What is breast cancer?
What Is Breast Cancer?
Breast cancer is a malignant tumor that starts from cells of the breast. A malignant tumor is a group of cancer cells that may 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 the American Cancer Society document, Breast Cancer in Men (http://www.cancer.org/docroot/CRI/content/CRI_2_4_4x_How_Is_Breast_Cancer_in_Men_Treated.asp?rnav=cri).
The normal breast
To understand breast cancer, it helps to have some basic knowledge about the normal structure of the breasts.
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).
Diagram of the breast (Figure 1)
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.
The lymph (lymphatic) system
The lymph system is important to understand because it is one of the ways in which breast cancers can spread. This system has several parts.
Lymph nodes are small, bean-shaped collections of immune system cells (cells that are important in fighting infections) that are connected by lymphatic vessels. Lymphatic vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) away from the breast. Lymph contains tissue fluid and waste products, as well as immune system cells. Breast cancer cells can enter lymphatic vessels and begin to grow in lymph nodes.
Most lymphatic vessels in the breast connect to lymph nodes under the arm (axillary nodes). Some lymphatic vessels connect to lymph nodes inside the chest (internal mammary nodes) and those either above or below the collarbone (supraclavicular or infraclavicular nodes).
Diagram of the lymph nodes (Figure 2)
It is important to find out if the cancer cells have spread to lymph nodes because, if they have, there is a higher chance that the cells could have also gotten into the bloodstream and spread (metastasized) to other sites in the body. The more lymph nodes that have breast cancer, the more likely it is that the cancer may be found in other organs as well. This is important to know because it could affect your treatment plan. Still, not all women with cancer cells in their lymph nodes develop metastases, and some women can have no cancer cells in their lymph nodes and later develop metastases.
Benign breast lumps
Most breast lumps are not cancerous; that is, they are benign. Still, some may need to be sampled and viewed under a microscope to prove they are not cancer.
Fibrocystic changes
Most lumps turn out to be fibrocystic changes. The term fibrocystic refers to fibrosis and cysts. Fibrosis is the formation of scar-like (fibrous) tissue and cysts are fluid-filled sacs. Fibrocystic changes can cause breast swelling and pain. This often happens just before a woman’s menstrual period is about to begin. Her breasts may feel lumpy and, sometimes, she may notice a clear or slightly cloudy nipple discharge.
Other benign breast lumps
Benign breast tumors such as fibroadenomas or intraductal papillomas are abnormal growths, but they are not cancerous and do not spread outside of the breast to other organs. They are not life threatening. Still, some benign breast conditions are important because women with these conditions have a higher risk of developing breast cancer.
For more information see the section, “What are the risk factors for breast cancer?” and the separate American Cancer Society document, Non-Cancerous Breast Conditions.