The work-up of unilateral breast masses in men involves a large differential diagnosis, including the following: unilateral gynecomastia, lipoma, inclusion cyst, oil cyst, abscess, panniculitis, hematoma, fat necrosis, ductal ectasia, intraductal papilloma, cysts, carcinoma, sarcoma and metastatic disease. All unilateral breast lumps in men aged 50 years or older deserve investigation.

The most common cause of a male breast mass is gynecomastia, which is usually unilateral at clinical presentation and bilateral at imaging. Up to 30% of middle-aged men and 60% of men in their seventh decade have benign histologic gynecomastia. Male breast carcinoma represents less than 1% of all male breast diseases. Although most male breast carcinomas are clinically apparent, distinguishing early breast cancer from unilateral gynecomastia is considered a difficult task.