Breast Cancer Prevention (PDQ®)

As a National Cancer Institute (NCI)-designated Comprehensive Cancer Center, a core part of our mission is to educate patients and the community about cancer. The following summary is trusted information from the NCI.

What is prevention?

Cancer prevention is action taken to lower the chance of getting cancer. By preventing cancer, the number of new cases of cancer in a group or population is lowered. Hopefully, this will lower the number of deaths caused by cancer.

To prevent new cancers from starting, scientists look at risk factors and protective factors. Anything that increases your chance of developing cancer is called a cancer risk factor; anything that decreases your chance of developing cancer is called a cancer protective factor.

Some risk factors for cancer can be avoided, but many cannot. For example, both smoking and inheriting certain genes are risk factors for some types of cancer, but only smoking can be avoided. Regular exercise and a healthy diet may be protective factors for some types of cancer. Avoiding risk factors and increasing protective factors may lower your risk but it does not mean that you will not get cancer.

Different ways to prevent cancer are being studied, including:

  • Changing lifestyle or eating habits.
  • Avoiding things known to cause cancer.
  • Taking medicines to treat a precancerous condition or to keep cancer from starting.

General Information About Breast Cancer

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.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:

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 decreased from 1999 to 2006. 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.

Breast Cancer Prevention

Avoiding cancer risk factors such as smoking, being overweight, and lack of exercise may help prevent certain cancers. 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, see Estimating Breast Cancer Risk: Questions and Answers or call 1-800-4-CANCER.

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.

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. 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. It is not known if this type of HRT/HT affects the risk of breast cancer. In women who have a uterus, estrogen-only therapy increases the risk of uterine cancer.

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

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

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.

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.

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 (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 lower the risk of new breast cancers in postmenopausal women with a history of breast cancer. In postmenopausal women, 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 osteoporosis and effects on brain function (such as talking, learning, and memory).

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.

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 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.

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

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.

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 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 website.

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

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 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.

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 PDQ Cancer Clinical Trials Registry for breast cancer prevention trials that are now accepting patients.



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