Myoepithelial carcinoma of the breast: a clinicopathological and immunohistochemical study of 15 diagnostically challenging cases.

Myoepithelial carcinoma (MEC) is a rare type of breast cancer composed purely of myoepithelial cells. Most often it presents with a spindle cell morphology that can mimic several benign and malignant lesions and may be misdiagnosed by the pathologist. We report 15 cases of MEC, which were sent to our consultation practice: Five of them were initially diagnosed as benign. The patients, all female, ranged from 45 to 86 years in age (mean 69.5) and-with one exception-presented with a breast mass. The tumor size measured between 1 and 4.8 cm (mean 2.6 cm). Microscopically, the tumors had infiltrative growth pattern most frequently with thin anastomosing cords of tumor cells associated with an intimately admixed reactive spindle cell stroma. The neoplastic myoepithelial cells were emanating from the myoepithelial cell layer of entrapped ductules in every case. The nuclei showed mild to moderate pleomorphism, and the mitotic activity ranged from 0 to 9/10 high power field. Immunohistochemical stains for p63, CD10, CK903, and CK5/6 reacted strongly and diffusely with the tumor cells, and mainly the reactive stroma had weak positivity for calponin, S-100, and smooth muscle actin. Estrogen receptor, progesteron receptor, and Her2 immunostains were negative, but strong epidermal growth factor receptor expression was observed. Follow-up was available for seven patients: All of them were alive at last contact; one patient had local recurrence, and one developed pulmonary metastases. MEC is a potentially aggressive malignant neoplasm sharing many features with metaplastic carcinomas. Morphologically, it is often difficult to distinguish it from benign spindle cell proliferations.