Pathologic examination of the mediastinal mass biopsy. (A) The tumor shows proliferation of oval cells separated by hemangiopericytomatous vessels (H&E X40). (B) Scattered positivity in tumor cells for pancytokeratin (MNF) can be seen (X20). (C) Cytoplasmic positivity for Bcl-2 is appreciated (X40). (D) Fli-1 nuclear stain is seen in this case (X40). CD99 was completely negative (not shown). (E) Abnormal cells hybridized with LSI SS18 (18q11.2) Dual Color, Break Apart Rearrangement Probe. The cells in this image show one fusion, one orange, and one green signal pattern indicative of a rearrangement of one copy of the SS18 gene region. These findings were consistent with the diagnosis of synovial sarcoma.