Nipple adenoma found in a mastectomy specimen: report of a case with special regard to the proliferation pattern

We report a case of nipple adenoma incidentally found in a mastectomy specimen, and describe its unique histological appearance and the immunohistochemical distribution of Ki-67 positive tumor cells. A 45-year-old woman with no symptoms or sign related to the nipple had a left mastectomy for invasive breast cancer. A small nipple adenoma, 7 mm in size, was incidentally recognized in the nipple of the resected breast. Histologically, the tumor in the nipple was composed of numerous proliferative ducts with a tubular and florid papillomatous appearance. Many demarcations between squamous cells of the epidermis and tumor cells were recognized in the summit as well as the lateral wall of the nipple. A high Ki-67 labeling index (20.3%) was recognized in the tumor cells in the superficial region, and a low labeling index (0.7%) was seen in the deeper region of the tumor. Based on these proliferative patterns, the symptoms and clinical signs related to the nipple that are often found in patients with nipple adenoma are thought to be associated with the destruction of the epidermis of the nipple by the invasion of benign tumor cells with high proliferative activity.