What is breast cancer?

What is breast cancer?

Breast cancer is a malignant tumor that starts in the cells of the breast. A malignant tumor
is a group of cancer cells that can grow into (invade) surrounding tissues or spread
(metastasize) to distant areas of the body. The disease occurs almost entirely in women,
but men can get it, too.
The remainder of this document refers only to breast cancer in women. For
information on breast cancer in men, see our document, Breast Cancer in Men.


The normal breast
To understand breast cancer, it helps to have some basic knowledge about the normal
structure of the breasts, shown in the diagram below.
The female breast is made up mainly of lobules (milk-producing glands), ducts (tiny
tubes that carry the milk from the lobules to the nipple), and stroma (fatty tissue and
connective tissue surrounding the ducts and lobules, blood vessels, and lymphatic
vessels).

Most breast cancers begin in the cells that line the ducts (ductal cancers). Some begin in
the cells that line the lobules (lobular cancers), while a small number start in other
tissues.