Myxoid synovial sarcoma: an underappreciated morphologic subset.

Focal myxoid change is a well-recognized feature of synovial sarcoma, but the presence of a predominantly myxoid stroma is rare. We describe seven cases of myxoid synovial sarcoma in which marked myxoid change initially obscured the diagnosis, leading to confusion (principally with malignant peripheral nerve sheath tumor). The median age (20 yr) and tumor location (four lower extremity, two upper extremity, and one head and neck region) were similar to those found in typical synovial sarcoma. Histologically, five cases were monophasic spindle cell lesions with a lacy appearance in areas with a prominent Alcian blue-positive myxoid stroma. Each case had foci with more typical features of synovial sarcoma, including a fascicular growth pattern with a variably collagenized stroma, stromal mast cells, a hemangiopericytoma-like vascular pattern, and calcification. Two cases showed small foci of glandular (biphasic) differentiation. Immunohistochemically, all of the seven cases were positive for epithelial membrane antigen, four of six were positive for pan-keratin, three of six were positive for S-100, two of four were positive for CD99, and six of six were negative for desmin. Clinical follow-up in six cases ranged from 8 to 48 months (median, 21 mo). Local recurrence developed in three patients at 9, 20, and 24 months, respectively. In one of these three patients, lung metastases developed at 13 months, and the patient died of disseminated disease at 23 months. In another of the three patients, lung metastases developed at 27 months. Three patients had no evidence of disease at 8, 15, and 15 months. Our data are too limited to indicate any clinical differences between myxoid synovial sarcoma and conventional synovial sarcoma Recognition of this rare histologic variant of synovial sarcoma is important because it can easily be mistaken for other myxoid spindle cell neoplasms, potentially resulting in suboptimal therapy.