Answer: Thymoma, WHO type AB

Histology: see below

Discussion: The tumor is lobulated with dense fibrous bands separating
cellular areas and appears to be encapsulated. No invasion into fibroadipose tissue is noted. The tumor exhibits predominantly type B areas with small round epithelial cells that are intermixed with small lymphocytes. A small cystic component with an intraluminal papillary and microcystic epithelial proliferation is present. The epithelial cells in these areas are spindled and lack pleomorphism.

Similar spindled epithelial cells are also noted focally in the surrounding thymoma. No Hassal’s corpuscles are noted. Immunostains demonstrate that the neoplastic epithelial cells (both spindle and epithelioid) are pancytokeratin positive, and CD5, CD10 and CD30 negative. The background CD3 positive lymphocytes show variable expression and CD5, CD10 and are negative for CD30. Ki67 stain shows low proliferative activity in the epithelial component. The neoplastic epithelial cells are focally EMA positive and CD4, CD8, TdT and CD1a negative. The majority of the background lymphocytes show variable expression of CD4, CD8 and TdT. Stains for WT-1 and calretinin are negative. This staining pattern supports the diagnosis. The tumor appears to be encapsulated but it was received as multiple fragments and therefore invasion cannot be evaluated.
Microcystic/papillary projections with or without cystic change can be seen in association with type AB thymomas. The cells lining the glandular structures have fairly dense chromatin and are cytologically similar to the cells of typical type A thymoma. These structures also may be confused with benign mesothelial proliferations. The type B areas of a type AB thymoma may exhibit a mixture of type B patterns and there typically no Hassals’s corpuscles in the background. The intermixed lymphoid cells benign T-cells.