Surgical Pathology Criteria

Type A 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 A Thymoma Solitary Fibrous Tumor
Plump spindle cells with appreciable cytoplasm Thin spindle cells without appreciable cytoplasm on H&E stain
Keratin positive, CD34 negative CD34 positive, keratin negative
Some immature T cells usually present Any lymphocytes present are mature T cells
Both may have hemangiopericytoma like vasculature

Type A Thymoma Thymic Spindle Cell Carcinoma
Mild cytologic atypia at worst Cytologically atypical
Epithelial cell mitotic figures rare Epithelial cell mitotic figures may be frequent
Some immature T cells usually present Lymphocytes are mature T and B cells
Usually well circumscribed, with only microscopic invasion Usually clearly invasive

Type A Thymoma, Adenomatoid/Reticular Variant Mediastinal Yolk Sac Tumor
Lacks hyaline globules and Schiller-Duval bodies Frequent hyaline globules and Schiller-Duval bodies
Mitotic figures infrequent or absent Mitotic figures may be frequent
No necrosis or hemmorhage Frequent necrosis and hemmorhage
Serum AFP negative Serum AFP positive (immunohistochemistry is not reliable)
SALL4 and glypican 3 not tested SALL4 and glypican 3 positive

Bibliography

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