Breast cancer is the growth of abnormal cells in one or both breasts. Male breast cancer usually develops in the breast tissue found behind the nipple. It usually causes a painless lump or swelling. Male breast cancer is often a type called invasive ductal carcinoma.

The exact cause isn't known, but some things increase the risk of male breast cancer. These include radiation exposure, liver disease, a family history of breast cancer, and inheriting a gene change such as BRCA. Male breast cancer mostly affects those older than 65.