Abstract

Solitary fibrous tumor (SFT), initially described by Klemperer and Rabin in 1931, is typically a benign mesenchymal tumor. A small percentage of cases (10% to 15%) may recur. Rarely (%) they have been reported to metastasize. Malignancy developing within subcutaneous SFT’s, as presented here, is exceptional.

An 80-year-old male presented with a large thigh mass of unknown duration. Histopathological examination revealed a large subcutaneous neoplasm composed of both round and spindled cells arranged in fascicles, accompanied by extensive areas of necrosis and numerous atypical mitotic figures. With immunohistochemistry, the neoplastic cells were positive for CD34, BCL2 and STAT-6. S-100, desmin, EMA, CK, SMA, CD117, CAM 5.2, and D2-40 were negative. INI-1 was retained.

Distinguishing malignant SFT from other spindle cell lesions in the extremities poses a diagnostic challenge. The differential diagnosis includes neurogenic tumors, fibrosarcoma, synovial sarcoma, and dermatofibrosarcoma protuberans (DFSP). The most helpful feature is the presence of residual recognizable pre-existing benign SFT. Because CD34 expression is commonly seen in SFTs as well as DFSP, STAT6 appears to be the most useful supportive marker for the diagnosis.

Malignant SFTs are extremely rare, even more so in the skin. We present this case to alert dermatopathologists to it’s morphology and unique immunohistochemical profile.