The atypical Type A thymoma is an uncommon tumor.
Type A features:
(1) focal or diffuse bland, spindle-shaped epithelial cells (cytokeratin positive)
(2) polygonal epithelial cells which can be CD20 positive
(3) no or few immature T cells (Tdt positive)
Atypical features:
(1) comedo-type tumor necrosis in polygonal cell areas
(2) increased mitotic count (> 4 per 2 square mm)
(3) nuclear crowding
The diagnosis of a thymic carcinoma should be excluded.
Type A thymomas may show a missense mutation in GTF2I gene at 7q11.23 (c.74146970T>A).
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Specialty: Hematology Oncology
