What is breast cancer?

Breast cancer is a condition in which breast cells grow abnormally and divide without control or order. Most breast cancers start in the lining of the milk ducts; these are called ductal cancers. Less commonly, breast cancers start in the milk lobules; these are called lobular cancers.

Cancer cells often begin “in situ,” or confined to the milk ducts or lobules. Sometimes, they then grow outside of the milk ducts, a process called invasion or infiltration. Once the cells become invasive, they might spread beyond the breast to lymph nodes and other organs through the bloodstream, a process referred to as metastasis.