Hypercellular Cystic Mucinous Carcinoma of the Breast With a Growth Pattern of Encapsulated Papillary Carcinoma

We report the case of an 82-year-old woman who developed a 70-mm cystic lesion in the breast. The cyst contained a papillary mass with numerous delicate fibrovascular stalks within a cystic space surrounded by a thick fibrous capsule. No myoepithelial cells were identified along the fibrovascular stalks. The neoplastic epithelial cells were arranged in micropapillary and cribriform structures. The tumor was composed of cells ranging from spindle-shaped to polygonal, with intermediate-grade nuclei. There were no densely cellular or solid nests of neoplastic cells. In some areas, tumor cells were also present in the cyst wall. Initially, the tumor was considered an encapsulated papillary carcinoma (EPC). However, more than 70% of the neoplastic cells were positive for synaptophysin and neuron-specific enolase (NSE), both markers of neuroendocrine differentiation. Therefore, the tumor was judged to represent a neuroendocrine neoplasm. Additional Alcian blue staining demonstrated a small amount of mucin in the extracellular space and lumens. Accordingly, the present case was diagnosed as a mucinous carcinoma (MC) of hypercellular type with neuroendocrine differentiation. To the best of our knowledge, a cystic MC of the breast exhibiting growth patterns of EPC has not been previously described.