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

Cutaneous syncytial myoepithelioma (CSM) is a recently recognized variant of myoepithelial neoplasms. CSM is characterized by an intradermal syncytial proliferation of spindled, ovoid, and histiocytoid cells. Immunohistochemically, tumor cells usually show strong positivity for S-100 and 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 positivity for 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 suggests that CSM could show extensive adipocytic metaplasia, which could make its histopathological diagnosis challenging. This article is protected by copyright. All rights reserved.