Cancers are a group of diseases that cause the cells in the body to change and grow out of control.
Most types of cancer cells form a lump or mass called a tumor, and are named after the part of the body where the tumor first starts. Breast cancer begins in the breast tissue, which is made up of glands called lobules that produce milk, and the ducts that connect lobules to the nipple. The remainder of the breast is made up of fatty, connective, and lymphatic tissue. Benign breast tumors are abnormal growths, but they do not grow and spread like cancer does, and are not life-threatening. Some breast tumors are cancerous, but are called in situ, because they have not spread beyond the area where they began.