Mucinous cystadenocarcinoma of the breast.

Four unusual cases of primary mammary mucinous cystadenocarcinoma composed predominantly of tall columnar cells with abundant intracytoplasmic mucin are reported; they were multicystic and appeared virtually identical to mucinous cystadenocarcinomas of the ovary and pancreas. Three of the women were white and one was black, they ranged in age from 49 to 67 years (average 58), and they had tumors that ranged from 0.8 to 19 cm in diameter. Microscopically, the tumors were characterized by cystic spaces lined by predominantly bland-appearing columnar mucinous cells with stratification, tufting, and papillary formations. Varying degrees of cytologic atypia were focally evident, with gradual loss of the intracytoplasmic mucin and transformation to an eosinophilic squamoid cell population. Multifocal invasion generally emanated from these eosinophilic, squamoid areas in all cases. All four tumors displayed immunoreactivity for MIB-1 (Ki-67) in a relatively high percentage of cells and failed to show immunoreactivity for estrogen receptors and progesterone receptors. All four stained positively with cytokeratin 7 (CK7) but were negative with cytokeratin 20 (CK20). Mastectomy and axillary lymph node dissection were performed in three cases and lumpectomy with lymph node dissection in the remaining case. Lymph node metastases, identified in only one patient, retained the distinctive morphology. Three of the patients are alive without evidence of disease 11, 22, and 24 months after the diagnosis; the fourth is a recent case. These tumors are a rare, clinicopathologically distinct type of primary breast carcinoma that should be distinguished from typical mucinous (colloid) carcinomas of the breast and, more importantly, metastases from other sites.