Epithelioid Monophasic Synovial Sarcoma

A 47-year-old man presented with a soft tissue mass of the distal right thigh near the knee. The tumor was highly vascular with epithelioid tumor cells growing in a peritheliomatous pattern, suggesting a soft tissue glomus tumor. Yet many tumor cells contained hemosiderin pigment and formed papillary structures suggestive of pigmented villonodular synovitis. Tumor cells were cytologically bland, and there was minimal mitotic activity. The tumor cells were strongly immunoreactive for cytokeratin, however, and contained true desmosomes, gland lumina, microvilli, tonofilaments, and well-developed basal lamina. These findings plus the absence perinuclear aggregates of intermediate filaments rule out malignant rhabdoid tumor and epithelioid sarcoma. Also, magnetic resonance imaging revealed no other lesions to suggest metastatic carcinoma. Thus this tumor appears to be a predominantly epithelioid form of monophasic synovial sarcoma. Recognition of this variant of synovial sarcoma is important for prognostication and therapeutic decision making because some studies indicate that this variant of synovial sarcoma follows a relatively benign clinical course.