What is cancer?

Cancer starts when cells in the body change (mutate) and grow out of control. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them, and die when your body doesn't need them any longer. Cancer is made up of abnormal cells that grow even though your body doesn’t need them. In most types of cancer, the abnormal cells grow to form a lump or mass called a tumor.

Understanding the breast

The breast is made up of lobules and ducts. The lobules are the glands that can make milk. The ducts are thin tubes that carry the milk from the lobules to the nipple. The breast is also made of fat, connective tissue, lymph nodes, and blood vessels.

What is breast cancer?

Breast cancer is cancer that starts in cells in the breast. The ducts and the lobules are the 2 parts of the breast where cancer is most likely to start. 

Breast cancer is one of the most common types of cancer in women in the U.S. Healthcare providers don't yet know exactly what causes it. Once breast cancer forms, cancer cells can spread to other parts of the body (metastasize), making it life-threatening. The good news is that breast cancer is often found early, when it's small and before it has spread.

There are many types of breast cancer. These are the most common types:

• Ductal carcinoma.  This is the most common type. It starts in the lining of the milk ducts. When breast cancer has not spread outside of the ducts, it's called ductal carcinoma in situ (DCIS) or intraductal carcinoma. This is the most common type of noninvasive breast cancer. Invasive ductal carcinoma is breast cancer that has spread beyond the walls of the breast ducts. It's the most common type of invasive breast cancer.

• Invasive lobular carcinoma. This type starts in the milk-producing glands (lobules) and spreads outside the lobules.

Here are a few less common types that you may hear about:

• Paget disease.  This is a very rare form of breast cancer that starts in the glands in the skin of the nipple. It grows slowly and occurs in only 1 nipple. Most people with Paget disease also have tumors in the same breast. This type causes symptoms that are like a skin infection, such as inflammation, redness, oozing, crusting, itching, and burning.

• Inflammatory breast cancer. This is a rare form of invasive breast cancer. Often there is no lump or tumor. Instead, this cancer makes the skin of the breast look red and feel warm. The breast skin also looks thick and pitted, like an orange peel. It tends to be found in younger women and grows and spreads quickly.

• Triple negative breast cancer.  This is a type of breast cancer that doesn’t have estrogen receptors and progesterone receptors. It also doesn’t have an excess of the HER2 protein on the cancer cell surfaces. This type of breast cancer is most often found in younger women and in African-American women. It tends to grow and spread faster than most other types of breast cancer. Because these cancer cells don't have hormone receptors or excess HER2, medicines that target these changes don't work. The most common kind is triple-negative invasive ductal carcinoma.

How breast cancer spreads

Breast cancer can spread by growing into nearby tissues in the breast. It can also spread when the cancer cells get into and travel through the blood or lymph systems. When this happens, cancer cells may be found in nearby lymph nodes, such as in the armpit. These lymph nodes are called axillary lymph nodes. They are often checked for cancer as part of the diagnosis process. If the cancer reaches these nodes, it may have spread to other parts of the body.

Breast cancer that has spread from the breast to other organs of the body is called metastatic breast cancer. When breast cancer spreads, it most often goes to the brain, bones, liver, or lungs.

A key factor in making a breast cancer diagnosis is finding out if it has spread:

• Noninvasive (in situ) cancer is only in the ducts and hasn’t spread to nearby areas. If not treated, it can grow into a more serious, invasive type of cancer over time. If you are diagnosed with noninvasive ductal carcinoma, your chances of surviving are very high if you don’t wait to treat it.

• Invasive (infiltrating) cancer has the potential to spread to nearby areas. This type is much more serious than noninvasive cancer. When it starts to spread, it often invades nearby lymph nodes first. It can then spread to other parts of your body through your bloodstream and lymphatic system. Treatment for invasive cancer is often a more difficult, long-term process.

Talking with your healthcare provider

If you have questions about breast cancer, talk with your healthcare provider. Your healthcare provider can help you understand more about this cancer.