A Case Report on Apocrine Encapsulated Papillary Carcinoma of the Breast with Frank Invasion: A Diagnostic Quandary.

Encapsulated papillary carcinoma is a special type of breast cancer defined in the fifth edition of the World Health Organization breast tumor classification guidelines. Apocrine encapsulated papillary carcinoma is extremely rare, and only 10 cases have been described previously. We encountered a case of apocrine encapsulated papillary carcinoma with frank invasion. The patient was a 77-year-old woman with a painless mass in her right breast. Core needle biopsy revealed that the tumor cells had voluminous eosinophilic cytoplasm and enlarged nuclei with prominent nucleoli. We diagnosed this lesion as carcinoma with apocrine differentiation and suggested the possibility of an encapsulated papillary carcinoma. The patient underwent a right-sided mastectomy. Gross examination of the resected specimen revealed a multilobulated tumor. Microscopically, the tumor cells, which had voluminous eosinophilic cytoplasm and enlarged nuclei with prominent nucleoli, proliferated in papillary fashion with fibrous stalks in the cystic space. Myoepithelial cells were not observed around the cystic space. Frank invasion was also observed around the encapsulated papillary carcinoma. Immunohistochemistry analysis revealed that the tumor cells were negative for estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 and positive for androgen receptor and gross cys-tic disease fluid protein 15. Based on these findings, we diagnosed this lesion as an apocrine encapsulated papillary carcinoma with frank invasion.