Immunotherapy-Based Combination Therapy for the Successful Treatment of Coexisting Primary and Axillary Accessory Breast Cancers: A Case Report

Accessory breast cancer is a relatively rare condition, and to our knowledge, no previous reports have described synchronous ipsilateral cancers of the breast and the axillary accessory breast. We report a unique case involving a 70-year-old female patient with luminal A subtype breast cancer and triple-negative accessory breast cancer. The axillary tumor, which had become ulcerative and exudative, was diagnosed as an accessory breast cancer with high Programmed Cell Death Ligand 1 (PD-L1) expression. Treatment with immune checkpoint inhibitors targeting PD-L1 was effective. This case highlights the clinical importance of considering accessory breast cancer as a distinct entity and demonstrates the potential utility of immunotherapy in such cases.