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), and the rest in other tissues.

Blood vessels (arteries and veins) carry plasma, red blood cells, white blood cells, and platelets to and from the breast. Lymphatic vessels are like veins, except that they carry lymph away from the breast, instead of blood. Lymph is a clear fluid that contains tissue fluid and waste products and immune system cells (cells that are important in fighting infections). Lymph nodes are small bean-shaped collections of immune system cells that are found along lymphatic vessels. Cancer cells can enter lymphatic vessels and begin to grow in lymph nodes. This becomes important when we talk about staging (see below).

Most lymphatic vessels in the breast connect to lymph nodes under the arm (axillary lymph nodes). Some lymphatic vessels connect to lymph nodes inside the chest (internal mammary nodes) and those either above or below the collarbone (supra- or infraclavicular nodes).

Knowing if the cancer cells have spread to lymph nodes is important because if it has, we know that there is a higher chance that the cells could have also gotten into the bloodstream and spread to other sites in the body. This spread is called metastasis. The more lymph nodes that are involved with the breast cancer, the more likely it is that the cancer will eventually be found in other organs as well. It is important to know if you have metastatic cancer when you are choosing a treatment plan. Not all women with lymph node involvement develop metastases, and it is not unusual for a woman to have negative lymph nodes and later develop metastases.