Primary ectopic breast carcinoma in the axilla: A rare presentation and review of the literature.

Ectopic breast tissue (EBT) develops as the result of the incomplete embryologic regression of the mammary ridge, which extends bilaterally from the anterior axilla folds to the inguinal folds in the fifth and sixth weeks of gestation. EBT is encountered in 0.3%-6% of women and 1%-3% of men and can be found anywhere along the milk line. It is subject to the same pathologic changes that affect the orthotopic breast. Primary carcinoma arising on the ectopic breast (PEBC) is a very rare occurrence accounting for 0.3%-0.6% of all breast cancers and is most frequently found in the axilla. Due to the rarity and atypical presentation of the disease, the diagnosis is often delayed. We herein present a very rare case of PEBC in the axilla along with a review of the literature. Any subcutaneous hypoechoic nodule located along the milk line without evidence of inflammation should raise concern for ectopic breast carcinoma. Accurate staging and differential diagnosis with detailed immunohistochemical analysis are required. No specific guidelines on diagnosis and treatment are available and the tumor is treated similarly to the orthotopic breast cancer of a similar stage. Physicians should be aware of this rare entity to avoid treatment delays.