Male Accessory Breast Cancer Successfully Treated With Single-Agent Trastuzumab: A Case Report

Male accessory breast cancer is an extremely rare tumor. There is no report about its monotherapy and subsequent outcome prior to 2022. The current study presents the case of a 76-year-old male patient with a hard mass in the left axilla. Histopathologic examination of an excisional specimen indicated an adenocarcinoma compatible with breast carcinoma. Immunohistochemical analysis demonstrated that the mass was estrogen receptor (ER) (−), progesterone receptor (PR)(−), and human epidermal growth factor receptor type 2 (HER2) (−). A diagnosis of breast cancer originating from the accessory mammary gland in the axilla was made. Two years following surgery, the patient presented with a pulmonary lesion. Core needle biopsy was performed, and the lesion was found to be ER (−), PR(−), and HER2 (3+). The patient was successfully treated with single-agent trastuzumab. Single-agent trastuzumab could be a reasonable regimen for metastatic accessory breast cancer patients with HER2 overexpression for whom chemotherapy and endocrine therapy are not suitable.