What is breast cancer, and what type do I have?

Cancer is a disease in which cells become abnormal and form more cells in an uncontrolled way. With breast cancer, the cancer begins in the tissues that make up the breasts. The cancerous cells may form a mass of tissue called a malignant tumor. The cells of a malignant tumor may spread to other parts of the body and threaten life. (Benign [buh-NYN] tumors are abnormal growths that are not cancer. The cells of a benign tumor do not spread to other parts of the body and do not threaten life.)
The most common types of breast cancer are:

• Lobular carcinoma (LAH-byuh-luhr KAR-sih-NOH-muh) — Cancer that begins in the glands of the breast that make milk. These milk-making glands are called lobules. About 1 in 10 breast cancers are this type.
• Ductal carcinoma — Cancer that begins in the milk ducts of the breast. Milk ducts are thin tubes that carry milk from the lobules to the nipple. About 8 in 10 breast cancers are this type.

In addition, there are two types of breast tumors that are not cancer but increase the risk of breast cancer:

• Ductal carcinoma in situ (DCIS) — DCIS is a condition in which abnormal cells are found in the lining of breast ducts. These cells have not spread outside the duct to the surrounding breast tissue. But some cases of DCIS become breast cancer over time. So DCIS is sometimes called Stage 0 breast cancer. Since it’s not possible to know which cases of DCIS will become breast cancer, it’s important to get treatment for DCIS. Women with DCIS often are treated with breast-sparing surgery and radiation therapy. Radiation therapy lowers the chance that DCIS will come back or develop into breast cancer. If a large area of DCIS is found or it is found in more than one location, some women will choose to have a mastectomy. Underarm lymph nodes usually are not removed in the treatment of DCIS. The drug tamoxifen, which stops the growth of breast tumors that depend on estrogen, is also sometimes used in the treatment of DCIS. Tamoxifen may decrease the risk of a breast cancer developing in the same breast after treatment or in the opposite breast.

• Lobular carcinoma in situ (LCIS) — LCIS is a condition in which abnormal cells are found in breast lobules. In contrast with DCIS, there is no evidence that the abnormal cells will become cancerous. However, having LCIS means that a woman has an increased risk of developing breast cancer in either breast. Despite this increased risk, most women with LCIS will never get breast cancer. Most women with LCIS are followed closely with regular checkups and mammograms. Some women choose to take tamoxifen to decrease their risk of developing breast cancer. Rarely, women with LCIS choose to have both breasts removed as a preventive measure, but most doctors think this approach is inappropriate.