Breast Mucoepidermoid Carcinoma: A Report of Two Cases.

Mucoepidermoid carcinoma (MEC) of the breast is a very rare salivary gland-type tumor that is often missed by pathologists and may pose significant diagnostic challenges. We report two cases of breast MEC which both presented as breast masses. Core needle biopsies revealed moderately differentiated carcinomas (based on intermediate nuclear atypia) with squamous features and focal intra- and extracellular mucin production. Immunohistochemical analysis demonstrated that the tumor cells were positive for CK7, CK5/6, p63, GATA3, and TRPS1. They were negative for SOX10, estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2. Both patients underwent breast-conserving surgery. One patient also received an axillary lymph node excision. Histologic examination of the resected masses showed cystic components lined by squamous cells and occasional mucous cells, whereas the solid areas were predominantly composed of intermediate cells. Molecular testing demonstrated MAML2 gene rearrangement in both tumors, confirming the diagnosis of breast MEC. No adjuvant therapy was administered in either of the patients. Post-operative surveillance over 24 months showed no evidence of locoregional recurrence or distant metastasis. Recognition of this rare entity is important to avoid misdiagnosis and potential overtreatment of breast MEC as conventional triple negative breast carcinoma.