Primary mammary mucinous cystadenocarcinoma: cytological and histological findings.

Mucinous cystadenocarcinoma (MCA), commonly encountered in the ovary or pancreas, is rare in the breast and was only recently described as a distinct variant of invasive ductal carcinoma of the breast. Only 11 cases of primary mammary MCA have been reported. In this article, we report a case of primary mammary MCA with focus on cytological and histological findings. A 65-year-old female noticed right palpable breast mass. Sonography showed an irregularly shaped 2.8 × 2.4 cm lesion in the upper outer quadrant of the right breast. Fine-needle aspiration cytology was performed on the right breast nodule, and cytopathologic examination suggested an adenocarcinoma composed of tall columnar cells with mucin. A partial mastectomy of the right breast and the axillary lymph nodes dissection was performed. The gross examination revealed a well-demarcated and mucus-filled tumor. Histologically, it had complex papillae, some of which were supported by delicate fibrovascular stroma lined by simple to slightly stratified columnar neoplastic epithelial cells with intracellular mucin, coexisting with MCA in situ and ordinary intraductal carcinoma component (ICC). Immunohistochemically, ICC was HER2-negative and estrogen receptor/progesterone receptor-positive, while MCA was triple negative. MCA might be derived from a metaplasia of ordinary ICC, but its pathogenesis and biologic behavior remains unclear. Despite the invasive nature of mammary MCA, these carcinomas appear to be associated with a good prognosis. The patient has remained well and disease-free for 6 months after the operation.