Breast cancer is a cancer that starts in breast tissue. It happens when cells in the breast change and grow out of control. The cells usually form a tumor. Sometimes the cancer does not spread any further. This is called "in situ." If the cancer spreads outside the breast, the cancer is called "invasive." It may just spread to nearby tissues and lymph nodes. Or the cancer may metastasize (spread to other parts of the body) through the lymph system or the blood. Breast cancer is the second most common type of cancer in women in the United States. Rarely, it can also affect men.
There are different types of breast cancer. The types are based on which breast cells turn into cancer. The types include: