Surgical Pathology Criteria

Type AB Thymoma

Definition

Alternate / Historical Names

Diagnostic Criteria

Supplemental Studies

Clinical

Differential Diagnosis

Type AB Thymoma Micronodular Thymoma with Lymphoid Stroma
Lymphocyte rich areas contain scattered epithelial cells Lymphocyte rich areas lack epithelial cells
Lymphocyte rich areas composed predominantly of T cells Lymphocyte rich areas contain many B cells, frequently with germinal centers

Type AB Thymoma Type B2 Thymoma
Epithelial cells have small oval to spindled nuclei with dispersed chromatin and inconspicuous nucleoli Epithelial cells have vesicular nuclei with prominent nucleoli
Areas of Type A are usually present Type A areas not present
Both may be composed of an essentially even mixture of epithelial cells and lymphocytes

Type AB 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
Lymphocyte poor areas of Type AB may be confused with Type B3

Bibliography

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