Myoepithelial carcinoma, also known as malignant myoepithelioma (MME), is a rare malignant tumor that occurs mainly in the salivary glands of all sizes. Those that occur in the breast are mostly benign myoepitheliomas, while malignant myoepitheliomas are extremely rare. Myoepithelial cells are found in the secretory part of human glands and ducts, and the tissue is ectodermal in origin and has epithelial characteristics. It is difficult to diagnose clinically, and there is no specific manifestation in imaging, so the diagnosis mainly relies on pathological examination. Microscopically: myoepithelial carcinoma has characteristic myoepithelium with large, columnar or ovoid cell bodies and vacuolated cytoplasm that appears watery and transparent. Immunohistochemistry showed that myoepithelial carcinoma had the same immunological characteristics as normal tissue myoepithelial cells, containing the same antigenic components, such as positive staining for S-100, SMA, CK and so on. In terms of biological behavior, myoepithelial carcinoma may metastasize bloodstream when the primary mass is >2 cm, and the metastatic sites are mainly lung, liver, bone, thyroid, etc. Surgical resection is the main means to treat myoepithelial carcinoma of breast. The surgical procedure is no different from other malignant tumors of the breast, mainly modified radical surgery for breast cancer, whose effect on chemotherapy and radiation therapy is still unclear. Diagnosis: 1. Clinical manifestations Myoepithelial carcinoma of the breast is mostly seen in women, but there are occasional reports of male cases, and the age of onset can be around 27-80. Most patients present with a single painless mass in the breast, which can be located in any area visible on mammography, and a few patients have nipple discharge. The average diameter of the mass varies and the duration of the disease is nonspecific, with the longest being several years. Adenosarcoma of the breast usually presents as a solitary, solid nodular mass with borders, and in a few cases the mass has poorly defined borders. 2. Differential diagnosis of carcinoma: (1) myoepithelial-rich carcinoma, (2) myoepithelial carcinoma, (3) saprophytic carcinoma, (4) adenomyelinating epithelial adenopathy, (5) intraductal papilloma, (6) clear cell tumor, (7) myofibroblastoma of breast, (8) smooth muscle tumor of breast 3. Malignant myoepithelial tumor of breast: The differential diagnosis of benign and malignant is difficult, except for the presence of metastatic foci in the tumor, which can be referred to: (1) significantly increased nuclear schizophrenia, >5/10 HPF; (2) Cell abundance with obvious heterogeneity; (3), tumor appears infiltrative growth and satellite foci; (4), necrosis within the tumor; (5), NA polyploidy analysis as aneuploidy.