Abstract
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.