Primary synovial epithelioid sarcoma of the knee: distinctly unusual location leading to its confusion with pigmented villonodular synovitis.

Epithelioid sarcoma (ES) is a rare malignant soft tissue tumor occurring in the distal extremities of young adults and is characterized histologically by nodules of epithelioid cells showing central necrosis. Intra-articular ES is extremely rare; only four cases have been reported, but their radiologic and histologic documentation of intra-articular origin have been imprecise. We report the first radiologically and histologically well-documented case of primary synovial ES. A 59-year-old woman presented with pain followed by swelling of her right knee for 6 months. MRI revealed an entirely intra-articular nodular synovial mass in the lateral part of the right knee joint in a background of diffusely thickened synovium. Synovectomy was performed under the clinical impression of pigmented villonodular synovitis (PVNS), a diagnosis erroneously confirmed by the reporting pathologist. The tumor rapidly recurred 3 months afterward and the diagnosis of primary synovial ES was made. Despite above-knee amputation, the tumor continued to spread proximally to the retroperitoneum. She developed multiple lung metastases and died 20 months after initial presentation. The nodular aggregates of tumor cells with central necrosis resulted in diffuse polypoid synovial thickening mimicking tuberculous synovitis and PVNS. The tumor cells showed positive staining for EMA, CK19, CD34, and complete loss of INI1 staining, establishing the diagnosis of primary synovial ES. The ES spread from the synovium to and along the joint capsule, and then extra-articularly into the soft tissue surrounding the knee joint, with lymphovascular permeation. Such pattern of spread calls for radical surgical excision as the treatment of choice.