What is breast cancer?

The breast consists of milk producing tissue called lobules, connected to the nipple by small ducts. The ducts and lobules are lined by a layer of cells that are gradually replaced to maintain a healthy lining.

Normal body cells have a uniform size and shape. Cancer develops when some of these cells stop obeying the body's instructions and start to multiply abnormally. A cancer starts from a single abnormal cell that produces millions of copies of itself.

Initially the abnormal cells are confined within the lining of the duct. This is called ductal carcinoma in situ.(DCIS)

After some time cancer cells may start to move out of the duct into the surrounding tissue. Once cells move from their normal location inside the duct we describe this as invasive breast cancer.

The cells heap up to form a lump and they also continue to move through the tissue.

Cells enter the small lymph vessels (shown in green) and veins running through the breast. Lymph nodes are filters connected to the lymph channels and are able to trap cancer cells. They are the first site at which cancer cells may be seen beyond the breast.

Eventually cancer cells bypass the lymph nodes and travel to distant parts of the body. They land up in areas that have a good blood supply where they multiply in great numbers.

Common sites are the liver, lungs and bone marrow.