A Case Report of Male Occult Breast Cancer First Manifesting as Axillary Lymph Node Metastasis With Part of Metastatic Mucinous Carcinoma

Occult breast cancer (OBC) is a type of breast cancer without any symptoms in the breast (no primary cancer lesion is found in either breast on a physical examination or imaging examination such as ultrasound and mammography). The incidence of OBC is rare in females, whereas in males, there are few cases of breast cancer, and the rate of OBC is very low. This is the first time report a case of male OBC first manifested as axillary metastasis, of which the pathological results showed moderately differentiated adenocarcinoma with part of metastatic mucinous carcinoma. A 40-year-old male patient presenting palpable masses in his left axillary on physical and imaging examination revealed unremarkable despite of multiple swollen lymph nodes in the left axillary, and the resected sample showed metastatic adenocarcinoma with part of metastatic mucinous carcinoma. Based on immunohistochemical analysis, positive of estrogen receptor (ER), progesterone receptor (PR), and human epidermal receptor 2 (Her-2), it was identified as an OBC. This is the fourth case report of male OBC in the literature; 1 case was reported in China in 2008, and it was metastatic infiltrating ductal carcinoma, and 2 cases were reported in Korea in 2012, one of which was reported as metastatic carcinoma and the other was metastatic adenocarcinoma; however, our case was a moderately differentiated adenocarcinoma with part of mucinous carcinoma. Our case of male OBC could metastasize to supraclavicular region and lung in addition to axillary lymph nodes, and the prognosis was relatively poor compared to the 3 cases reported before. The aim of this case report is to introduce the imaging, pathological features, and management of a rare male OBC.