Dermatofibrosarcoma protuberans with unusual sarcomatous transformation: a series of 4 cases with molecular confirmation.

Dermatofibrosarcoma protuberans (DFSP) is a superficial sarcoma of intermediate malignancy usually composed of monotonous short spindle cells with storiform architecture. The tumor cells are diffusely reactive for CD34 and characterized by a translocation involving chromosomes 17 and 22 or a supernumerary ring chromosome that results in the fusion of exon 2 of platelet-derived growth factor beta (PDGFβ; 22q13) to various exons of collagen type 1 alpha 1 (COL1A1; 17q22). In some tumors, fibrosarcomatous transformation can occur and is characterized by a monotonous spindle cell proliferation arranged in fascicles or a herringbone-type pattern. We report 4 DFSPs with unusual and pleomorphic sarcomatous transformation. They occurred on the back, scalp, shoulder, and forehead of women (ages 31,48, 48, and 27 year). In addition to areas of conventional DFSP that strongly expressed CD34, 2 cases showed pleomorphic areas mimicking undifferentiated pleomorphic sarcoma/malignant fibrous histiocytoma: 1 case had a patternless area and 1 case had combined round/spindled cells with myxoid areas. Reverse transcription--polymerase chain reaction was performed in 1 case, confirming the presence of a COL1A1-PDGFβ fusion transcript. The remaining three cases were found to be positive for a PDGFβ gene rearrangement by fluorescence in situ hybridization. This series illustrates that sarcomatous transformation in DFSP may occasionally display areas, which can mimic undifferentiated pleomorphic or unclassified sarcoma. Ancillary diagnostic testing may be helpful to confirm the diagnosis, especially in small biopsies.