Myxoid dermatofibrosarcoma protuberans: clinicopathologic, immunohistochemical, and molecular analysis of eight cases.

Dermatofibrosarcoma protuberans (DFSP) represents a locally aggressive mesenchymal neoplasm of skin and subcutis with characteristic clinicopathologic, immunohistochemical, and molecular findings. In addition to typical cases, morphologic variants such as pigmented, fibrosarcomatous, myofibroblastic, and granular cell DFSP have been described. Purely or predominantly myxoid DFSP is extremely rare, and may cause considerable diagnostic problems. Eight cases of predominantly myxoid DFSP were studied. Paraffin-embedded blocks and slides were retrieved from the files of the authors. Clinical data were obtained from the referring pathologists and dermatologists. Immunohistochemistry was performed using the ABC method, and three cases were studied by polymerase chain reaction technique. There were six male and two female patients (age range: 29 to 74 years). Locations included the inguinal area (three cases), thigh, upper arm, shoulder, abdominal wall, and back (one each). The patients were treated by wide excision as well as reexcision. Tumor size ranged from 1.5 to 12 cm. Histologically, a nodular growth with peripheral diffuse infiltration, as well as a diffusely infiltrating growth of relatively uniform spindled and stellated tumor cells containing slightly enlarged nuclei, was noted. Three cases were entirely myxoid, and in five cases more than 80% of the tumor area showed myxoid stromal changes. In two cases each, focal fibrosarcomatous and focal giant cell fibroblastoma-like changes were present. At least focally, hypocellular areas were evident in one case. Scattered enlarged tumor cells were seen in two cases. The mitotic rate ranged from 1 to 10 mitoses in 10 high-power fields. Numerous blood vessels with slightly fibrosed vessel walls were seen in seven cases. Immunohistochemically, tumor cells in all cases stained positively for CD34, and in one case each a focal expression of alpha-smooth muscle actin and epithelial membrane antigen (EMA) was noted. The remaining antibodies (CD99, CD31, S-100, Factor XIIIa) were all negative. Polymerase chain reaction technique showed in one case the characteristic COL1A1-PDGFB fusion gene. Follow-up information in seven cases (range: 2 months to 10 years; mean: 62 months; median: 48 months) revealed a local recurrence at 5 years. In conclusion, myxoid DFSP represents a very rare morphologic variant with characteristic changes that has to be distinguished from benign and malignant myxoid mesenchymal neoplasms as superficial angiomyxoma, superficial acral fibromyxoma, myxoid solitary fibrous tumor, myxoid perineurioma, low-grade myxofibrosarcoma, low-grade fibromyxoid sarcoma, myxoid liposarcoma, and myxoid synovial sarcoma.