Surgical Pathology Criteria

Type B3 Thymoma

Definition

Alternate / Historical Names

Diagnostic Criteria

Supplemental Studies

Clinical

Differential Diagnosis

Type A Thymoma Type B3 Thymoma
Predominantly spindle cells Predominantly polygonal cells
Lacks any epidermoid features Frequently has epidermoid differentiation (but lacks overt keratinization and bridges)
Frequent glandular structures lined by cuboidal to flat cells Frequent perivascular spaces surrounded by palisaded epithelial cells
Lacks cytologic atypia Mild to moderate cytologic atypia
Epithelial cells may be CD20 positive Epithelial cells CD20 negative
Epithelial cells EMA and CD5 negative Epithelial cells may be focally positive for EMA or CD5
Prominent reticulin within epithelial areas Lacks prominent reticulin around epithelial cells

Type B2 Thymoma Type B3 Thymoma
Both lymphocytes and epithelial cells are easy to see Predominantly epithelial with few lymphocytes
Lacks any epidermoid features Frequently has epidermoid differentiation (but lacks overt keratinization and bridges)
Epithelial CD5 negative Epithelial cells may be focally positive for CD5
Epithelial cell nuclei are large round vesicular with prominent nucleoli

Epithelial cell nuclei moderate in size, not typically large and vesicular


Type B3 Thymoma Thymic Low Grade Squamous Carcinoma
Cytologic atypia usually mild, moderate at worst Cytologically atypical
Epithelial cell mitotic figures usually <2/10hpf Epithelial cell mitotic figures may be frequent
Lymphocytes few, but with immature T phenotype Lymphocytes are mature T and B cells
Overt squamous differentiation usually scant and focal Overtly squamous, with intercellular bridges usually identifiable
Lobular growth pattern Infiltrative growth pattern
Frequent perivascular spaces surrounded by palisaded epithelial cells Lacks perivascular spaces and palisading
Epithelial cells CD117 negative to rare Epithelial cells CD117 65-80% positive
Epithelial cells CD5 negative to at most focally positive Epithelial cells CD5 70% positive

Bibliography

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