Primary male neuroendocrine adenocarcinoma involving the nipple simulating Merkel cell carcinoma - a diagnostic pitfall.

Male breast cancer is a rare entity accounting for < 1% of all breast cancer cases in the United States, but with a rate that has been rising over the last 25 years. Nipple skin/subcutaneous tumors in men are even rarer. Likewise, true neuroendocrine carcinoma of the breast, defined as > 50% of tumor cells staining for either chromogranin or synaptophysin, is not a common entity, usually occurring in older women. We present the case of a 70-year-old man with a slowly growing nipple mass that had enlarged over the previous 1.5 years. The histology consisted of nests, trabeculae and sheets of basaloid cells with rare abortive gland formation and a pushing edge. The case was originally misdiagnosed as a Merkel cell carcinoma, based largely on histologic morphology. Strong staining for synaptophysin (in greater than 50% of cells), CD56, keratins AE1 : AE3 and Cam 5.2, as well as estrogen receptor and progesterone receptor was noted. Myoepithelial cells within in situ areas were identified using stains for calponin and 4A4, supporting a primary mammary duct origin. Additionally, a substantial portion of cells stained for Gross Cystic Disease Fluid Protein-15 (GCDFP-15), confirming some overlap with sweat duct differentiation. To the best of our knowledge, although reported in the male breast, no case of primary nipple neuroendocrine carcinoma in a male patient has been reported in the literature. The gender of the patient and association with the skin of the chest wall probably contributed to the original misdiagnosis of Merkel cell carcinoma in this patient.