Primary Cutaneous Spindle Cell Sarcoma With FN1::FGFR1 Fusion.

We report a case of a primary cutaneous spindle cell sarcoma (SCS) with FN1::FGFR1 fusion. The tumor lacked the typical histologic and immunohistochemical features associated with other FN1-rearranged neoplasms, such as phosphaturic mesenchymal tumors (PMT) and calcified chondroid mesenchymal neoplasms (CCMN). Unlike PMTs, which often feature a cartilaginous matrix and are associated with tumor-induced osteomalacia (TIO), the present case lacked these characteristics and did not show FGF23 mRNA expression. Immunohistochemically, the tumor cells showed patchy staining for CD34 but were negative for markers such as ERG, desmin, S100, and pan-TRK. The fusion event in this case involves the loss of the FGFR1 Ig1 (D1) domain, a mechanism proposed to drive oncogenesis by releasing FGFR1 from autoinhibition. Despite the preservation of other FGFR1 domains, no evidence of FGF23 signaling was detected, and the patient had no clinical history of TIO. This case underscores the complexity of oncogenesis in FN1::FGFR1-rearranged neoplasms, a form of "promiscuous" gene fusion, where similar fusions lead to diverse tumor phenotypes. It emphasizes the importance of incorporating molecular testing in diagnosing spindle cell sarcomas, particularly those occurring in acral sites, to identify this underrecognized entity.