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

FUS::TFCP2-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::TFCP2 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::TFCP2 fusion was detected. Myogenin was negative, but MyoD1 was positive. Detection of FUS signals using FISH led to the diagnosis of FUS::TFCP2-rearranged rhabdomyosarcoma. In cases of ALK positivity and spindle cell or epithelioid cell morphology, FUS::TFCP2-rearranged rhabdomyosarcoma should be considered in the differential diagnosis using staining for rhabdomyoblastic markers other than desmin.