Breast cancer

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Risk factors for breast cancer

A risk factor is something that increases the risk of developing cancer. It could be a behaviour, substance or condition. 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. The risk of developing breast cancer increases with age. Breast cancer mostly occurs in women between 50 and 69 years of age.

Risk factors

Personal history of breast cancer

Family history of breast and other cancers

BRCA gene mutations

Certain genetic conditions

Other gene mutations

Ashkenazi Jewish ancestry

Dense breasts

Reproductive history

Exposure to ionizing radiation

Hormone replacement therapy

Oral contraceptives

Atypical hyperplasia

Alcohol

Being obese

Physical inactivity

High socio-economic status

Tall adult height

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 breast cancer can develop in the same breast as the first cancer or in the other breast. Women who had ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS) have a higher risk of developing a second breast cancer, but 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 linked with an inherited BRCA1 or BRCA2 mutation is highest in families that have:

  • a history of family members with breast or ovarian cancer
  • breast cancer that developed before age 50 in 1 or more female relatives
  • a single relative diagnosed with both breast and ovarian cancer
  • family members who develop cancer in both breasts
  • a male relative diagnosed with breast cancer

Studies show 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. If they develop cancer in one breast, they have a greater 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 testing.

Certain genetic conditions

The following rare inherited genetic conditions are linked with a higher risk for breast cancer.

Li-Fraumeni syndrome increases the 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) is caused by a mutation of the ATM gene. This gene is responsible for repairing damaged DNA. Certain families with a high rate of breast cancer have mutations of this gene.

Cowden syndrome is caused by a mutation in the PTEN gene, which is normally a tumour suppressor gene. People with this condition are more likely to develop breast cancer, gastrointestinal cancers and thyroid cancer.

Peutz-Jeghers syndrome may be related to a mutation of the STK11 (also known as LKB1) gene. This gene appears to normally function as a tumour suppressor gene. Peutz-Jeghers syndrome increases the risk of developing gastrointestinal, breast, ovarian and testicular cancers.

Other gene mutations

The following gene mutations are linked with a higher risk for breast cancer.

CHEK2 gene mutation has been identified in some families with Li-Fraumeni syndrome. It is normally a tumour suppressor gene. When it is mutated, it increases breast cancer risk.

PALB2 gene mutation means that it loses its function. It normally protects against breast cancer by helping repair changes to DNA and preventing tumour growth. When this gene loses its function, there is a higher risk for breast cancer.

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.

Dense breasts

Dense breasts have more connective tissue, glands and milk ducts than fatty tissue. Breast density is an inherited trait. Some studies show that women with dense breast tissue in 75% or more of their breasts are 4–6 times more likely to develop breast cancer than women with little or no dense breast tissue.

Breast density can only be seen on a mammogram, but 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.

Reproductive history

Estrogen is the main hormone associated with breast cancer. Estrogen affects the growth of breast cells. Experts believe that it plays 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 that breast cancer will develop.

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 for breast cancer. Likewise, menopause at a younger age decreases the length of time breast tissue is exposed to estrogen and other hormones. Early menopause is 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 for 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 developing 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, acne or an enlarged thymus gland. Women who were given medical radiation therapy to the chest area for these diseases have a greater risk of developing breast cancer.

Women exposed to ionizing radiation during atomic bomb blasts during the Second World War also have a higher risk of developing breast cancer, especially if they were exposed during puberty.

Many women fear that regular mammography will increase their risk for 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 a higher risk for breast cancer.

The Women’s Health Initiative (WHI) study showed the risk for 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 higher 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 trend 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 higher risk disappears after the woman stops taking oral contraceptives. However, current and recent (less than 10 years since last use) users have a slightly greater risk compared with women who have never used oral contraceptives.

Atypical hyperplasia

Atypical hyperplasia is a non-cancerous (benign) condition where there is a greater number of abnormal (atypical) cells in the breast tissue. Atypical hyperplasia increases a woman’s risk of developing breast cancer.

Alcohol

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 the link between alcohol and breast cancer 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 vitamin B), vitamin A and vitamin C.

Being obese

Obesity increases the risk for breast cancer in post-menopausal women. Studies show 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 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 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.

Physical inactivity

Physical inactivity increases the risk of breast cancer in both premenopausal and post-menopausal women. A number of studies are currently looking into the role of exercise in breast cancer.

High socio-economic status

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

Tall adult height

Research shows 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 for sure that they are risk factors. More research is needed to clarify the role of these factors for 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 lowering your weight would reduce the risk for breast cancer.

Smoking and second-hand smoke

Recent studies show 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 developing breast cancer before menopause. A greater birth weight means the fetus is exposed to more estrogen during pregnancy, which may increase the risk of developing breast cancer later in life.

Night shift work

Researchers think that night work, and being exposed to artificial light, lowers the amount of melatonin in the body. In women, melatonin lowers the amount of estrogen in the body, and it may slow the growth of breast cancer cells. Some studies suggest that women who work shifts, particularly night shifts, have a slightly higher risk of developing breast cancer. Other studies show no increased risk.

Some non-cancerous breast conditions

Most non-cancerous (benign) breast conditions do not increase a woman’s risk of developing breast cancer. But 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 linked with a greater number (overgrowth) of cells. Non-cancerous breast conditions that may increase the risk for breast cancer include:

No link to breast cancer

Significant evidence shows no link between antiperspirants, deodorants, abortion, breast implants or bras and a higher risk of breast cancer.

Questions to ask your healthcare team

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

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