Invasive ductal carcinoma arising in ectopic breast tissue of the axilla. Case report and review of the literature.

Mammary carcinoma arising in ectopic breast tissue is an uncommon occurrence. Most reported cases have involved ductal carcinoma, but other types, such as medullary, papillary, and lobular carcinomas, have been described. For pathologists, the diagnosis of mammary carcinoma arising in ectopic breast tissue can be difficult, especially in the axilla, where carcinoma of adnexal origin must be excluded. We describe a 51-year-old woman who developed invasive secretory ductal carcinoma in ectopic left axillary breast tissue and micrometastatic carcinoma in an ipsilateral axillary lymph node. The carcinoma arose in a left axillary mass that had been present for several years, from which she had secreted fluid during prior menstrual periods.