Biphasic dermatofibrosarcoma protuberans with a labyrinthine plexiform high‐grade fibrosarcomatous transformation

Several variants of dermatofibrosarcoma protuberans, a low-grade superficial sarcoma, are well recognized. The most prognostically important is the fibrosarcomatous variant. We report a case of biphasic dermatofibrosarcoma protuberans in which the high-grade component exhibited a previously undescribed plexiform pattern. A clinicopathological study complemented with immunohistochemical, ultrastructural, reverse transcription polymerase chain reaction and fluorescence in situ hybridization analyses of this unique case. Histopathologically, a conventional low-grade dermatofibrosarcoma protuberans was admixed with intratumoral high-grade areas showing a striking labyrinthine plexiform pattern characterized by a higher cellularity of larger and slightly atypical tumor cells. CD34 expression was present in both components, while Ki-67 immunostaining was significantly higher in the plexiform high-grade areas. Focal epithelial membrane antigen and claudin-1 immunostaining was present at the interphase between high- and low-grade areas. COL1A1-PDGFB fusion transcripts, with breakpoints at exon 25 of COL1A1 and exon 2 of PDGFB, were present in both components, being more numerous, as the extra copies of both genes, in the high-grade areas. A previously undescribed histopathologic pattern of high-grade sarcomatous transformation of dermatofibrosarcoma protuberans is reported: a biphasic tumor with a labyrinthine plexiform high-grade component.