Breast cancer is a malignant tumor that has developed from cells of the breast. A malignant tumor is a group of cancer cells that may invade surrounding tissues or spread to distant areas of the body. The disease occurs mostly in women, but men can get it, too.

The female breast is made up mainly of milk-producing glands called lobules, ducts, which are milk passages that connect the lobules to the nipple and fatty tissue and connective tissue surrounding the ducts and lobules, blood vessels and lymphatic vessels, called stroma. Most breast cancers begin in the ducts, some in the lobules, and the rest in other tissues.

Lymphatic vessels are like veins, except that they carry lymph instead of blood. Lymph is a clear fluid that contains tissue fluid and waste products and immune system cells. Lymph nodes are small bean-shaped collections of immune system cells that are found along lymphatic vessels. Cancer cells can enter lymphatic vessels and spread to lymph nodes.

Almost all lymphatic vessels in the breast connect to lymph nodes under the arm. Some lymphatic vessels connect to lymph nodes inside the chest and either above or below the collar bone.

When breast cancer cells reach the axillary or underarm lymph nodes, they may continue to grow, often causing the lymph nodes in that area to swell. If breast cancer cells have spread to the underarm lymph nodes, they are more likely to have spread to other organs of the body as well. This is why it is important to find out if breast cancer has spread to your axillary lymph nodes when you are choosing a treatment.