Two cases of spindle cell tumors with S100 and CD34 co-expression showing novel RAF1 fusions

BACKGROUND: Recently, a novel group of CD34 and S100 co-expression spindle cell tumors with distinctive stromal and perivascular hyalinization harboring recurrent gene fusions involving RET, RAF1, BRAF, and NTRK1/2 gene has been identified. CASE PRESENTATION: In this study, we reported two Chinese male patients with soft tissue tumors presenting in the right knee joint and the left thigh, respectively. For both patients, the tumors were completely excised with clear margin. Microscopically, case 1showed morphological overlap with neurofibroma, and case 2 showed overlap with lipomatous solitary fibrous tumor. Both tumors showed co-expression of S100 and CD34, and absence of SOX10. Genomic profiling with DNA-based next-generation sequencing (NGS) assay was performed and revealed KIF5B-RAF1 (K16:R8) and TLN2-RAF1 (T54:R8) rearrangements. RNA-based NGS and RT-PCR were performed to confirm the gene fusion. CONCLUSIONS: Though systemic therapy was not indicated in these two patients, identification of targetable kinase fusions may help to refine tumors with an ambiguous immunoprofile, and provides suggestions for targeted therapy in rare aggressive cases. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13000-022-01263-y.