Breast cancer starts when cells in the breast begin to grow out of control. These cells usually form a tumor that can often be seen on an x-ray or felt as a lump. The tumor is malignant (cancerous) if the cells can grow into (invade) surrounding tissues or spread to distant areas of the body. Breast cancer occurs almost entirely in women, but men can get it, too. Although many types of breast cancer can cause a lump in the breast, not all do. There are other symptoms of breast cancer you should watch out for and report to a health care provider.