (sarcoid-like granulomas) may be present; necrosis is uncommon unless large tumor

─ Absence of overt glandular/squamous differentiation or high-grade pleomorphism (unlike embryonal ca or choriocarcinoma)

─ Embryonal Carcinoma:

─ Cells are large, highly pleomorphic, epithelioid, appearing in cohesive clusters, sheets, or singly; may form abortive glandular or papillary structures

─ Cytoplasm is moderate, amphophilic or basophilic, often vacuolated

─ Nuclei show large, irregular features with coarse, clumped chromatin and prominent, often multiple and irregular, macronucleoli; high N:C ratio; frequent mitoses, including atypical forms

─ Background shows often necrotic, hemorrhagic

─ Yolk Sac Tumor (Endodermal Sinus Tumor):

─ Cells are variable, from bland cuboidal/flattened cells lining microcysts or papillae to more pleomorphic cells; Schiller-Duval bodies (glomeruloid structures with a central vessel lined by tumor cells) are pathognomonic but rarely seen on cytology

─ Cytoplasm is moderate, clear to eosinophilic, may contain hyaline globules (PAS-D positive, AFP positive)

─ Nuclei show vesicular chromatin, prominent nucleoli

─ Background shows often myxoid or bloody; hyaline globules may be extracellular

─ Choriocarcinoma:

─ Cells are a biphasic population of malignant syncytiotrophoblasts (large, multinucleated giant cells with dense eosinophilic/amphophilic cytoplasm, multiple pleomorphic/hyperchromatic nuclei) and cytotrophoblasts (smaller, mononuclear cells with clear cytoplasm, round nuclei, distinct nucleoli); both components must be present for diagnosis

─ Background shows extensive hemorrhage and necrosis are characteristic

Ancillary studies ─

─ Teratoma: IHC not usually needed if mature elements from all three germ layers are identified.

─ Seminoma:

─ IHC (+): OCT3/4 (nuclear), SALL4 (nuclear), PLAP (placental alkaline phosphatase, membranous), c-Kit (CD117, membranous), D2-40 (podoplanin, variable)

─ IHC (-): Cytokeratins (AE1/AE3 usually negative or only focal dot-like), EMA, CD30, AFP, hCG (unless syncytiotrophoblasts present), SOX2

─ Embryonal Carcinoma:

─ IHC (+): Cytokeratins (AE1/AE3, CAM5.2), OCT3/4 (nuclear), SALL4 (nuclear), CD30 (membranous/Golgi), SOX2 (nuclear)

─ IHC (-): c-Kit (usually), PLAP (often weaker/patchier than seminoma), AFP (unless yolk sac component), hCG (unless syncytiotrophoblasts present)

─ Yolk Sac Tumor:

─ IHC (+): AFP (alpha-fetoprotein, cytoplasmic), Glypican-3, SALL4 (nuclear), Cytokeratins (AE1/AE3, CAM5.2)

─ IHC (-): OCT3/4, PLAP, CD30, hCG

─ Choriocarcinoma:

─ IHC (+): hCG (strong in syncytiotrophoblasts), Cytokeratins (AE1/AE3, CAM5.2); GATA3 may be positive in trophoblastic cells

─ IHC (-): AFP, PLAP, OCT3/4

─ Serum Markers: AFP (elevated in yolk sac tumor, some embryonal ca/mixed GCTs), hCG (elevated in choriocarcinoma, some seminomas with syncytiotrophoblasts, some embryonal ca), LDH (often elevated in seminoma and other GCTs)

DDx ─

(Depends on specific GCT type suspected)

─ Teratoma: Metastatic carcinoma with diverse differentiation (e g , adenosquamous), thymoma with cystic change (dual population of epithelial cells and lymphocytes)

─ Seminoma: Lymphoma (especially DLBCL, anaplastic large cell lymphoma; lymphoid markers CD45, CD20, CD30 positive, GCT markers negative), thymoma (lymphocyte-rich; epithelial cells CK/p63 positive), poorly differentiated carcinoma (CK positive, GCT markers negative)

─ Embryonal Ca: Poorly differentiated carcinoma/adenocarcinoma (CK positive, but OCT3/4, SALL4 negative; TTF-1 may be positive in lung adeno), thymic carcinoma (different CK profile, CD5, c-Kit often positive), melanoma (S100/melanoma markers positive)

─ Yolk Sac Tumor: Clear cell adenocarcinoma (lung primary or metastatic; TTF-1 or site-specific markers positive, AFP negative), mesothelioma with clear cell change (mesothelial markers positive)

─ Choriocarcinoma: Poorly differentiated carcinoma with bizarre giant cells (hCG negative), metastatic choriocarcinoma (clinical history of gonadal or gestational primary)

Prognosis ─

─ Mature teratoma: Benign, excellent prognosis after complete excision

─ Immature teratoma: Prognosis depends on grade (amount of immature neuroepithelium) and presence of other malignant GCT components

─ Seminoma: Highly radiosensitive and chemosensitive; excellent prognosis even with metastatic disease (>90% cure rate)

─ Non-seminomatous GCTs (embryonal ca, yolk sac, choriocarcinoma, mixed): More aggressive than seminoma, but modern cisplatin-based chemotherapy has significantly improved outcomes; prognosis depends on stage, tumor markers, and specific components

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Primary angiosarcoma of the lung

An exceedingly rare, highly aggressive malignant vascular neoplasm arising from endothelial cells of pulmonary blood vessels. Most angiosarcomas involving the lung are metastatic.

Clinical ─

─ Extremely rare as a primary lung tumor

─ Can occur at any age, but often in adults; may be associated with prior radiation or foreign bodies

─ Site: Can be solitary or multifocal/diffuse within the lung parenchyma; may involve pleura

─ Clin: Aggressive course; presents with hemoptysis, dyspnea, chest pain, anemia; diffuse forms (pulmonary angiosarcomatosis) can mimic diffuse alveolar damage or hemorrhage

─ Imaging: Variable; solitary or multiple nodules/masses, diffuse infiltrates, ground-glass opacities, pleural effusion; often hemorrhagic

Cytology ─

(BAL, FNA, or pleural fluid cytology)

─ Cells are epithelioid, spindle-shaped, or pleomorphic, appearing singly or in loose, poorly cohesive clusters; may line or be associated with vascular channels if tissue fragments are obtained; "floret-like" or pseudo-acinar arrangements can be seen

─ Cytoplasm is scant to moderate, eosinophilic or amphophilic, sometimes vacuolated (intracytoplasmic lumina, which may contain erythrocytes); hemosiderin pigment may be present

─ Nuclei show marked atypia, pleomorphism, hyperchromasia, and irregular nuclear contours; nucleoli are often prominent and can be large and irregular; mitotic figures, including atypical forms, may be frequent

─ Background shows often extensively hemorrhagic; necrotic debris may be present; hemosiderin-laden macrophages

─ Absence of definitive keratinization, glandular mucin, or neuroendocrine features (unless a combined tumor, which is exceptionally rare for primary angiosarcoma)

Ancillary studies ─

─ IHC (+): Vascular endothelial markers: CD31 (membranous, most sensitive and specific), ERG (nuclear, highly sensitive and specific), Factor VIII-related antigen (von Willebrand factor, cytoplasmic), CD34 (variable, less specific), FLI1 (nuclear)

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2 usually, though epithelioid angiosarcomas can rarely show focal CK positivity, creating a diagnostic pitfall), TTF-1, Napsin A, p40, S100 protein, Melanoma markers, Lymphoid markers, Calretinin

─ Mol: MYC amplification is common in radiation-induced and chronic lymphedema-associated angiosarcomas, but less so in primary sporadic angiosarcomas; KDR mutations or FLT4 amplifications reported in some cases

DDx ─

─ Poorly differentiated carcinoma (especially adenocarcinoma or squamous cell carcinoma with epithelioid or spindle features; CK positive, vascular markers negative)

─ Melanoma, epithelioid or spindle cell (S100, SOX10, Melan-A/HMB45 positive, vascular markers negative)

─ Sarcomatoid mesothelioma (pleural-based, may have epithelioid/spindle cells, mesothelial markers may be focally positive, BAP1 loss)

─ Other sarcomas with epithelioid or spindle features (e g , epithelioid sarcoma which is CK positive and INI1 lost, synovial sarcoma which is TLE1 positive; specific sarcoma markers and molecular genetics crucial)

─ Organizing pneumonia/diffuse alveolar damage with reactive endothelial atypia (clinical context, less severe atypia, no true invasion if tissue fragments seen)

─ Kaposi sarcoma (HHV8 positive, often in immunocompromised patients, different morphology with slit-like vascular spaces and extravasated RBCs)

─ Epithelioid hemangioendothelioma (another vascular tumor, but generally lower grade, cells often embedded in myxohyaline stroma, WWTR1-CAMTA1 or YAP1-TFE3 fusions)

Prognosis ─

─ Extremely poor; highly aggressive with rapid local invasion and early distant metastasis (often to other parts of lung, liver, bone, brain)

─ Median survival is typically very short, measured in months, even with multimodality treatment

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Main Document

Serous Fluids

Table of Contents

Introuction ─ General Concepts

Types of cells

Benign Mesothelial cells

Reactive mesothelial cells

Histiocytes (Macrophages)

Introuction ─ General Concepts

Types of cells

Serous effusions can contain a variety of cells, including mesothelial cells (benign, reactive, or malignant), inflammatory cells (neutrophils, lymphocytes, eosinophils, plasma cells), macrophages (histiocytes), and, in cases of malignancy, tumor cells metastatic to the serous cavity or arising from it (mesothelioma). Other elements like blood, fibrin, and sometimes specific non-cellular materials (e g , psammoma bodies, Curschmann's spirals) can also be present.

Benign Mesothelial cells

Mesothelial cells are the normal lining cells of serous cavities (pleura, peritoneum, pericardium). In effusions, benign mesothelial cells are commonly seen and represent exfoliated native lining cells, which may or may not show reactive changes.

Clinical ─

─ Presence of benign mesothelial cells is a normal finding in serous fluid samples

─ They can increase in number and show reactive changes in response to various stimuli (inflammation, infection, irritation, systemic disease, malignancy elsewhere not directly involving the serosa)

Cytology ─

─ Cells are usually seen singly or in small, flat, monolayered sheets or loose clusters; "windows" or clear spaces between adjacent cells are characteristic, formed by their long microvilli

─ Cytoplasm is moderate to abundant, pale, dense, or finely vacuolated, often with a "two-tone" appearance (denser endoplasm, paler ectoplasm or "lacy skirt"); cell borders are generally well-defined

─ Nuclei show round to oval features, typically centrally located (but can be eccentric), with smooth nuclear membranes; chromatin is finely granular and evenly distributed; nucleoli are usually small and inconspicuous, but can become more prominent in reactive states

─ Background shows often clean or may contain a sparse population of inflammatory cells

─ Absence of significant atypia, large complex three-dimensional clusters, or features of malignancy

Ancillary studies ─

─ IHC (+): Calretinin (nuclear and cytoplasmic), WT1 (nuclear, especially in peritoneal and pleural mesothelial cells), CK5/6 (cytoplasmic), Podoplanin (D2-40, membranous), EMA (epithelial membrane antigen, often membranous but can be cytoplasmic; reactive cells may be weaker or patchy compared to mesothelioma), Desmin (often positive in reactive mesothelial cells)

─ IHC (-): Carcinoma markers such as CEA (monoclonal), MOC31, Ber-EP4, TTF-1, Napsin A, PAX8 (except in some Mullerian inclusions or Walthard nests which can be PAX8+)

DDx ─

─ Reactive mesothelial cells (show more pronounced atypia, see below)

─ Malignant mesothelioma (shows overt malignant features, see specific entry)

─ Adenocarcinoma (forms true glands, expresses carcinoma markers, negative for most mesothelial markers)

─ Histiocytes/Macrophages (kidney-bean shaped nuclei, foamy cytoplasm, CD68/CD163 positive, negative for keratins and most mesothelial markers)

Prognosis ─

─ Finding only benign mesothelial cells is consistent with a benign effusion or an effusion not directly involved by malignancy; the underlying cause of the effusion dictates prognosis

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Reactive mesothelial cells

Mesothelial cells that exhibit cytologic atypia due to irritation, inflammation, or other stimuli, but lack definitive features of malignancy. Distinguishing florid reactive changes from well-differentiated malignant mesothelioma can be challenging.

Clinical ─

─ Seen in response to a wide variety of conditions including infections (pneumonia, peritonitis), inflammation (e g , pancreatitis, collagen vascular diseases like SLE or rheumatoid arthritis), pulmonary embolism/infarction, cardiac failure, cirrhosis, uremia, post-surgical or post-radiation changes

─ The presence of reactive mesothelial cells is nonspecific and indicates an underlying pathological process causing the effusion

Cytology ─

─ Cells are often more numerous than benign resting mesothelial cells, appearing singly, in sheets, or in small to moderately sized clusters, which may sometimes appear three-dimensional or papillary-like; "windows" between cells are usually maintained

─ Cytoplasm is often abundant, dense, eosinophilic or basophilic, and may show vacuolization (fine or large, single or multiple vacuoles); "lacy skirt" ectoplasm may be prominent

─ Nuclei show mild to moderate atypia, including nuclear enlargement (often 2-3x normal), variation in nuclear size (anisonucleosis), binucleation or multinucleation (common); nuclear membranes are usually smooth but can be slightly irregular; chromatin is typically finely granular and evenly distributed, though it can appear more open or slightly coarsened; nucleoli are often prominent, single or multiple, and round (macronucleoli can be seen but are usually smooth)

─ Background shows may contain inflammatory cells (neutrophils, lymphocytes, eosinophils), fibrin, or evidence of the underlying cause (e g , LE cells in lupus, rheumatoid nodules/debris in rheumatoid arthritis)

─ Absence of overt malignant criteria such as very large, complex, berry-like clusters with deep nuclear crowding, significant nuclear membrane irregularity, very coarse chromatin, or bizarre mitotic figures; BAP1 nuclear expression is retained

Ancillary studies ─

─ IHC (+): Similar to benign mesothelial cells: Calretinin, WT1, CK5/6, Podoplanin (D2-40), EMA (often patchy or weaker than in mesothelioma), Desmin; Ki-67 proliferation index is typically low (<5-10%)

─ IHC (-): Carcinoma markers (CEA, MOC31, Ber-EP4, TTF-1, Napsin A, PAX8); BAP1 expression is retained (loss of nuclear BAP1 is a marker for mesothelioma)

─ Note: Ancillary tests are primarily used to exclude adenocarcinoma or confirm mesothelial origin if atypia is marked, rather than to definitively distinguish reactive from neoplastic mesothelial cells based on IHC alone (BAP1 loss and CDKN2A/p16 FISH are exceptions for diagnosing mesothelioma)

DDx ─

─ Malignant mesothelioma, epithelioid, well-differentiated (can be very difficult; mesothelioma often shows larger, more complex clusters, more significant atypia, higher Ki-67, and may show BAP1 loss or p16 deletion by FISH)

─ Adenocarcinoma (true gland formation, mucin, positive carcinoma markers, negative for most mesothelial markers)

─ Viral cytopathic effect (e g , herpes, CMV can cause enlarged, atypical cells, but specific viral inclusions should be sought)

Prognosis ─

─ Depends on the underlying cause of the reactive changes and the effusion

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Histiocytes (Macrophages)

Phagocytic cells of monocytic lineage commonly found in serous effusions, playing a role in inflammation and cleanup of debris. Their appearance can vary depending on what they have phagocytosed.

Clinical ─

─ Presence is common and usually nonspecific

─ Increased numbers seen in various inflammatory conditions, infections (especially chronic, e g , TB), hemorrhage, or in association with malignancy (tumor-associated macrophages)

─ Can be the predominant cell type in some conditions (e g , resolving inflammation, some granulomatous diseases)

Cytology ─

─ Cells are typically seen singly, but can form loose aggregates or occasionally small clusters, especially if epithelioid

─ Cytoplasm is usually abundant, pale, grey-blue (Giemsa) or amphophilic (Pap), often finely vacuolated (foamy) or granular; may contain phagocytosed material such as red blood cells (erythrophagocytosis), hemosiderin (golden-brown pigment), lipids, cellular debris, or microorganisms; cell borders are often indistinct

─ Nuclei show typically eccentric placement, often kidney-bean shaped (reniform) or oval, with smooth nuclear contours; chromatin is usually fine and vesicular; nucleoli are generally inconspicuous or small; multinucleated forms (giant cells) can be seen, especially in granulomatous inflammation or foreign body reactions

─ Background shows varies depending on the cause of the effusion; may be clean, inflammatory, or hemorrhagic

─ Absence of epithelial features (cohesive clusters with distinct cell junctions, keratinization, true gland formation); absence of mesothelial features ("windows", two-tone cytoplasm, calretinin/WT1 positivity)

Ancillary studies ─

─ IHC (+): CD68 (cytoplasmic, granular), CD163 (more specific for M2 macrophages), Lysozyme

─ IHC (-): Cytokeratins (AE1/AE3, CAM5.2), Calretinin, WT1, TTF-1, Napsin A, S100 protein (except Langerhans cells or some other histiocytic disorders)

DDx ─

─ Mesothelial cells (especially reactive or degenerated ones with vacuolated cytoplasm; mesothelial cells are CK, calretinin, WT1 positive)

─ Adenocarcinoma, signet ring cell or clear cell type (malignant nuclear features, mucin positive, specific carcinoma markers positive)

─ Melanoma (if pigment is present; melanoma cells are S100/SOX10/melanocytic markers positive)

─ Langerhans cell histiocytosis (Langerhans cells have characteristic grooved nuclei, S100/CD1a/Langerin positive)

─ Malignant lymphoma, large cell types (malignant lymphoid features, CD45 and specific lymphoid markers positive)

Prognosis ─

─ Presence of histiocytes itself is not prognostic; prognosis depends on the underlying condition causing their accumulation

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Table of Contents