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Malignant myoepithelioma of the breast

Malignant myoepithelioma of the breast is rare. A 50‐year‐old Japanese woman was admitted to our hospital because of a right breast tumor (11 × 10 × 5.5 cm). Core needle biopsy revealed malignant spindle cells. A mastectomy was performed. The tumor consisted of malignant spindle, round, pleomorphic and giant cells with many mitotic figures and necrotic areas. Tumor and osteoclast‐like giant cells were scattered. Much lymphovascular permeation was seen. In a few areas, particularly on the tumor periphery, there were merges between the tumor cells and myoepithelial cells of the non‐tumorous ducts, as if the tumor emanated from the duct myoepithelium. The tumor was invasive into the skin and pectoral muscle. Immunohistochemically, the tumor cells were diffusely positive for vimentin, CD10, α‐smooth muscle antigen, and Ki‐67 (labeling = 95%). The significant areas of the tumor were positive for S100 protein, p63, p53, CD68, caldesmon, desmin and TGFβ1. A few areas were positive for pancytokeratin (AE1/3), cytokeratin (CK) 5/6, and CK 34βE12. In contrast, the tumor cells were negative for pancytokeratins (WSS, CAM5.2), CK7, CK8, CK14, CK18, CK19, CK20, EMA, CEA, bcl‐2, myoglobin, CD34, CD56, CD45, HMB45, GFAP, α‐1‐antitrypsin, synaptophysin, estrogen receptor, progesterone receptor, HER2/neu, MUC1, MUC2, MUC5AC and MUC6. The author diagnosed the tumor as malignant myoepithelioma, as myoepithelial markers (C10, p63, S100 protein, α‐smooth muscle actin, caldesmon) were positive, and also because there was a transition between the tumor cells and myoepithelium of non‐tumorous ducts. The grade of the tumor was high. The patient was treated with chemoradiation and was free of disease 5 months after the operation.