Case Report: CD34‐Negative, S100‐Positive Spindle Cell Tumor With a MYH10-RET Fusion

With the application of next‐generation sequencing (NGS) in soft tissue tumors, NTRK gene rearrangement is becoming known to pathologists as a molecular hallmark of spindle cell tumors, and other spindle cell tumors with related kinases are being reported. However, cases of RET rearranged spindle cell tumors not associated with NTRK rearrangements are rarely reported. Here, we describe a case of RET rearranged spindle cell tumor in a 3‐year‐old girl who presented with swelling and pain in the arm. Histologically, the tumor consisted of a fascicular arrangement of monomorphic spindle cells infiltrating and growing in adipose and muscle tissue with visible mitotic activity. Immunohistochemistry showed spindle cells negative for CD34, positive for S100 protein, and focal staining for Pan‐TRK. MYH10-RET fusion was identified by NGS. Fluorescence in situ hybridization (FISH) analysis confirmed the RET gene rearrangement but did not detect the NTRK1/2/3 gene rearrangement. In conclusion, we describe a rare case of CD34‐negative, S100‐positive spindle cell tumor with MYH10-RET fusion.