A case of cutaneous syncytial myoepithelioma with extensive adipocytic metaplasia: Usefulness of EWSR1‐PBX3 gene fusion analysis

AbstractCutaneous syncytial myoepithelioma (CSM) is a recently recognized variant of myoepithelioma characterized by an intradermal syncytial proliferation of spindled, ovoid, and histiocytoid cells. Immunohistochemically, tumor cells usually show strong expression of S‐100 protein and epithelial membrane antigen (EMA). Here we report a case of CSM in the thigh of a 51‐year‐old Japanese woman. Histopathological findings showed a sheet‐like growth of ovoid cells and histiocytoid cells with an eosinophilic syncytial cytoplasm, and adipocytic metaplasia was widely observed in the tumor. Immunohistochemical staining revealed a diffuse, strong pattern for EMA, smooth muscle actin (SMA), and HHF35, and variable expression of S‐100 protein and p63 in ovoid and histiocytoid cells without significant mitotic figures or pleomorphism. In addition, EWSR1‐PBX3 gene fusion, which is characteristic of CSM, was observed in the tumor. Based on these findings, we diagnosed the patient as having CSM. Our case shows that CSM can exhibit extensive adipocytic metaplasia, which could make its histopathological diagnosis challenging.