Platelet-Derived Growth Factor-D Fusion-Positive Dermatofibrosarcoma Protuberans: Case Report of an Atypical Breast Mass and Literature Review.

Dermatofibrosarcoma protuberans (DFSP) is a rare, CD34+ mesenchymal neoplasm that classically involves the dermis. A COL1A1::PDGFB t(17;22) translocation is present in 91.4% to 96% of cases, resulting in aberrant proliferation due to tyrosine kinase hyperactivity. Here, we present a postmenopausal woman with a CD34-positive spindle cell neoplasm of the breast without cutaneous involvement, lacking muscle marker expression, STAT6 expression, and 13q14 deletion by fluorescence in situ hybridization (FISH). Although the classic PDGFB translocation was not detected by FISH, the overall features were highly suspicious for DFSP. Subsequent RNA-based next-generation sequencing revealed an EMILIN2::PDGFD fusion. A literature review showed that PDGFD fusions can be detected in up to 55% PDGFB FISH negative cases, with EMILIN2::PDGFD fusion highly associated with fibrosarcomatous transformation. This holds important diagnostic and prognostic information as fibrosarcomatous-DFSP is associated with higher recurrence and metastatic potential. The tumor was completely resected with clear margins, showed no fibrosarcomatous areas, and no evidence of recurrence is documented 2 years since resection. This review and case report adds to the literature regarding PDGFD -translocation positive DFSP as a differential diagnosis of CD34-positive spindle cell tumors of the breast, while emphasizing the prognostic importance of EMILIN2::PDGFD fusions.