Breast cancer starts when cells in the breast begin to grow out of control and can spread when cancer cells enter the blood or lymph system and are carried to other parts of the body. These cells usually form a tumor that can often be seen on an X-ray or felt as a lump. A tumor is malignant (cancer) if the cells can grow into surrounding tissues or spread to distant areas of the body. Breast cancer occurs almost entirely in women, but men can get breast cancer, too.