Soft tissue tumor with BRAF and NRAS mutations sharing features with NTRK‐rearranged spindle cell neoplasm: A case report expanding the spectrum of spindle cell tumor with kinase gene alterations

Abstract NTRK ‐rearranged spindle cell neoplasm is a group of tumors characterized by NTRK 1/2/3 gene fusion. Recently, tumors with other kinase fusion genes were reported to exhibit similar morphologies. Herein, we discuss an adult‐onset soft tissue tumor with similar histologic patterns as kinase gene fusion‐related tumors but with BRAF and NRAS mutations. A female in her 40s had a 40 mm tumor with an unclear border in the soft tissue of her foot joint. Short spindle‐shaped tumor cell proliferation with abundant capillaries and collagen fiber bundles were observed. The tumor infiltrated the subcutaneous adipose tissue, exhibiting a lipofibromatosis‐like pattern. Immunohistochemically, the tumor cells coexpressed CD34, S‐100, and BRAF V600E. Whole‐exome sequencing revealed BRAF p.V600E and NRAS p.Q61K mutations. Since BRAF activation occurs in BRAF fusion gene tumors and BRAF mutations, they could share a similar mechanism in tumorigenesis. This case suggests the further expansion of kinase‐related spindle cell tumors.