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.

NTRK-rearranged spindle cell neoplasm is a group of tumors characterized by NTRK1/2/3 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.