Breast cancer starts when cells in the breast begin to grow out of control. These cells usually form a tumor that can often be seen on an x-ray or felt as a lump. The tumor is malignant (cancerous) if the cells can grow into (invade) surrounding tissues or spread (metastasize) to distant areas of the body. Breast cancer occurs almost entirely in women, but men can get it, too.

Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how all cancers start and spread, see What Is Cancer?

This information refers only to breast cancer in women. For information on breast cancer in men, see Breast Cancer in Men.

Breast cancers can start from different parts of the breast. Most breast cancers begin in the ducts that carry milk to the nipple (ductal cancers). Some start in the glands that make breast milk (lobular cancers). There are also other types of breast cancer that are less common.

A small number of cancers start in other tissues in the breast. These cancers are called sarcomas and lymphomasand are not really thought of as breast cancers.

Although many types of breast cancer can cause a lump in the breast, not all do. There are other symptoms of breast cancer you should watch out for and report to a health care provider.

It’s also important to understand that most breast lumps are not cancer, they are benign. 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. Any breast lump or change needs to be checked by a health care provider to determine whether it is benign or cancer, and whether it might impact your future cancer risk. For more information see “What are the risk factors for breast cancer?” and Non-cancerous Breast Conditions.

How Breast Cancer Spreads

Breast cancer can spread through the lymph system.

The lymph system includes lymph nodes, lymph vessels and lymph fluid found throughout the body. Lymph nodes are small, bean-shaped collections of immune system cells that are connected by lymph (or lymphatic) vessels. Lymph 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 lymph vessels and begin to grow in lymph nodes.

Most of the lymph vessels of the breast drain into:

• Lymph nodes under the arm (axillary nodes).
• Lymph nodes around the collar bone (supraclavicular and infraclavicular lymph nodes)
• Lymph nodes inside the chest near the breast bone (internal mammary lymph nodes)

If cancer cells have spread to your lymph nodes, there is a higher chance that the cells could have spread (metastasized) to other sites in your body. The more lymph nodes with breast cancer cells, the more likely it is that the cancer may be found in other organs as well. Because of this, finding cancer in one or more lymph nodes often affects your treatment plan. Usually, surgery to remove one or more lymph nodes will be needed to know whether the cancer has spread there.

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.