What Is Breast Cancer?

Having breast cancer means that some cells in your breast have changed and are growing out of control. Learning about the different types and stages of breast cancer can help you take an active role in your treatment.

Changes in your breast

Your entire body is made of living tissue. This tissue is made up of tiny cells. You can't see these cells with the naked eye. Normal cells divide (reproduce) in a controlled way. They grow when your body needs them, and die when your body doesn't need them any longer. When you have cancer, some cells change or become abnormal. These cells may divide quickly, don't die when they should, and can spread into other parts of the body.

Cross section of breast.

Cross section of breast showing noninvasive breast cancer.

Normal breast tissue is made of healthy cells. They reproduce new cells that look and work the same. The lobules make milk and the ducts carry it to the nipple.

Noninvasive breast cancer (carcinoma in situ) happens when cancer cells are only in the ducts or lobules.

Cross section of breast showing invasive breast cancer.

Outline of woman from head to mid-thigh. Arrows show breast cancer spreading to multiple organs.

Invasive breast cancer happens when cancer cells move out of the ducts or lobules into nearby breast tissue.

Metastasis happens when cancer cells move into the lymph nodes or bloodstream and travel to another part of the body.

Tests are used to measure the size of a tumor and learn how far it has spread. This is called staging. The stage of your cancer will help to decide which treatment plan is best for you. The stages of breast cancer are:

  • Stage 0. The cancer is noninvasive. Cancer cells are found only in the ducts (called ductal carcinoma in situ or DCIS).

  • Stage I. The tumor is 2 cm (about 3/4 of an inch) or less in diameter. It has invaded nearby breast tissue, and tiny amounts of cancer cells may be found in the underarm lymph nodes.

  • Stage II. The tumor is any size and has not spread to lymph nodes. Or the cancer is less than 5 cm across and has spread to lymph nodes.

  • Stage III. The tumor is any size, and there's cancer in your underarm lymph nodes, or it has spread to other lymph nodes. Or the tumor is any size and has spread to the skin, chest wall, and maybe to nearby lymph nodes.

  • Stage IV. The tumor has spread beyond the breast to the bones, lungs, liver, brain, or lymph nodes far away from the breast.

Recurrent breast cancer is cancer that comes back after treatment.