Accessory Breast Carcinoma Masquerading as a Sebaceous Cyst in a Breast Cancer Survivor: A Diagnostic Pitfall in Axillary Assessment.

Accessory breast carcinoma is an uncommon malignancy and is often diagnosed late because it could mimic a benign axillary lesion. We report the case of a 63-year-old woman with a prior history of left breast cancer treated with breast-conserving surgery and radiotherapy who later developed a palpable right axillary lump. The lesion had initially been assessed clinically as a sebaceous cyst and was not definitively investigated for 23 months. During routine breast surveillance, mammography and targeted ultrasound revealed a new suspicious lesion arising within accessory axillary breast tissue, which was later proven to be invasive breast carcinoma. Staging investigations, including fluorodeoxyglucose (FDG) PET-CT and contrast-enhanced mammography, showed no synchronous lesions in either orthotopic breast and no evidence of distant metastasis. The patient underwent wide local excision with sentinel lymph node biopsy, followed by adjuvant radiotherapy and hormonal therapy. At seven months' follow-up, she remained disease-free. This case underscores the diagnostic difficulty of accessory breast carcinoma and shows how malignancy may be overlooked even during regular breast cancer surveillance. Persistent axillary masses should prompt consideration of accessory breast tissue cancer in all female patients, particularly in those with a history of breast cancer, to enable timely diagnosis and appropriate treatment.