Breast Cancer

What are the causes of breast cancer?

Breast cancer is the most common cause of cancer in women and the second most common cause of cancer death in women in the U.S. While the majority of new breast cancers are diagnosed as a result of an abnormality seen on a mammogram, a lump, or change in consistency of the breast tissue can also be a warning sign of the disease. Heightened awareness of breast cancer risk in the past decades has led to an increase in the number of women undergoing mammography for screening, leading to detection of cancers in earlier stages and a resultant improvement in survival rates. Still, breast cancer is the most common cause of death in women between 45-55 years of age. Although breast cancer in women is a common form of cancer, male breast cancer does occur and accounts for about 1% of all cancer deaths in men.

Research has yielded much information about the causes of breast cancers, and it is now believed that genetic and/or hormonal factors are the primary risk factors for breast cancer. Staging systems have been developed to allow doctors to characterize the extent to which a particular cancer has spread and to make decisions concerning treatment options. Breast cancer treatment depends upon many factors, including the type of cancer and the extent to which it has spread. Treatment options for breast cancer may involve surgery (removal of the cancer alone or, in some cases, mastectomy), radiation therapy, hormonal therapy, and/or chemotherapy.

With advances in screening, diagnosis, and treatment, the death rate for breast cancer has declined. In fact, about 90% of women newly diagnosed with breast cancer will survive for at least five years. Research is ongoing to develop even more effective screening and treatment programs.

MedicineNet Medical Author: Melissa Conrad Stöppler, MD
MedicineNet Medical Editor: William C. Shiel Jr., MD, FACP, FACR


The following information is provided by the National Cancer Institute.

The breasts

Inside a woman's breast are 15 to 20 sections called lobes. Each lobe is made of many smaller sections called lobules. Lobules have groups of tiny glands that can make milk. After a baby is born, a woman's breast milk flows from the lobules through thin tubes called ducts to the nipple. Fat and fibrous tissue fill the spaces between the lobules and ducts.

The breasts also contain lymph vessels. These vessels are connected to small, round masses of tissue called lymph nodes. Groups of lymph nodes are near the breast in the underarm (axilla), above the collarbone, and in the chest behind the breastbone.

Picture of the anatomy of the breast
Picture of the anatomy of the breast

Cancer cells

Cancer begins in cells, the building blocks that make up tissues. Tissues make up the breasts and other parts of the body.

Normal cells grow and divide to form new cells as the body needs them. When normal cells grow old or get damaged, they die, and new cells take their place.

Sometimes, this process goes wrong. New cells form when the body doesn't need them, and old or damaged cells don't die as they should. The buildup of extra cells often forms a mass of tissue called a lump, growth, or tumor.

Tumors in the breast can be benign (not cancer) or malignant (cancer). Benign tumors are not as harmful as malignant tumors:

Benign tumors:

  • are rarely a threat to life


  • can be removed and usually don't grow back


  • don't invade the tissues around them


  • don't spread to other parts of the body

Malignant tumors:

  • may be a threat to life


  • often can be removed but sometimes grow back


  • can invade and damage nearby organs and tissues (such as the chest wall)


  • can spread to other parts of the body

Breast cancer cells can spread by breaking away from the original tumor. They enter blood vessels or lymph vessels, which branch into all the tissues of the body. The cancer cells may be found in lymph nodes near the breast. The cancer cells may attach to other tissues and grow to form new tumors that may damage those tissues.

The spread of cancer is called metastasis.




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