Diagnostic challenges and management of primary accessory axillary breast cancer.

A perimenopausal woman presented with a slowly enlarging right axillary mass initially suspected to be a sebaceous cyst. An incisional biopsy revealed high-grade invasive ductal carcinoma arising from accessory axillary breast tissue. Imaging showed no orthotopic breast lesion and staging was cT1N0M0. She underwent axillary lumpectomy and sentinel lymph node biopsy, confirming pT1bN0M0 invasive carcinoma. Adjuvant therapy included whole breast radiation, hormonal therapy with anastrozole and goserelin. At 2 years, she remains disease-free. This case highlights the diagnostic challenge of accessory axillary breast cancer (AABC), a rare entity often missed on routine imaging. Early recognition and application of standard breast cancer treatment protocols can result in excellent outcomes. Clinicians should maintain a high index of suspicion for AABC in axillary masses, especially in patients with no primary breast findings on imaging to ensure timely diagnosis and appropriate management.