Malignant Biphasic Adenomyoepithelioma: A Rare Entity

Objectives Adenomyoepithelioma of the breast is an uncommon biphasic breast lesion characterized by a cytologically bland epithelial and myoepithelial proliferation. It is often considered benign and thought to represent a variant of intraductal papilloma. However, rare cases of adenomyoepitheliomas have been reported to show malignant transformation. Features that favor malignancy include increased mitoses, necrosis, atypia, cellular pleomorphism, and infiltrative borders. Most reported cases of malignant adenomyoepithelioma demonstrate carcinomas arising in a benign adenomyoepithelioma. Methods A 74-year-old woman presented to the hospital with an 8-month history of a palpable right breast mass. Mammogram demonstrated an irregular marginated mass, and a needle core biopsy of the mass showed atypical sclerosing adenosis. A lumpectomy revealed a firm, tan-white, ill-defined 2.7-cm mass abutting the anterior and posterior margins. Results Histologic examination showed an infiltrative proliferation of epithelial-lined tubules surrounded by myoepithelial cells with clear cytoplasm. There were focal areas of hypercellularity with marked cytologic atypia in both the epithelial and myoepithelial components. Focal necrosis and increased mitoses (12/10 HPF) were also identified. On immunohistochemical studies, the epithelial cells were positive for CK7 while the myoepithelial cells were highlighted by p63, SMMHC, SMA, and CK5/6. Ki-67 showed an increased proliferative index (35%). ER, PR, and HER-2 immunostains were negative. The overall histologic and immunohistochemical findings support a diagnosis of malignant biphasic adenomyoepithelioma. No in situ carcinoma was identified. After the diagnosis was reported, a follow-up reexcision with axillary sentinel lymph node excision was performed. Both specimens were negative for malignancy. Conclusion Malignant adenomyoepithelioma of the breast is an uncommon entity and it is strongly associated with local recurrence and distant metastases. Awareness of its distinctive histologic and immunohistochemical features will be helpful in initiating the appropriate medical management for patients with this entity, which is crucial for improving prognosis and overall survival.