Primary Merkel Cell Carcinoma of the Breast - A Report of an Extremely Rare Case.

Neuroendocrine carcinoma of the breast is a very rare malignancy, and when it presents with characteristics similar to Merkel cell carcinoma (MCC), it is even more uncommon. This report discusses a case of small cell carcinoma exhibiting Merkel cell-like features in an 80-year-old woman who had a history of rheumatoid arthritis under treatment with immunosuppressive drugs. Microscopic analysis showed tissue fragments with areas of necrosis, as well as focal regions containing neoplastic cells. These cells were small to medium in size, with poorly defined borders, scant cytoplasm, and hyperchromatic, atypical nuclei. Immunohistochemical staining revealed perinuclear (dot-like) positivity for cytokeratin 18/8 and strong positivity for cytokeratin 20 and cluster of differentiation 56. The cell proliferation marker Ki-67 was positive in over 80% of the cells. What is extremely rare in this case is the presentation of MCC of the breast as a retro areolar lesion, located behind the areola and associated with nipple discharge, without evidence of a primary cutaneous lesion.