Breast cancer

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Risk factors for breast cancer

A risk factor is something (such as a behaviour, substance or condition) that increases the risk of developing cancer. Most cancers are the result of many risk factors, but sometimes breast cancer develops in women who don’t have any of the risk factors described below.

Most breast cancers occur in women. The main reason women develop breast cancer is because their breast cells are exposed to the female hormones estrogen and progesterone. These hormones, especially estrogen, are linked 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. Breast cancer mostly occurs in women between the ages of 50 and 69.

Risk factors are generally listed in order from most to least important. But in most cases, it is impossible to rank them with absolute certainty.

*You may wonder about antiperspirants, deodorants, abortion, breast implants and bras. There is significant evidence showing that there is no association between these factors and breast cancer.

Known risk factors

There is convincing evidence that the following factors increase your risk for breast cancer.

Personal history of breast cancer

Women who had breast cancer in the past have a higher risk of developing breast cancer again. The new primary breast cancer can develop in the same breast as the first cancer or in the opposite breast. While women who had ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS) have a higher risk of developing a second breast cancer, most women who had these cancers do not develop breast cancer again.

Find out more about ductal carcinoma in situ (DCIS) and lobular carcinoma in situ (LCIS).

Family history of breast and other cancers

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, genes passed from parents to children or a combination of these factors.

The risk of developing breast cancer is higher if:

  • one or more first-degree relatives (such as a mother, sister or daughter) had breast cancer, especially if they were diagnosed before menopause
  • second-degree relatives (such as a grandmother, aunt or niece) from either the mother’s or the father’s side of the family had breast cancer
  • a relative had cancer in both breasts (called 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

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.The risk with second-degree relatives is not as much as the risk with first-degree relatives.

Find out more about genetic risk and cancer.

BRCA gene mutations

Genetic mutations are changes to a gene. Some gene changes can increase the risk of developing certain types of cancer. Inherited gene mutations are passed on from a parent to a child. Only a small number of breast cancers (about 5%–10%) are caused by an inherited gene mutation.

BRCA1 (breast cancer gene 1) and BRCA2 (breast cancer gene 2) are normally found in the body. They are called tumour suppressor genes because they appear to play a role in controlling the growth of cancer cells. Mutations in the BRCA1 or BRCA2 genes can affect them so they no longer control cancer growth. These mutations are rare. They occur in about 1 in 500 people.

Both men and women can inherit a mutated BRCA gene from either their mother or father. People who have the gene mutation can also pass it on to their children. If one parent has the mutation in 1 of the 2 copies of the BRCA gene, a child has a 50% chance of inheriting the gene mutation. This also means there is a 50% chance that a child will not inherit the gene mutation.

The likelihood that breast or ovarian cancer is associated with an inherited BRCA1 or BRCA2 mutation is highest in families that have:

  • a history of family members with breast or ovarian cancer
  • early onset breast cancer in 1 or more female relatives (before age 50)
  • both breast and ovarian cancer in a single relative
  • family members who develop cancer in both breasts (bilateral breast cancer)
  • a male relative with breast cancer

Studies have shown that women with inherited BRCA1 or BRCA2 gene mutations have up to an 85% chance of developing breast cancer in their lifetime. Women with these inherited mutations also have a higher risk of developing breast cancer at a younger age (usually before menopause) than other women. Women with a BRCA gene mutation also have a higher 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. Having BRCA gene mutations also increases a woman’s risk of developing ovarian cancer at any age.

If several of your family members develop breast cancer, talk to your doctor about hereditary breast cancer and genetic risk assessment.

Dense breasts

Breasts that have larger amounts of connective, gland and milk duct tissues than fatty tissue are considered dense. Breast density is an inherited trait. Some studies have shown that women with dense breast tissue in 75% or more of their breasts have a 4–6 times greater risk of breast cancer than women with little or no dense breast tissue.

Breast density can only be seen on a mammogram. Unfortunately, dense breasts also make a mammogram harder to read. On a mammogram, fatty tissue looks dark, while dense tissue looks white, like tumours, so it can hide a tumour.

Ashkenazi Jewish ancestry

Women of Ashkenazi Jewish descent have a higher 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, while only 1 in 500 women in the general population have it.

Rare genetic conditions

The following rare inherited genetic conditions have been linked with an increased risk for breast cancer.

Li-Fraumeni syndrome

People with Li-Fraumeni syndrome have an increased risk of developing certain types of cancer, including breast cancer, osteosarcoma, soft tissue sarcoma and leukemia. Most people with Li-Fraumeni syndrome have inherited a mutation in the TP53 gene, which is normally a tumour suppressor gene.

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 disease, have a mutation in the tumour suppressor gene PTEN. Cowden syndrome is a rare disease that makes it more likely that people will develop 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. This gene appears to normally 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 normally a tumour suppressor gene. When it is mutated, it increases breast cancer risk. CHEK2 mutations have been identified in some families with Li-Fraumeni syndrome.

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 the growth of breast cancer cells as well. The type of exposure and how long cells are exposed to estrogen affects the chances of breast cancer developing.

Early menarche

The start of menstruation is called menarche. Early menarche is when menstruation starts at an early age (11 or younger). Starting your period early means that your cells are exposed to estrogen and other hormones for a greater amount of time. This increases the risk of breast cancer.

Late menopause

Menopause occurs as the ovaries stop making hormones and the level of hormones (mainly estrogen and progesterone) in the body drops. This causes a woman to stop menstruating. If you enter menopause at a later age (after age 55), it means that your cells are exposed to estrogen and other hormones for a greater amount of time. 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. Early menopause has been linked with a lower risk of breast cancer.

Late pregnancy or no pregnancies

Pregnancy interrupts the exposure of breast cells to circulating estrogen. It also lowers the total number of menstrual cycles a woman has in her lifetime.

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.

Exposure to ionizing radiation

Women who have received radiation therapy to the chest, neck and armpit area (called the mantle radiation field) have a higher risk of developing breast cancer. This increased risk has been particularly noted in women who received treatment to these areas for Hodgkin lymphoma.

The risk of breast cancer is higher if mantle radiation therapy for Hodgkin lymphoma was given before the age of 30. The 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. But 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 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 have an increased risk of developing breast cancer.

Women exposed to ionizing radiation during for the atomic bomb blasts during the Second World War also have 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. The benefits of mammography outweigh the risks of exposure to radiation.

Hormone replacement therapy

Research shows that taking hormone replacement therapy (HRT) for a long time increases the risk of breast cancer. This is especially true for HRT that uses estrogen plus progestin (called combined HRT).

Researchers looked at the data from numerous studies. Their analysis showed that current or recent users of combined HRT for 5 years or longer have an increased risk of breast cancer.

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. The study also found that the risk was increased even with comparatively short-term use of combined HRT compared to a placebo. The increased risk appears to disappear a few years after stopping HRT.

The WHI study also showed that there was a significant drop in the rate of new cases of breast cancer from 2002 to 2004 among Canadian women aged 50–69 years. This drop 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.

Find out more about hormone replacement therapy (HRT) and the Women’s Health Initiative (WHI) study.

Oral contraceptives

Oral contraceptives that contain both estrogen and progesterone can slightly increase the risk for breast cancer, especially among women who have used oral contraceptives for 10 or more years. The increased risk disappears after the woman stops taking 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.

Atypical hyperplasia

Atypical hyperplasia is a benign 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.


Drinking alcohol increases a woman’s risk for breast cancer. Even low levels of alcohol consumption (just over 1 drink per day) can increase a woman’s risk. 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 lower levels of some essential nutrients that protect against cell damage, such as folate (a type of B vitamin), vitamin A and vitamin C.

Being obese

Obesity increases the risk of breast cancer in post-menopausal women. Studies have shown that women who have never taken hormone replacement therapy and who have a body mass index (BMI) of 31.1 or higher have a 2.5 times greater risk of developing breast cancer than those witha 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 overall dose of estrogen the breast tissue receives over time. 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 so increase the chance that breast cancer will develop.

Find out more about body weight and cancer risk.

High socio-economic status

Breast cancer risk is slightly higher for women with higher incomes. This may be because of lifestyle factors such as having children later in life or having fewer children. These lifestyle factors are linked to an increased risk of breast cancer.

Tall adult height

Research has shown that tall women have a slightly higher risk of developing 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 a higher risk of developing breast cancer before menopause, but more research is needed to confirm this.

Possible risk factors

The following factors have been linked with breast cancer, but there is not enough evidence to show 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 in both premenopausal and post-menopausal women. The research suggests that physical inactivity is probably a risk factor for breast cancer. A number of studies are currently 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 to show 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 the rate of new cases of breast cancer, death rates and the relationship between genetics and the risk of smoking.

Birth weight

Some evidence suggests that a greater birth weight may increase the risk of premenopausal breast cancer. A greater birth weight means the fetus is exposed to more maternal estrogen, which may increase the risk of developing breast cancer later in life

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 shifts, particularly night shifts, have a slightly higher risk of developing breast cancer. Other studies have shown no increased risk.

Some benign breast conditions

Most non-cancerous, or benign, breast conditions do not increase a woman’s risk of developing breast cancer. However, there may be a relationship between some non-cancerous breast conditions and a family history of breast cancer. Some non-cancerous breast conditions may slightly increase a woman’s risk of breast cancer if they are associated with an increased number (overgrowth) of cells. Non-cancerous breast conditions that may increase the risk for breast cancer include:

Unknown risk factors

It isn’t known whether or not the following factors are linked with breast cancer. It may be that researchers can’t show a definite link or that studies have had different results. Further study is needed to see if the following are risk factors for breast cancer:

  • diethylstilbestrol (DES)
  • diet, including diets that are low in vegetables and fruit or high in fat, soy, red meat or cadmium
  • thyroid disease
  • psychological or emotional stress
  • history of melanoma

Questions to ask your healthcare team

To make the decisions that are right for you, ask your healthcare team questions about risks.


Dr Lillian Sung Improving supportive care for children with cancer

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