Abstract
Ectopic breast cancer (EBC) is a rare condition, and there have been no reports of axillary EBC as a third primary breast cancer after bilateral pectoral breast cancer (PBC) surgery. A woman presented with a mass in her axilla, which was located just beneath the skin. On ultrasound (US) examination, it appeared oval-shaped with uniform internal echoes and relatively clear boundaries. Pathologically, it was an oval-shaped, well-defined tumor and was diagnosed as encapsulated papillary carcinoma with invasion. EBC rises directly under the axillary skin, where axillary ectopic breast tissue exists. This area is not generally included in the resection field during mastectomy and axillary lymph node dissection, as illustrated in this case. US findings of EBC do not consistently show an irregularly shaped mass with indistinct boundaries, as these depend on the tumor's histological structure, similar to PBC. When axillary masses are observed, it is crucial to consider the possibility of EBC based on their superficial location in the axilla and to predict histological architecture and type from US findings, similar to the approach for PBC.