A 55-year-old man presents with a solitary breast mass and biopsy reveals malignant cells. Immunohistochemical staining show that the tumor cells are positive for estrogen receptor (ER) and negative for Her2/neu. What is the most likely diagnosis?

Cancer in the male breast is uncommon and accounts for less than 1% of all cases of breast cancer. The most common subtype is infiltrating (invasive) ductal carcinoma. Because there is less fat in the male breast, invasion of chest wall muscles is more frequent at the time of diagnosis. More than 90% of breast cancers in males are of luminal type, while triple negative breast cancer and Her2/neu enriched cancers are very rare (<5%). The most impoant familial factor conferring an increased risk for male breast cancer is germline mutation of the BRCA2 tumor suppressor gene. Choice A is a skin tumor and the other choices are rare in the male breast.