Cells in the body normally divide at a regular, steady rate. New cells form and take the place of old or injured cells. However, the cells sometimes divide abnormally, growing rapidly and forming a lump. The lump, also called a tumor, can be either benign or malignant. Benign tumors are not cancer and do not invade surrounding tissue or spread to other parts of the body.
Malignant tumors are cancer.
These cells can invade surrounding tissue and spread, or metastasize, to other parts of the body. Lobular carcinomas originate in the small sacs in the breast that produce milk. Ductal carcinomas, the more common, begin in the lining of the ducts that carry milk from the lobules to the nipple. Within these two categories there are many more specific types of breast cancer. Seventy-five percent are invasive ductal cancers or combinations of invasive ductal cancers and other less common types.