A low-grade malignant soft tissue tumor with S100 and CD34 co-expression showing novel CDC42SE2-BRAF fusion with distinct features.

Recently, a novel group of spindle cell tumors defined by S100 and CD34 co-expression harboring recurrent fusions involving RET, RAF1, BRAF, and NTRK1/2 gene has been identified. Morphologically, they are characterized by monomorphic neoplasm cells, "patternless" growth pattern, stromal, and perivascular hyalinization, lacked necrosis. We reported a 52-year-old Chinese female patient with a S100 and CD34 co-expression sarcoma presenting in the right proximal forearm. The forearm mass initially emerged 19 months ago when it was misdiagnosed as a solitary fibrous tumor and was surgically removed without further treatment. Microscopically, the primary and the recurred tumors share the same features, resembling the morphology of the recently characterized group. Nevertheless, some distinct features, such as predominantly epithelioid tumor cells and focally staghorn vessels, were also present in our case. Genomic profiling with clinical next-generation sequencing was performed and revealed CDC42SE2-BRAF gene fusion, MET amplification, and CDKN2A/B deletion. Both FISH and nested RT-PCR were performed to confirm the gene fusion. The patient was treated with crizotinib for two cycles but showed no obvious benefit. The presented case adds to the spectrum of the novel, characterized solid tumors, and provides suggestions for emerging therapeutic strategies for precision medicine involving targeted kinase inhibitors.