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A case of monophasic synovial sarcoma of the prostate in a 37-year-old man is reported. Histologically, the tumor was chiefly composed of uniform spindle and oval cells, which often formed interlacing fascicles resembling those of fibrosarcoma. In some areas, the compact fascicles of tumor cells alternated with hypocellular myxoid tissue bearing a superficial resemblance to peripheral nerve sheath tumors, whereas small portions of the tumor showed a pericytomatous pattern consisting of polygonal cells arranged around dilated, thin-walled blood vessels. By immunohistochemistry, vimentin was detected in most cells, and a focal reactivity for epithelial membrane antigen was also observed. The tumor cells, however, were negative for keratin, S-100 protein, neuron-specific enolase, CD34, desmin, muscle-specific actin, and alpha-smooth muscle actin. Cytogenetic analysis and fluorescence in situ hybridization (FISH) using the cultured tumor cells demonstrated a translocation t(X;18)(p11.2;q11.2), an aberration specific for synovial sarcoma. To the authors' knowledge, this is the first report of a primary prostatic synovial sarcoma confirmed by cytogenetic analysis.

To elucidate the precise origin and characteristics of the proliferating cells in malignant fibrous histiocytoma (MFH), the authors analyzed 33 MFH tumors, using immunohistochemical techniques with a panel of 12 antibodies. All three types of MFH cells (spindle cells, polygonal cells, and bizarre giant cells) stained positively for mesenchymal antigens (FU3 and vimentin) but did not stain for macrophage/histiocyte markers (HAM 56 and CD68). Therefore, the MFH cells may not represent true histiocytes, although they may be mesenchymal-derived cells behaving as "facultative histiocytes" with superficial resemblance to actual histiocytes. Normal histiocytes in the stroma tested positive for macrophage/histiocyte antigens; the most common cells were HAM 56-positive cells constituting 30-80% of nonneoplastic stromal cells, followed by those positive for CD68 (10-50%), Mac 387 (less than 2%), and S-100 protein (less than 1%). Our results indicate the presence of heterogeneity of "histiocytic" cells in MFH. Proliferating-cell nuclear antigen (PCNA) was expressed not only in the spindle and polygonal MFH cells but also in the bizarre giant cells. These findings suggest that all three types of MFH cells participate in the proliferative compartment of MFH. Uneven PCNA staining of the irregular nuclear segments of the bizarre giant cells may result in abnormal DNA synthesis, possibly contributing to the marked diversity of nuclear morphology in MFH. Touton-type and osteoclast-like giant cells did not stain for PCNA but stained positively for histiocytic markers. Therefore, these giant cells may lack proliferative activity and probably result from normal histiocytes fusing together.