Mucinous lesions of the breast: potpourri of old and new

Mucinous lesions of the breast encompass a range of benign and malignant entities characterized by extracellular mucin production. Increased sampling of mammary calcifications has identified a range of mucocele-like lesions, which are associated with benign proliferative and atypical intraductal epithelial proliferation ranging in architectural complexity from flat epithelial atypia to ductal carcinoma in situ. Mucinous carcinoma is a unique histologic subtype of breast cancer with a good prognosis. Mucinous carcinoma, especially in males and older females, can arise in association with solid papillary carcinoma. Recent molecular studies reveal distinct genomic changes in mucinous carcinoma. Micropapillary mucinous carcinoma is a recently described histologic variant of breast cancer with aggressive clinical features and poorer outcome. Not all mucinous lesions are of ductal epithelial phenotype; rarely, lobular carcinoma can present with extracellular mucin. This concise review discusses clinical features, histological findings, and differential diagnoses of mucinous lesions of the breast, including common and some rare lesions as well as recently described entities.