A Case of Axillary Fibroadenoma That Grew Rapidly from Axillary Accessory Breast Tissue over 40 Days

Axillary accessory breast (AAB) occurs in 2%–6% of women. Like normal breast tissue, ABB can undergo changes, including periodic enlargement that can result in a palpable axillary mass. Fibroadenoma is the most common benign subcutaneous tumor of the breast: it occurs in approximately 25% of women and accounts for 50% of all breast biopsies. However, fibroadenoma in AAB is rare (2.6%). Here, we describe the case of a patient who was diagnosed first with left AAB on the basis of clinical and magnetic imaging resonance findings, and then 40 days later with fibroadenoma in left AAB by histopathology of the resected mass. The tumor, which had been undetectable at the initial visit, had transformed into a clinically obvious, hard, protruding mass at surgery. Thus, fibroadenomas originating from AAB can grow quickly, and imaging-based diagnosis should be confirmed with histology. Treatment should involve complete excision of the fibroadenoma and surrounding AAB.