Breast cancer is a disease in which malignant (cancer) cells form in the tissues of the breast.

The breast is made up of lobes and ducts. Each breast has 15 to 20 sections called lobes, which have many smaller sections called lobules. Lobules end in dozens of tiny bulbs that can produce milk. The lobes, lobules, and bulbs are linked by thin tubes called ducts.

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Anatomy of the female breast. The nipple and areola are shown on the outside of the breast. The lymph nodes, lobes, lobules, ducts, and other parts of the inside of the breast are also shown.

Each breast also has blood vessels and lymph vessels. The lymph vessels carry an almost colorless fluid called lymph. Lymph vessels lead to organs called lymph nodes. Lymph nodes are small bean-shaped structures that are found throughout the body. They filter lymph and store white blood cells that help fight infection and disease. Clusters of lymph nodes are found near the breast in the axilla (under the arm), above the collarbone, and in the chest.

See the following PDQ summaries for more information about breast cancer:

• Breast Cancer Screening
• Breast Cancer Treatment
• Genetics of Breast and Ovarian Cancer

Breast cancer is the second most common type of cancer in American women.

Women in the United States get breast cancer more than any other type of cancer except skin cancer. The number of new cases of breast cancer has stayed about the same since 2003. Breast cancer is second to lung cancer as a cause of cancer death in American women. However, deaths from breast cancer have decreased a little bit every year for the past several years. Breast cancer also occurs in men, but the number of new cases is small.

Avoiding risk factors and increasing protective factors may help prevent cancer.

Avoiding cancer risk factors may help prevent certain cancers. Risk factors include smoking, being overweight, and not getting enough exercise. Increasing protective factors such as quitting smoking, eating a healthy diet, and exercising may also help prevent some cancers. Talk to your doctor or other health care professional about how you might lower your risk of cancer.

NCI's Breast Cancer Risk Assessment Tool uses a woman's risk factors to estimate her risk for breast cancer during the next five years and up to age 90. This online tool is meant to be used by a health care provider. For more information on breast cancer risk, call 1-800-4-CANCER.

The following risk factors may increase the risk of breast cancer:

Estrogen (endogenous)

Endogenous estrogen is a hormone made by the body. It helps the body develop and maintain female sex characteristics. Being exposed to estrogen over a long time may increase the risk of breast cancer. Estrogen levels are highest during the years a woman is menstruating. A woman's exposure to estrogen is increased in the following ways:

• Early menstruation: Beginning to have menstrual periods at age 11 or younger increases the number of years the breast tissue is exposed to estrogen.
• Late menopause: The more years a woman menstruates, the longer her breast tissue is exposed to estrogen.
• Late pregnancy or never being pregnant: Because estrogen levels are lower during pregnancy, breast tissue is exposed to more estrogen in women who become pregnant for the first time after age 35 or who never become pregnant.

Hormone replacement therapy/Hormone therapy

Hormones that are made outside the body, in a laboratory, are called exogenous hormones. Estrogen, progestin, or both may be given to replace the estrogen no longer produced by the ovaries in postmenopausal women or women who have had their ovaries removed. This is called hormone replacement therapy (HRT) or hormone therapy (HT) and may be given in one of the following ways:

• Combination HRT/HT is estrogen combined with progesterone or progestin. This type of HRT/HT increases the risk of developing breast cancer. Women taking combination HRT/HT also may be more likely to have an abnormal mammogram. Studies show that when women stop taking estrogen combined with progesterone, the risk of getting breast cancer decreases.
• Estrogen-only therapy may be given to women who have had a hysterectomy. Clinical trials studying whether estrogen-only therapy affects the risk of breast cancer have had mixed results. In women who have a uterus, estrogen-only therapy increases the risk of uterine cancer.

Exposure to Radiation

Radiation therapy to the chest for the treatment of cancers increases the risk of breast cancer, starting 10 years after treatment and lasting for a lifetime. The risk of developing breast cancer depends on the dose of radiation and the age at which it is given. The risk is highest if radiation treatment was used during puberty. For example, radiation therapy used to treat Hodgkin disease by age 16, especially radiation to the chest and neck, increases the risk of breast cancer.

Radiation therapy to treat cancer in one breast does not appear to increase the risk of developing cancer in the other breast.

For women who are at risk of breast cancer due to inherited changes in the BRCA1 and BRCA2 genes, exposure to radiation, such as that from chest x-rays, may further increase the risk of breast cancer, especially in women who were x-rayed before 20 years of age.

Obesity

Obesity increases the risk of breast cancer in postmenopausal women who have not used hormone replacement therapy.

Alcohol

Drinking alcohol increases the risk of breast cancer. The level of risk rises as the amount of alcohol consumed rises.

Inherited Risk

Women who have inherited certain changes in the BRCA1 and BRCA2 genes have a higher risk of breast cancer, and the breast cancer may develop at a younger age.

The following protective factors may decrease the risk of breast cancer:

Exercise

Exercising four or more hours a week may decrease hormone levels and help lower breast cancer risk. The effect of exercise on breast cancer risk may be greatest in premenopausal women of normal or low weight. Care should be taken to exercise safely, because exercise carries the risk of injury to bones and muscles.

Estrogen (decreased exposure)

Decreasing the length of time a woman's breast tissue is exposed to estrogen may help prevent breast cancer. Exposure to estrogen is reduced in the following ways:

• Pregnancy: Estrogen levels are lower during pregnancy. The risk of breast cancer appears to be lower if a woman has her first full-term pregnancy before she is 20 years old.
• Breast-feeding: Estrogen levels may remain lower while a woman is breast-feeding.
• Ovarian ablation: The amount of estrogen made by the body can be greatly reduced by removing one or both ovaries, which make estrogen. Also, drugs may be taken to lower the amount of estrogen made by the ovaries.
• Late menstruation: Beginning to have menstrual periods at age 14 or older decreases the number of years the breast tissue is exposed to estrogen.
• Early menopause: The fewer years a woman menstruates, the shorter the time her breast tissue is exposed to estrogen.

Selective estrogen receptor modulators

Selective estrogen receptor modulators (SERMs) are drugs that act like estrogen on some tissues in the body, but block the effect of estrogen on other tissues. Tamoxifen is a SERM that belongs to the family of drugs called antiestrogens. Antiestrogens block the effects of the hormone estrogen in the body. Tamoxifen lowers the risk of breast cancer in women who are at high risk for the disease. This effect lasts for several years after drug treatment is stopped.

Taking tamoxifen increases the risk of developing other serious conditions, including endometrial cancer, stroke, cataracts, and blood clots, especially in the lungs and legs. The risk of developing these conditions increases with age. Women younger than 50 years who have a high risk of breast cancer may benefit the most from taking tamoxifen. Talk with your doctor about the risks and benefits of taking this drug.

Raloxifene is another SERM that helps prevent breast cancer. In postmenopausal women with osteoporosis (decreased bone density), raloxifene lowers the risk of breast cancer for women at both high risk and low risk of developing the disease. It is not known if raloxifene would have the same effect in women who do not have osteoporosis. Like tamoxifen, raloxifene may increase the risk of blood clots, especially in the lungs and legs, but does not appear to increase the risk of endometrial cancer.

Other SERMs are being studied in clinical trials.

Aromatase inhibitors

Aromatase inhibitors lower the risk of a new breast cancer in women who have an increased risk of breast cancer. Women with an increased risk of breast cancer include the following:

• Postmenopausal women with a personal history of breast cancer.
• Women with no personal history of breast cancer who are 60 years and older, have a history of ductal carcinoma in situ with mastectomy, or have a high risk of breast cancer based on the Gail model tool (considers many risk factors for breast cancer).

In women with an increased risk of breast cancer, taking aromatase inhibitors decreases the amount of estrogen made by the body. Before menopause, estrogen is made by the ovaries and other tissues in a woman's body, including the brain, fat tissue, and skin. After menopause, the ovaries stop making estrogen, but the other tissues do not. Aromatase inhibitors block the action of an enzyme called aromatase, which is used to make all of the body's estrogen.

Possible harms from taking aromatase inhibitors include muscle and joint pain, osteoporosis, hot flashes, and feeling very tired.

Prophylactic mastectomy

Some women who have a high risk of breast cancer may choose to have a prophylactic mastectomy (the removal of both breasts when there are no signs of cancer). The risk of breast cancer is lowered in these women. However, it is very important to have a cancer risk assessment and counseling about all options for possible prevention before making this decision. In some women, prophylactic mastectomy may cause anxiety, depression, and concerns about body image.

Prophylactic oophorectomy

Some women who have a high risk of breast cancer may choose to have a prophylactic oophorectomy (the removal of both ovaries when there are no signs of cancer). This decreases the amount of estrogen made by the body and lowers the risk of breast cancer. However, it is very important to have a cancer risk assessment and counseling before making this decision. The sudden drop in estrogen levels may cause the onset of symptoms of menopause, including hot flashes, trouble sleeping, anxiety, and depression. Long-term effects include decreased sex drive, vaginal dryness, and decreased bone density. These symptoms vary greatly among women.

Fenretinide

Fenretinide is a type of vitamin A called a retinoid. When given to premenopausal women who have a history of breast cancer, fenretinide may lower the risk of forming a new breast cancer. Taken over time, fenretinide may cause night blindness and skin disorders. Women must avoid pregnancy while taking this drug because it could harm a developing fetus.

The following have been proven not to be risk factors for breast cancer or their effects on breast cancer risk are not known:

Abortion

There does not appear to be a link between abortion and breast cancer.

Oral Contraceptives

Taking oral contraceptives ("the pill") may slightly increase the risk of breast cancer in current users. This risk decreases over time. The most commonly used oral contraceptive contains estrogen.

Progestin-only contraceptives that are injected or implanted do not appear to increase the risk of breast cancer.

Environment

Studies have not proven that being exposed to certain substances in the environment (such as chemicals, metals, dust, and pollution) increases the risk of breast cancer.

Diet

Diet is being studied as a risk factor for breast cancer. It is not proven that a diet low in fat or high in fruits and vegetables will prevent breast cancer. For more information on diet and health, see the Fruits and Veggies Web site.

Active and passive cigarette smoking

It has not been proven that either active cigarette smoking or passive smoking (inhaling secondhand smoke) increases the risk of developing breast cancer.

Statins

Studies have not found that taking statins (cholesterol -lowering drugs) affects the risk of breast cancer.

Cancer prevention clinical trials are used to study ways to prevent cancer.

Cancer prevention clinical trials are used to study ways to lower the risk of developing certain types of cancer. Some cancer prevention trials are conducted with healthy people who have not had cancer but who have an increased risk for cancer. Other prevention trials are conducted with people who have had cancer and are trying to prevent another cancer of the same type or to lower their chance of developing a new type of cancer. Other trials are done with healthy volunteers who are not known to have any risk factors for cancer.

The purpose of some cancer prevention clinical trials is to find out whether actions people take can prevent cancer. These may include exercising more or quitting smoking or taking certain medicines, vitamins, minerals, or food supplements.

New ways to prevent breast cancer are being studied in clinical trials.

Clinical trials are taking place in many parts of the country. Information about clinical trials can be found in the Clinical Trials section of the NCI Web site. Check NCI's list of cancer clinical trials for breast cancer prevention trials that are now accepting patients.