Abstract
KEY CLINICAL MESSAGE: Epithelial-myoepithelial carcinoma of the breast is an extremely rare biphasic tumor. This report documents the first case of epithelial-myoepithelial carcinoma presenting in the location of a previously treated ductal carcinoma in order to increase the awareness of this entity as a potential differential for recurrent breast lesions. ABSTRACT: Epithelial-myoepithelial carcinoma of the breast is an exceedingly rare biphasic tumor, seldom documented in medical literature. This report describes the first known case of this entity at the site of a previously treated neoplasm in a 75-year-old female with a history of high-grade ductal carcinoma in situ who presented with a new breast mass. Imaging demonstrated an oval shaped mass with microlobulated borders and hypoechoic echogenicity on ultrasound. Following multidisciplinary discussion, she underwent a mastectomy, revealing epithelial-myoepithelial carcinoma with metaplastic squamous cell carcinoma. The patient began chemotherapy but discontinued due to poor tolerance and neurological complications. Generally, prognosis for epithelial-myoepithelial carcinoma (World Health Organization Classification of Breast Tumors 2019, 8562/3) is highly variable, with limited available data suggesting that epithelial-myoepithelial carcinoma may follow a course similar to that of breast adenocarcinomas with both hematogenous and lymphatic spread. Treatment typically involves curative excision, though the role of axillary lymph node sampling remains under discussion. This case underscores the need for vigilance in post-treatment surveillance for breast cancer and highlights the importance of recognizing this entity in recurrent breast pathology.