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What Is Breast Cancer?

Breast cancer is a malignant (cancerous) tumor that starts from cells of the breast. The disease occurs mostly in women, but men can get breast cancer as well. The information here refers only to breast cancer in women.

A woman’s breast is made up of lobules (glands that make breast milk), ducts (small tubes that connect lobules to the nipple), fatty and connective tissue, blood vessels, and lymph (“limf”) vessels. Most breast cancers begin in the ducts (ductal), some in the lobules (lobular), and the rest in other tissues.

Lymph vessels are like veins, except that they carry lymph fluid instead of blood. Lymph is a clear fluid that contains immune system cells and waste products. Lymph vessels lead to small, bean-shaped collections of tissue called lymph nodes. Most lymph vessels of the breast lead to lymph nodes under the arm. These are called axillary (AX-uh-lair-ee) nodes.

If breast cancer cells reach the underarm lymph nodes and continue to grow, they cause the nodes to swell. Once cancer cells have reached these nodes they are more likely to spread to other organs of the body as well.

Benign Breast Lumps

Most breast lumps are benign; that is, they are not cancer. Benign breast tumors are abnormal growths, but they do not spread outside of the breast and they are not life threatening. But some benign breast lumps can increase a woman’s risk of getting breast cancer.

Some lumps aren’t really tumors at all. These lumps are often caused by fibrocystic (fi-bro-SIS-tik) changes. Cysts are fluid-filled sacs. Fibrosis is the formation of scar-like tissue. Such changes can cause breast swelling and pain. The breasts may feel lumpy, and sometimes there is a clear or slightly cloudy nipple discharge. For more detail, the American Cancer Society (ACS) has a separate document on "Non-Cancerous Breast Conditions" that you can get through our toll-free number or on our Web site.

Main Types of Breast Cancer

Understanding key words as they relate to breast cancer can be a challenge. Here are the most common types of breast cancer:

Carcinoma in situ (in SIGH-to): This term is used for early stage cancer, when it is confined to the place where it started. In breast cancer, it means that the cancer is confined to the ducts or the lobules, depending on where it started. It has not gone into the fatty tissues in the breast nor spread to other organs in the body.

Ductal carcinoma in situ (DCIS): This is the most common type of noninvasive breast cancer. DCIS means that the cancer is confined to the ducts. It has not spread through the walls of the ducts into the fatty tissue of the breast. Nearly all women with cancer at this stage can be cured. The best way to find DCIS early is with a mammogram.

Lobular carcinoma in situ (LCIS): This condition begins in the milk-making glands but does not go through the wall of the lobules. Although not a true cancer, having LCIS increases a woman's risk of getting cancer later. For this reason, it's important that women with LCIS to follow the screening guidelines for breast cancer (see below).

Infiltrating (invasive) ductal carcinoma (IDC): This cancer starts in a milk passage or duct, breaks through the wall of the duct, and invades the fatty tissue of the breast. From there it can spread to other parts of the body. IDC is the most common type of breast cancer. It accounts for about 80% of invasive breast cancers.

Infiltrating (invasive) lobular carcinoma (ILC): This cancer starts in the milk glands or lobules. It can spread to other parts of the body. About 10% of invasive breast cancers are of this type.

There are also several other less common types of breast cancer. You can get information about these through our toll-free number or on our Web site.

American Cancer Society website: