The female breast has two main components; glandular tissue (lobes and ducts) and connective tissue. The breast is divided into 15 to 20 lobes that radiate outwards from the nipple and contain clusters of milk-producing glands. The lobes are further divided into smaller compartments called lobules. Each cluster drains into a duct, which connects the lobules and the nipple. The breast is held together by fatty connective tissue, which provides support and contains nerves as well as blood and lymphatic vessels.
Adenocarcinoma is a general term that refers to a cancer that starts in glandular tissues anywhere in the body. The majority of breast cancers start in glandular tissue and therefore are classified as adenocarcinomas. Those that originate in lobules are known as lobular carcinoma and those that begin in ducts are ductal carcinomas. The term "noninvasive breast cancer" refers to adenocarcinomas that are confined to lobules or ducts. Another term used to describe these cancers is in situ. Invasive breast cancer refers to a carcinoma that has spread from lobules or ducts to fatty connective tissue.