Any substance or condition that increases cancer risk is referred to as a risk factor. There isn't a known, single cause of breast cancer. Most cancers are the result of many risk factors. However, some women with breast cancer do not have any identifiable risk factors.
*Risk factors are generally listed in order from most significant to least significant. In most cases, it is impossible to rank the relative significance of individual risk factors with absolute certainty.
Most breast cancers occur in women. The main reason women develop breast cancer is because of the effects of hormones – the breast cells of women are exposed to the female hormones estrogen and progesterone. These hormones, especially estrogen, are associated with breast cancer and encourage the growth of some breast cancers.
Breast cancer is more common in high income, developed countries such as Canada, the United States and some European countries like Great Britain.
The risk of developing breast cancer increases with age. The longer a woman lives, the greater her chances of developing breast cancer. Breast cancer mostly occurs in women between the ages of 50 and 69.
The following factors are known to increase the risk of developing breast cancer.
Personal history of breast cancer
Women who have had breast cancer are at an increased risk of developing breast cancer again. This new primary breast cancer can develop in the same breast where the first cancer started or in the opposite breast.
History of carcinoma in situ
Women who have a history of carcinoma in situ are at an increased risk of developing breast cancer:
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Family history of breast cancer
A family history of breast cancer means that one or more close blood relatives have or had breast cancer. Some families have more cases of breast cancer than would be expected by chance. Sometimes it is not clear whether the family's pattern of cancer is due to chance, shared lifestyle factors, an inherited (hereditary) factor that has been passed on from parents to children through genes or a combination of these.
The risk of developing breast cancer is increased if:
- one or more first-degree relatives (such as a mother, sister or daughter) had breast cancer, especially if they were diagnosed before menopause
- Having one first-degree relative with breast cancer approximately doubles a woman's risk.
- The more first-degree relatives with breast cancer, the greater the risk.
- second-degree relatives (such as a grandmother, aunt or niece) from either side of the family had breast cancer
- The risk with second-degree relatives is not as much as the risk with first-degree relatives.
- a relative had cancer in both breasts (bilateral breast cancer) before menopause
- 2 or more relatives had breast cancer or ovarian cancer
- a relative had both breast cancer and ovarian cancer
- a male relative had breast cancer
- a relative had other multiple cancers
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Dense breasts
Breasts that have larger amounts of connective, gland and milk duct tissues than fatty tissue are considered dense. Breast density can only been seen on a mammogram. On a mammogram, fatty tissue looks dark, while dense tissue looks white, like tumours, so it can hide a cancer.
Women with dense breasts have a 4 to 6 times greater risk of breast cancer compared to women with little or no dense breast tissue.
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BRCA gene mutations
Genetic mutations are changes to a gene, which can increase the risk of developing cancer. Inherited gene mutations are passed on from a parent to a child. Only a small portion of breast cancers (about 5–10%) are caused by inheriting a gene mutation.
Mutations in BRCA1 (breast cancer gene 1) and BRCA2 (breast cancer gene 2) make some women more susceptible to developing breast cancer. These genes appear to play a role in controlling the growth of cancer cells. They are called tumour suppressor genes.
BRCA1 and BRCA2 mutations are rare, occurring in approximately 1 in 500 people. Studies have shown that:
- Women with inherited BRCA1 or BRCA2 gene mutations have up to an 80% lifetime risk of developing breast cancer.
- Women with an inherited mutation of these genes have an increased risk of developing breast cancer at a younger age (usually before menopause) compared to other women.
- Women with a BRCA gene mutation also have an increased risk of developing cancer in both breasts (bilateral breast cancer). If they develop cancer in one breast, they have an increased risk of developing cancer in the other breast.
- BRCA gene mutations also increase the risk of a woman developing ovarian cancer at any age.
Hereditary breast cancer caused by a genetic mutation may be suspected if several family members develop breast cancer. Women who are concerned about hereditary breast cancer should talk to their doctor. Genetic risk assessment and genetic testing may be an option for some women.
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Ashkenazi Jewish ancestry
Women of Ashkenazi Jewish descent (Eastern European ancestry) have an increased risk of developing breast cancer. This is because BRCA1 and BRCA2 gene mutations are more common in Ashkenazi women. About 1 in 40 Ashkenazi Jewish women carry a BRCA gene mutation, compared with 1 in 500 in the general population.
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Rare genetic conditions
There are some rare inherited genetic conditions that are associated with an increased breast cancer risk:
- Li-Fraumeni syndrome – People with Li-Fraumeni syndrome have an increased risk of developing several specific types of cancer, such as breast cancer, osteosarcoma, soft tissue sarcoma and leukemia. Generally, people with Li-Fraumeni syndrome have inherited a mutation in the tumour suppressor gene TP53.
- ataxia-telangiectasia (AT) – People with AT have a mutation of the ATM gene. This gene is responsible for repairing damaged DNA. Certain families with a high rate of breast cancer have been found to have mutations of this gene.
- Cowden syndrome – People with Cowden syndrome (or Cowden's disease) have a mutation in the tumour suppressor gene PTEN. Cowden syndrome is a rare disease that predisposes people to breast cancer, gastrointestinal cancers and thyroid cancer.
- Peutz-Jeghers syndrome – People with Peutz-Jeghers syndrome may have a mutation of the STK11 (also known as LKB1) gene, which appears to function as a tumour suppressor gene. This disorder increases the risk of developing gastrointestinal, breast, ovarian and testicular cancers.
- CHEK2 gene mutation – The CHEK2 gene is a tumour suppressor gene that increases breast cancer risk when it is mutated. CHEK2 mutations have been identified in some families with Li-Fraumeni syndrome.
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Reproductive history
Estrogen is the main hormone associated with breast cancer. Estrogen affects the growth of breast cells. It is believed to play an important role in breast cancer cell growth. The type and duration of exposure to estrogen influences the chances of breast cancer developing.
Early menarche
Beginning to menstruate (menarche) at an early age (11 or younger) increases the length of time women are exposed to estrogen and other hormones. This increases the risk of breast cancer.
Late menopause
Menopause occurs as the ovaries stop producing hormones and the level of hormones (mainly estrogen and progesterone) in the body drops. This causes a woman to stop menstruating. Later age at menopause (after age 55) increases the length of time women are exposed to estrogen and other hormones. This increases the risk of breast cancer. Likewise, menopause at a younger age decreases the length of time breast tissue is exposed to estrogen and other hormones and has been associated with a lower risk of breast cancer.
Late pregnancy or no pregnancies
Pregnancy interrupts the exposure of breast cells to circulating estrogen and reduces the total number of lifetime menstrual cycles.
- Women who have their first full-term pregnancy after the age of 30 have a slightly higher risk of breast cancer than women who have at least one full-term pregnancy at an earlier age. Becoming pregnant at an early age (such as before age 20) reduces breast cancer risk.
- The more children a woman has, the greater the protection against breast cancer.
- Not becoming pregnant at all (called nulliparity) increases the risk of breast cancer.
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Exposure to ionizing radiation
Women who receive radiation therapy to the lymph nodes in the chest, neck and armpit (mantle radiation field) for Hodgkin lymphoma have an increased risk of breast cancer. The increase in risk is relative to the women's age during treatment.
The risk of breast cancer is increased if mantle radiation therapy for Hodgkin lymphoma was given before the age of 30. Risk is further increased if the radiation treatment was given during puberty. Breast cancer risk is greatly increased if chemotherapy is combined with radiation therapy to treat Hodgkin lymphoma before the age of 15 years. The benefit of treating the cancer usually far outweighs the risk of developing a second cancer from radiation therapy treatment.
In the past, medical radiation therapy was used to treat health problems as such as tuberculosis, postpartum mastitis, acne or an enlarged thymus gland. Women who were given medical radiation therapy to the chest area for these diseases are at an increased risk of developing breast cancer.
Women who were exposed to ionizing radiation during the atomic bomb blasts are also at an increased risk of developing breast cancer, especially if they were exposed during puberty.
Many women fear that regular mammography will increase their risk of breast cancer. Modern mammography equipment uses very low doses of radiation compared to the dose used for treating cancer.
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Hormone replacement therapy
Research shows that prolonged use of hormone replacement therapy (HRT), especially estrogen plus progestin (called combined HRT), increases the risk of breast cancer.
Data were pooled from numerous studies, and the analysis showed an increased risk of breast cancer with long-term (5 years or longer) use of combined HRT for current or recent users.
The findings of the Women's Health Initiative (WHI) study showed the risk of breast cancer went up by about 1% for every year that women took estrogen alone and about 8% for every year that they took combined HRT. Increased risk was even seen with comparatively short-term use of combined HRT compared to placebo. The increased risk appears to disappear a few years after stopping HRT.
A Canadian study showed that a significant drop in breast cancer incidence from 2002 to 2004 among Canadian women aged 50–69 coincided with a drop in combined HRT use. This was also seen in a number of other countries around the world, including the United States, Australia, Germany, the Netherlands, Switzerland and Norway.
Researchers now believe that the risks of long-term use of combined HRT outweigh the benefits.
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Hormonal contraceptives
Hormonal contraceptives, such as oral contraceptives, that contain both estrogen and progesterone cause a small increase in the risk of breast cancer, especially among women who have used oral contraceptives for 10 or more years. The increased risk disappears after stopping oral contraceptives. However, current and recent (less than 10 years since last use) users have a slight increased risk compared with women who have never used oral contraceptives.
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Atypical hyperplasia
Atypical hyperplasia is a condition where there is an increased number of abnormal (atypical) cells in the breast tissue.
- Atypical hyperplasia increases a woman's risk of developing breast cancer.
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Alcohol
Drinking alcohol increases a woman's risk of breast cancer. Even low levels of alcohol consumption (just over 1 drink per day) can increase a woman's risk of breast cancer. The risk increases with the amount of alcohol consumed.
One possible reason for this is that alcohol is thought to cause higher levels of estrogen. Alcohol may also decrease some essential nutrients that protect against cell damage, such as folate (a type of B vitamin), vitamin A and vitamin C.
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Being obese
Obesity increases the risk of breast cancer in post-menopausal women.
Studies have shown that women (who had never taken hormone replacement therapy) with a body mass index (BMI) of 31.1 or higher had a 2.5 times greater risk of developing breast cancer than those with a BMI of 22.6 or lower.
Ovarian hormones, estrogens in particular, play an important role in breast cancer. Many of the risk factors for breast cancer are believed to result from the cumulative dose of estrogen to the breast tissue. The ovaries make most of the body's estrogen, but after menopause fat tissue produces a small amount of estrogen. Having more fat tissue can increase estrogen levels and increase the chances of breast cancer developing.
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High socio-economic status
Breast cancer risk is slightly increased in women with higher incomes. This may be because they tend to start having children later in life or have fewer children. They may also be more likely to take hormone replacement therapy after menopause. These lifestyle factors are linked to an increased risk of breast cancer.
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Tall adult height
Research has shown that women who are tall have a small increased risk of breast cancer after menopause. It is thought that energy intake and diet early in life, which affect adult height, are the factors that increase the risk, rather than just being tall.
Tall women may also have an increased risk of developing breast cancer before menopause, but more research is needed to confirm this.
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Possible risk factors
The following factors have some association with breast cancer, but there is not enough evidence to say they are known risk factors. Further study is needed to clarify the role of these factors for breast cancer.
- physical inactivity – Studies show that greater physical activity is associated with a lower risk of breast cancer, for both premenopausal and post-menopausal breast cancer. The research evidence suggests very strongly that physical inactivity is a risk factor for breast cancer. There are a number of studies that are looking into the role of exercise in breast cancer.
- adult weight gain – There is consistent evidence from studies that adult weight gain is a probable cause of post-menopausal breast cancer. It is not certain whether reducing weight would reduce the risk of breast cancer.
- smoking and second-hand smoke – Research shows that there may be a link between smoking and second-hand smoke and breast cancer. Recent studies have shown that active smoking is related to breast cancer in both premenopausal and post-menopausal women. There is also a link between second-hand smoke and breast cancer, particularly in younger, mainly premenopausal women who have never smoked. There is not enough evidence regarding a link between second-hand smoke and breast cancer risk in post-menopausal women. More research is needed to determine the impact of active smoking and second-hand smoke on breast cancer incidence, death rates and the relationship between genetics and the risk of smoking.
- birth weight – Evidence suggests that exposures early in life may have important effects on cancer risk – such as high concentrations of maternal estrogens during pregnancy and increased breast cancer development later in life. Greater birth weight raises circulating maternal estrogen levels, exposing the fetus to more estrogen and possibly increasing the risk of premenopausal breast cancer.
- night shift work – It is thought that night work, and being exposed to artificial light, reduces the amount of melatonin in the body. In women, melatonin reduces the amount of estrogen in the body, and it may slow the growth of breast cancer cells. Some studies have suggested that women who work shift work, particularly at night, are at a slight increased risk of developing breast cancer. Other studies have shown no increased risk.
- some benign breast conditions – Most benign (non-cancerous) breast conditions do not increase a woman's risk of developing breast cancer. However, there may be a relationship between benign breast conditions, such as fibrocystic breast changes, and family history of breast cancer. Some benign breast conditions may slightly increase a woman's risk of breast cancer if they are associated with an increased number (overgrowth) of cells:
- hyperplasia
- complex fibroadenoma
- sclerosing adenosis
- papillomatosis
- radial scar
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Factors not associated with breast cancer
The following are not considered to be risk factors for breast cancer because there is enough/significant evidence showing that there is no association.
- antiperspirants and deodorants
- abortion
- stress
- breast implants
- bras
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Unknown risk factors
The following are factors for which there is not enough evidence or the evidence is inconclusive. In other words, it can't be determined for sure whether these risk factors are or are not associated with breast cancer.
- diethylstilbestrol (DES)
- high-fat diet
- soy
- environmental exposures
- a diet low in vegetables and fruit
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See a list of questions to ask your doctor about risks. Find out how to reduce the risk of breast cancer.