Myoepithelioma of soft tissue: A cytological-pathological correlation with literature review

Abstract Myoepitheliomas of soft tissue are rare tumors with variable morphologic, immunohistochemical and molecular profiles and therefore are diagnostically challenging for pathologists. We report a case in a 60-year-old male with a painless slowly growing 3 cm mass on left medial forefoot. Core biopsy of the mass showed a neoplastic proliferation of plasmacytoid tumor cells, consistent with myoepithelioma of soft tissue. Immunohistochemical stains demonstrated positivity of the tumor cells for cytokeratin AE1/AE3, CK18, S-100 protein and myosin heavy chain (SMMS-1), supporting the diagnosis. Fine-needle aspiration was performed intraoperatively before the resection of the mass. Air-dried Diff-Quik stained cytology slides showed singly scattered and loosely cohesive clusters of plasmacytoid and spindle cells with dense basophilic cytoplasm, distinct cytoplasmic borders, and round to oval mildly pleomorphic nuclei with smooth nuclear membrane. Scattered naked nuclei, binucleated tumor cells, as well as tumor cells with wispy elongated cytoplasm were also seen. Occasional clusters of cells were intimately associated with metachromatic fibrillary stromal material. Histologic examination of the resected tumor confirmed the diagnosis of myoepithelioma. Molecular studies showed absence of EWSR1 rearrangement. Cytological differential diagnosis, clinical, histological, immunohistochemical, and molecular features of soft tissue myoepitheliomas are discussed in this study.