Intra-articular epithelioid sarcoma of the knee: a diagnostic challenge.

Epithelioid sarcoma is a rare mesenchymal neoplasm characterized by aggregates of epithelioid cells. Intra-articular occurrence is exceedingly rare with only few reports described in the literature. A 22 year-old man presented a progressive mechanical knee pain. Initially, the investigation revealed a non-infectious unspecific synovitis. The patient gradually presented increasing knee enlargement and functional impairment. Intra-articular nodular proliferation with bone invasion was later observed on magnetic resonance imaging reevaluation. Pigmented villonodular synovitis hypothesis was considered. The biopsy ultimately revealed nodules of epithelioid cells with an immunoprofile compatible with epithelioid sarcoma diagnosis. The patient underwent neoadjuvant chemotherapy and radiotherapy and an above-knee amputation was performed.