A case of intracystic apocrine papillary tumor: diagnostic pitfalls for malignancy.

Intraductal/intracystic papillary carcinoma (IPC) of the breast is defined as a malignant non-invasive papillary tumor arising from the ductal-lobular system. Based on the presence of myoepithelial cells in the cystic wall, IPC is distinguished from encapsulated papillary carcinoma (EPC). Here, we report a case of an intracystic apocrine papillary tumor in the breast of a 49-year-old woman. Histopathologic examination revealed that the entire papillary structures and cyst wall were comprised of apocrine cells, some of which showed nuclear atypia with macronucleoli. Immunohistochemical examination revealed a lack of myoepithelial cells in the papillary fronds and cyst wall. Although the dense proliferation of apocrine cells mimicked a cribriform pattern, detailed examination identified a delicately intermingled interstitium in the cribriform-like growth area in the present case. We judged the current case to be benign apocrine papilloma. Only a few apocrine variants of IPC or EPC have been reported to be malignant or potentially malignant. Since even benign apocrine lesions are known to lack myoepithelial cells, histopathologic evaluation regarding malignant potential requires caution in apocrine variants. The detection of clearly benign areas and knowledge of the "pseudo-cribriform" pattern should provide clues to distinguish between benign and malignant apocrine papillary tumors.