Mucinous Cystadenocarcinoma of the Breast with Axillary Lymph Node Metastasis

An Entity with an Unusual Clinical Course

Abstract

This is a case of a 54-year-old, perimenopausal, Asian, woman, who presented with an enlarging left breast mass associated with whitish to bloody nipple discharge. A core needle biopsy, done in another institution, showed histologic findings of a mucinous carcinoma with triple negative “basal-like” biomarker status (ER, PR, HER2/neu). Six cycles of neoadjuvant chemotherapy were given after which the subsequent modified radical mastectomy revealed a centrally located, 10.0 cm, well-circumscribed, nodular, ovoid mass on gross examination. Microscopic findings showed tall columnar cells in stratification, tufts and papillary formations, with surrounding abundant extracellular mucin. The individual tumor cells exhibit enlarged, hyperchromatic, basally located nuclei with prominent nucleoli, abundant amphophilic and occasionally oncocytic cytoplasm which contains intracytoplasmic mucin. Based on the histologic features, “basal-like” biomarker expression, and additional immunohistochemical studies (positive CK7, negative CK20 and CDX2), this case demonstrates a pure mucinous cystadenocarcinoma of the breast. In addition to the rare histologic type, this case is exceptional since, despite multiple cycles of neoadjuvant chemotherapy, presence of extensive lymphovascular invasion and axillary lymph node involvement with extranodal extension remain evident.