Background: Ectopic breast cancer is a rare entity and an important diagnostic and treatment challenge. Case Presentation: 46-year-old female with previous history of resection of ectopic axillary breast tissue who was referred to our Institution after a second resection following an increase in left axillary volume. Histology reported a ductal ectopic breast tissue carcinoma that invaded the deep layer of the dermis and was positive for estrogen and progesterone receptors and Human Epidermal growth factor Receptor-type 2. After being referred to our Institution, surgical margin re-excision and sentinel node biopsy were performed. The patient initiated hormone therapy and underwent adjuvant radiotherapy to the breast and axilla (50 Gy/25 fractions, in daily 2 Gy fractions with a 3D conformal radiation therapy technique) and boost with bolus to the axilla (to complete 60 Gy). She remains in follow-up, with no recurrence to date. Conclusion: A high index of suspicion is required for the timely diagnosis of this rare condition. Since there are no specific guidelines, treatment remains a challenge.

Keywords: Breast cancer, breast ectopic carcinoma, breast ectopic tissue, axilla, adjuvant radiotherapy, case report