Intraarticular synovial sarcoma.

A 43-year-old man presented with decreased range of motion in his left knee and a painful medial joint mass that was grossly visible. Arthroscopy demonstrated a mobile, flat mass 3 cm in diameter in the knee joint that seemed to be loosely tethered to the synovium. The mass was excised, and light microscopic examination demonstrated a biphasic synovial sarcoma. There was no transition with the attached normal synovium. Immunohistochemically, the epithelial component was intensely positive for epithelial membrane antigen and cytokeratins (CAM 5.2 and AE 1/AE 3), and the spindle cell component was focally positive for these markers. The patient has no evidence of disease 9 years after only local excision. Although the term synovial sarcoma suggests a relationship to normal synovium, only rarely has truly intraarticular disease been reported.