Breast cancer begins in the cells of the breast. Breast cancer may start in the lobules, which are milk-producing glands, or in the ducts, which are tubes that carry milk to the nipple. Occasionally, breast cancer starts in the stroma, the fatty and connective tissues in the breast. Breast cancer may begin as a small tumor in breast, but may eventually grow and spread to nearby tissue. Once the tumor reaches the blood vessels or lymph nodes, it can spread to other parts of the body. There, it can form new tumors on other tissues and organs.