Fibroadenoma in axillary accessory breast mimicking carcinoma of unknown primary; a case report with literature review

Introduction: Accessory breast (AB) is extra and ectopic breast tissue. Fibroadenoma (FA) in AB is a rare finding. This study aims to present a case of FA in axillary AB mimicking carcinoma of unknown primary (CUP). Case report: A 38-year-old female presented with a mass in her right axilla. She had a mass in her right breast for 5 years. She previously had a left breast lumpectomy for a benign condition. The lump in her axilla was palpable and hard. Ultrasound showed an oval lymph node in the level I axilla (12*6mm) with blurred and unclear fatty hilum, suspicious for malignancy. Magnetic resonance imaging revealed an oval lesion (12*7mm) in the level I axilla with no fatty hilum and with heterogeneous enhancements, suggesting abnormal nodes. Fine needle aspiration of the axillary mass suspected CUP. But core biopsy resembled FA. Both masses in the right axilla and breast were surgically excised. Histopathology confirmed FA in both masses. Discussion: Although pathologies in AB are uncommon, it is still susceptible to the same malignant and benign transformations that are found in normal breasts. The axilla is the most frequent location for FA in AB and often affects young women. Imaging techniques can be inconclusive and only histopathology can conclude a definitive diagnosis. Conclusion: FA in axillary AB is a rare condition that causes a diagnostic dilemma as it can be mistaken for other benign or malignant pathologies.