Clinicopathological and molecular analysis of a dermatofibrosarcoma protuberans with fibrosarcomatous transformation in the breast.

Dermatofibrosarcoma protuberans (DFSP) is a rare soft tissue sarcoma with low potential for metastases. To date, only a few cases of DFSP in the breast have been reported, and none have reported comprehensive genetic signatures. We report, for the first time, the largest DFSP in the breast with multidisciplinary tumor board discussion and comprehensive molecular analysis. This DFSP is 18.5 cm with fibrosarcomatous transformation, distinct gene rearrangement translocation t(17;22) (q22;q13), COL1A1::PDGFB fusion, and subsequent segmental amplification of chromosomes 17q and 22q. The high-level of amplifications of genomic regions containing the COL1A1 and PDGFB genes (6 copies) compared to the reported DFSP cases (2-3 copies) supports the gene dosage effect correlated with aggressive behavior and high-grade tumor histology. This case highlights the importance of precise genetic alteration in the clinicopathological and phenotypic expression of DFSP with fibrosarcomatous transformation, and it helps guide clinical management and informs the role of targeted therapy.