Myxoid dermatofibrosarcoma protuberans: a rare variant analyzed in a series of 23 cases.

The myxoid variant of dermatofibrosarcoma protuberans (DFSPs) is uncommon. It often presents a diagnostic challenge and is important to recognize to prevent both undertreatment and overtreatment. To better characterize this unusual variant of DFSP, 23 myxoid DFSPs (DFSP with greater than 50% myxoid stroma) were retrieved from the authors' consult files. 13 patients were male and 10 were female (median age 40 years; range 9 months to 72 years of age). Tumor size ranged from 1.5 to 11 cm (median 2.8 cm). The most frequent sites were the extremities (9) and head and neck (7), followed by the trunk (4) and anogenital region (3). Grossly, the tumors were white/tan/gray to yellow, firm to gelatinous soft tissue masses. Histologically, tumor stroma ranged from 50 to 100% myxoid (median 80%). The majority of cases displayed an infiltrative sheet-like proliferation of bland spindle cells with palely eosinophilic cytoplasm and stellate nuclei without pleomorphism. The stroma was myxoid with prominent thin-walled vessels. All cases displayed honeycomb infiltration of fat and 16 cases showed cellular areas of more typical DFSP. Four tumors contained pigmented dendritic cells (Bednar variant), 1 showed areas of giant cell fibroblastoma and 1 showed progression to fibrosarcomatous DFSP. Mitoses ranged from 0 to 5 per 10 high power fields. 95% of cases (21 out of 22) were positive for CD34 and all cases were negative for S100 and muscle markers. Clinical follow-up in 8 cases, ranging from 3-21 years, (median follow-up 6 years), revealed local recurrence in 2 cases and no evidence of metastasis. All patients were free of disease following wide excision or excision followed by radiotherapy. In summary, these low-grade lesions are clinically similar to typical DFSP, but their unusual morphology is easily confused with a variety of other tumor types.