Answer: Thymoma, Spindle cell type

Histology: This is a spindle cell predominant lesion which has a hemangiopericytomatous vasculature. The lesion has a fibrous capsule surrounding it, but sends small microscopic extensions into the capsule and into the adjacent thymic tissue. This would qualify as minimal invasion in the thymoma, which has essentially no prognostic significance. The lesional cells are fairly bland and have indistinct cytoplasm. There are small foci where the cells become more polygonal and have more abundant eosinophilic cytoplasm. There is a sprinkling of immature lymphocytes throughout the lesion. The lesional cells show strong immunoreactivity for cytokeratin and p63, while the lymphocytes label for CD99, supporting the diagnosis of thymoma.

Discussion: A synovial sarcoma could show a similar bland spindle cell morphology and hemangiopericytomatous vasculature; however, it should not label diffusely for cytokeratin, and rarely labels for p63. In addition, synovial sarcoma should show a higher mitotic rate than the current lesion. Ewing sarcoma suggested by presence of rosette like structures within lesion; however, this cytology of the cells is wrong for Ewing sarcoma in that the cells are more spindled and have open chromatin, as opposed to the primitive small round blue cell appearance of a Ewing sarcoma. Solitary fibrous tumor would show more variation in cellularity with greater collagen deposition, and would label for CD34 and not for cytokeratin.