Cutaneous FUS::TFCP2‐Rearranged Rhabdomyosarcoma Initially Misdiagnosed as ALK‐Rearranged Mesenchymal Neoplasm: A Case Report

FUS::TFCP 2‐rearranged rhabdomyosarcoma is a recently identified malignant neoplasm characterized by immunohistochemical evidence of the co‐expression of rhabdomyoblastic markers and ALK. Herein, we report a case of cutaneous spindle cell/sclerosing rhabdomyosarcoma with FUS::TFCP 2 fusion that was initially interpreted as an ALK‐rearranged mesenchymal neoplasm in a 43‐year‐old male due to negative desmin expression, a rhabdomyoblastic marker. RNA sequencing was performed to detect ALK fusion counterparts; however, no ALK counterpart fusion was observed, and FUS::TFCP 2 fusion was detected. Myogenin was negative, but MyoD1 was positive. Detection of FUS signals using FISH led to the diagnosis of FUS::TFCP 2‐rearranged rhabdomyosarcoma. In cases of ALK positivity and spindle cell or epithelioid cell morphology, FUS::TFCP 2‐rearranged rhabdomyosarcoma should be considered in the differential diagnosis using staining for rhabdomyoblastic markers other than desmin.