Over most of the body there is a small amount of fat between the surface skin and the underlying muscles. Rather than being a single gland, each breast consists of from 15 to 20 lobes, or segments, and the duct of each lobe opens separately on the nipple. At the very fine ends of the branches are the glands which produce milk and empty it into the duct system. Cancer is a disease in which abnormal cells in some organs or tissue go out of control, growing and increasing in number. Normal cells reproduce themselves throughout life, but in an orderly and controlled manner.
Normal growth occurs, worn out tissues are replaced and wounds heal.
When cells grow out of control and form a mass, the mass is called a tumor. Other tumors not only enlarge locally but also have the potential to invade and destroy the normal tissue around them and to spread to distant parts of the body. Such tumors are called malignant tumors, or cancer. Cancer of the breast can have all the features which are characteristics of cancer in general. The tumour usually arises from the cells of milk sacs or milk ducts, although malignant tumours occasionally arise from the supporting structure.