Accessory breast cancer: surgery, radiation, endocrine therapy, no recurrence

What happened, in summary

A 53-year-old woman came to a breast clinic after noticing a lump in her right armpit. She had first felt the mass 2 years earlier, and it had grown over the previous 6 months. On examination, both armpits contained accessory breast tissue, and the right-sided lump was within this extra breast tissue rather than the main breast. Mammography showed no suspicious findings in either breast, while ultrasound found a small irregular mass in the right axillary accessory breast tissue. Biopsy showed low-grade invasive ductal carcinoma with estrogen receptor and progesterone receptor expression and no HER2 expression. Breast MRI did not find a primary tumor in the breast itself, and chest/abdominal CT plus bone scan showed no distant metastases. She underwent wide total excision of the right accessory breast, including the tumor, overlying skin, a rim of normal fat, and fascia over the pectoralis major muscle. Sentinel lymph node mapping was performed with radionuclide tracer and indigo-carmine dye. Six sentinel lymph nodes were removed, and none contained cancer. Final pathology described a 0.7 × 0.6 cm invasive tubular carcinoma with low nuclear and histologic grade, ER 2+/>95%, PR 3+/>95%, HER2-negative status, and Ki-67 of 2%. Margins were negative, and the tumor was staged as Stage I. Because both native breasts were clear on imaging, treatment focused on the accessory breast cancer in the axilla. She started anastrozole 1 mg daily and received 5,000 cGy of adjuvant radiation to the right breast and axilla. As of March 2025, 26 months after surgery, she remained on anastrozole with no recurrence on breast MRI.