Male Accessory Breast Mucinous Carcinoma: A Case Report and Literature Review

Accessory breast cancer is a special type of breast cancer, accounting for approximately 0.3–0.6% of all breast cancers, of which male accessory breast cancer is even rarer. This condition typically manifests as a progressively enlarging mass, most frequently located in the axillary or inguinal regions, and may present with early lymph node metastasis. Pathological examination serves as the definitive diagnostic standard, while ultrasound, mammography and MRI are valuable ancillary diagnostic tools. Currently, the treatment for accessory breast cancer primarily references the treatment of breast cancer, employing surgical resection as the cornerstone of treatment, supplemented by endocrine therapy, chemotherapy, and radiotherapy. Here, we report a case of a 72-year-old male with accessory breast cancer. The patient was diagnosed with right axillary accessory breast cancer with right axillary lymph node metastatic cancer in 2021 and underwent right accessory mastectomy plus right axillary lymph node dissection at our hospital. Postoperatively, he has been supplemented with chemotherapy and endocrine therapy to this day. In 2025, a follow-up examination revealed a new mass in the medial aspect of the right upper arm near the axilla, and post-excisional pathology suggested it to be mucinous breast cancer. We aim to increase clinical awareness and understanding of male accessory breast cancer by detailing the discovery, clinical course, and management of this exceptionally rare case.