A risk factor is anything that affects your chance of getting a
disease, such as cancer. Different cancers have different risk factors.
For example, exposing skin to strong sunlight is a risk factor for skin
cancer. Smoking is a risk factor for cancers of the lung, mouth, larynx
(voice box), bladder, kidney, and several other organs.
But risk factors don’t tell us everything. Having a risk factor, or
even several, does not mean that you will get the disease. Most women
who have one or more breast cancer risk factors never develop the
disease, while many women with breast cancer have no apparent risk
factors (other than being a woman and growing older). Even when a woman
with risk factors develops breast cancer, it is hard to know just how
much these factors might have contributed.
Some risk factors, like a person’s age or race, can’t be changed.
Others are linked to cancer-causing factors in the environment. Still
others are related to personal behaviors, such as smoking, drinking, and
diet. Some factors influence risk more than others, and your risk for
breast cancer can change over time, due to factors such as aging or
lifestyle.
Risk factors you cannot change
Gender
Simply being a woman is the main risk factor for developing breast
cancer. Men can develop breast cancer, but this disease is about 100
times more common among women than men. This is probably because men
have less of the female hormones estrogen and progesterone, which can
promote breast cancer cell growth
Aging
Your risk of developing breast cancer increases as you get older.
About 1 out of 8 invasive breast cancers are found in women younger than
45, while about 2 of 3 invasive breast cancers are found in women age
55 or older.
Genetic risk factors
About 5% to 10% of breast cancer cases are thought to be hereditary,
meaning that they result directly from gene defects (called
mutations)
inherited from a parent. See the section, “Do we know what causes
breast cancer?” for more information about genes and DNA and how they
can affect breast cancer risk.
BRCA1 and BRCA2: The most common cause of hereditary breast cancer is an inherited mutation in the
BRCA1 and
BRCA2
genes. In normal cells, these genes help prevent cancer by making
proteins that keep the cells from growing abnormally. If you have
inherited a mutated copy of either gene from a parent, you have a high
risk of developing breast cancer during your lifetime.
Although in some families with
BRCA1 mutations the lifetime
risk of breast cancer is as high as 80%, on average this risk seems to
be in the range of 55 to 65%. For
BRCA2 mutations the risk is lower, around 45%.
Breast cancers linked to these mutations occur more often in younger
women and more often affect both breasts than cancers not linked to
these mutations. Women with these inherited mutations also have an
increased risk for developing other cancers, particularly ovarian
cancer.
In the United States
BRCA mutations are more common in Jewish
people of Ashkenazi (Eastern Europe) origin than in other racial and
ethnic groups, but they can occur in anyone.
Changes in other genes: Other gene mutations can also lead to
inherited breast cancers. These gene mutations are much rarer and often
do not increase the risk of breast cancer as much as the BRCA genes.
They are not frequent causes of inherited breast cancer.
- ATM: The ATM gene normally helps repair damaged DNA.
Inheriting 2 abnormal copies of this gene causes the disease
ataxia-telangiectasia. Inheriting 1 mutated copy of this gene has been
linked to a high rate of breast cancer in some families.
- TP53: The TP53 gene gives instructions for making a
protein called p53 that helps stop the growth of abnormal cells.
Inherited mutations of this gene cause Li-Fraumeni syndrome
(named after the 2 researchers who first described it). People with this
syndrome have an increased risk of developing breast cancer, as well as
several other cancers such as leukemia, brain tumors, and sarcomas
(cancer of bones or connective tissue). This is a rare cause of breast
cancer.
- CHEK2: The Li-Fraumeni syndrome can also be caused by inherited mutations in the CHEK2 gene. Even when it does not cause this syndrome, it can increase breast cancer risk about twofold when it is mutated.
- PTEN: The PTEN gene normally helps regulate cell growth. Inherited mutations in this gene can cause Cowden syndrome,
a rare disorder in which people are at increased risk for both benign
and malignant breast tumors, as well as growths in the digestive tract,
thyroid, uterus, and ovaries. Defects in this gene can also cause a
different syndrome called Bannayan-Riley-Ruvalcaba syndrome that is not
thought to be linked to breast cancer risk.
- CDH1: Inherited mutations in this gene cause hereditary diffuse gastric cancer,
a syndrome in which people develop a rare type of stomach cancer at an
early age. Women with mutations in this gene also have an increased risk
of invasive lobular breast cancer.
- STK11: Defects in this gene can lead to Peutz-Jeghers syndrome.
People with this disorder develop pigmented spots on their lips and in
their mouths, polyps in the urinary and gastrointestinal tracts, and
have an increased risk of many types of cancer, including breast cancer.
Genetic testing: Genetic tests can be done to look for mutations in the
BRCA1 and
BRCA2
genes (or some other genes linked to breast cancer risk). Although
testing may be helpful in some situations, the pros and cons need to be
considered carefully. For more information, see the section, “Can breast
cancer be prevented?”
Family history of breast cancer
Breast cancer risk is higher among women whose close blood relatives have this disease.
Having one first-degree relative (mother, sister, or daughter) with
breast cancer approximately doubles a woman’s risk. Having 2
first-degree relatives increases her risk about 3-fold.
The exact risk is not known, but women with a family history of
breast cancer in a father or brother also have an increased risk of
breast cancer. Altogether, less than 15% of women with breast cancer
have a family member with this disease. This means that most (over 85%)
women who get breast cancer
do not have a family history of this disease.
Personal history of breast cancer
A woman with cancer in one breast has a 3- to 4-fold increased risk
of developing a new cancer in the other breast or in another part of the
same breast. This is different from a recurrence (return) of the first
cancer.
Race and ethnicity
Overall, white women are slightly more likely to develop breast
cancer than are African-American women, but African-American women are
more likely to die of this cancer. However, in women under 45 years of
age, breast cancer is more common in African- American women. Asian,
Hispanic, and Native-American women have a lower risk of developing and
dying from breast cancer.
Dense breast tissue
Breasts are made up of fatty tissue, fibrous tissue, and glandular
tissue. Someone is said to have dense breast tissue (as seen on a
mammogram) when they have more glandular and fibrous tissue and less
fatty tissue. Women with dense breasts have a higher risk of breast
cancer than women with less dense breasts. Unfortunately, dense breast
tissue can also make mammograms less accurate.
A number of factors can affect breast density, such as age,
menopausal status, the use of drugs (such as menopausal hormone
therapy), pregnancy, and genetics.
Certain benign breast conditions
Women diagnosed with certain benign breast conditions might have an
increased risk of breast cancer. Some of these conditions are more
closely linked to breast cancer risk than others. Doctors often divide
benign breast conditions into 3 general groups, depending on how they
affect this risk.
Non-proliferative lesions: These conditions are not associated
with overgrowth of breast tissue. They do not seem to affect breast
cancer risk, or if they do, it is to a very small extent. They include:
- Fibrosis and/or simple cysts (this used to be called fibrocystic disease or changes)
- Mild hyperplasia
- Adenosis (non-sclerosing)
- Ductal ectasia
- Phyllodes tumor (benign)
- A single papilloma
- Fat necrosis
- Periductal fibrosis
- Squamous and apocrine metaplasia
- Epithelial-related calcifications
- Other benign tumors (lipoma, hamartoma, hemangioma, neurofibroma, adenomyoepthelioma)
Mastitis (infection of the breast) is not a lesion, but is a
condition that can occur that does not increase the risk of breast
cancer.
Proliferative lesions without atypia: These conditions show
excessive growth of cells in the ducts or lobules of the breast tissue.
They seem to raise a woman’s risk of breast cancer slightly (1½ to 2
times normal). They include:
- Usual ductal hyperplasia (without atypia)
- Fibroadenoma
- Sclerosing adenosis
- Several papillomas (called papillomatosis)
- Radial scar
Proliferative lesions with atypia: In these conditions, there
is an overgrowth of cells in the ducts or lobules of the breast tissue,
with some of the cells no longer appearing normal. They have a stronger
effect on breast cancer risk, raising it 3½ to 5 times higher than
normal. These types of lesions include:
- Atypical ductal hyperplasia (ADH)
- Atypical lobular hyperplasia (ALH)
Women with a family history of breast cancer and either hyperplasia
or atypical hyperplasia have an even higher risk of developing a breast
cancer.
For more information on these conditions, see our document,
Non-cancerous Breast Conditions.
Lobular carcinoma in situ
In lobular carcinoma in situ (LCIS) cells that look like cancer cells
are growing in the lobules of the milk-producing glands of the breast,
but they do not grow through the wall of the lobules. LCIS (also called
lobular neoplasia)
is sometimes grouped with ductal carcinoma in situ (DCIS) as a
non-invasive breast cancer, but it differs from DCIS in that it doesn’t
seem to become an invasive cancer if it isn’t treated.
Women with this condition have a 7- to 11-fold increased risk of
developing invasive cancer in either breast. For this reason, women with
LCIS should make sure they have regular mammograms and doctor visits.
Menstrual periods
Women who have had more menstrual cycles because they started
menstruating early (before age 12) and/or went through menopause later
(after age 55) have a slightly higher risk of breast cancer. The
increase in risk may be due to a longer lifetime exposure to the
hormones estrogen and progesterone.
Previous chest radiation
Women who, as children or young adults, had radiation therapy to the
chest area as treatment for another cancer (such as Hodgkin disease or
non-Hodgkin lymphoma) have a significantly increased risk for breast
cancer. This varies with the patient’s age when they had radiation. If
chemotherapy was also given, it may have stopped ovarian hormone
production for some time, lowering the risk. The risk of developing
breast cancer from chest radiation is highest if the radiation was given
during adolescence, when the breasts were still developing. Radiation
treatment after age 40 does not seem to increase breast cancer risk.
Diethylstilbestrol exposure
From the 1940s through the 1960s some pregnant women were given the
drug diethylstilbestrol (DES) because it was thought to lower their
chances of miscarriage (losing the baby). These women have a slightly
increased risk of developing breast cancer. Women whose mothers took DES
during pregnancy may also have a slightly higher risk of breast cancer.
For more information on DES see our document,
DES Exposure: Questions and Answers.
Lifestyle-related factors and breast cancer risk
Having children
Women who have had no children or who had their first child after age
30 have a slightly higher breast cancer risk. Having many pregnancies
and becoming pregnant at a young age reduce breast cancer risk.
Pregnancy reduces a woman’s total number of lifetime menstrual cycles,
which may be the reason for this effect.
Birth control
Oral contraceptives: Studies have found that women using oral
contraceptives (birth control pills) have a slightly greater risk of
breast cancer than women who have never used them. This risk seems to go
back to normal over time once the pills are stopped. Women who stopped
using oral contraceptives more than 10 years ago do not appear to have
any increased breast cancer risk. When thinking about using oral
contraceptives, women should discuss their other risk factors for breast
cancer with their health care team.
Depot-medroxyprogesterone acetate (DMPA; Depo-Provera
®)
is an injectable form of progesterone that is given once every 3 months
as birth control. A few studies have looked at the effect of DMPA on
breast cancer risk. Women currently using DMPA seem to have an increase
in risk, but the risk doesn’t seem to be increased if this drug was used
more than 5 years ago.
Hormone therapy after menopause
Hormone therapy with estrogen (often combined with progesterone) has
been used for many years to help relieve symptoms of menopause and to
help prevent osteoporosis (thinning of the bones). Earlier studies
suggested it might have other health benefits as well, but these
benefits have not been found in more recent, better designed studies.
This treatment goes by many names, such as
post-menopausal hormone therapy (PHT),
hormone replacement therapy (HRT), and
menopausal hormone therapy (MHT).
There are 2 main types of hormone therapy. For women who still have a
uterus (womb), doctors generally prescribe both estrogen and
progesterone (known as
combined hormone therapy or
HT).
Progesterone is needed because estrogen alone can increase the risk of
cancer of the uterus. For women who no longer have a uterus (those
who’ve had a hysterectomy), estrogen alone can be prescribed. This is
commonly known as
estrogen replacement therapy (ERT) or just
estrogen therapy (ET).
Combined hormone therapy: Using combined hormone therapy after
menopause increases the risk of getting breast cancer. It may also
increase the chances of dying from breast cancer. This increase in risk
can be seen with as little as 2 years of use. Combined HT also increases
the likelihood that the cancer may be found at a more advanced stage.
The increased risk from combined hormone therapy appears to apply
only to current and recent users. A woman’s breast cancer risk seems to
return to that of the general population within 5 years of stopping
combined treatment.
The word
bioidentical is sometimes used to describe versions
of estrogen and progesterone with the same chemical structure as those
found naturally in people. The use of these hormones has been marketed
as a safe way to treat the symptoms of menopause. It is important to
realize that although there are few studies comparing “bioidentical” or
“natural” hormones to synthetic versions of hormones, there is no
evidence that they are safer or more effective. The use of these
bioidentical hormones should be assumed to have the same health risks as
any other type of hormone therapy.
Estrogen therapy (ET): The use of estrogen alone after
menopause does not appear to increase the risk of developing breast
cancer. In fact, some research has suggested that women who have
previously had their uterus removed and who take estrogen actually have a
lower risk of breast cancer. Women taking estrogen seem to have more
problems with strokes and other blood clots, though. Also, when used
long term (for more than 10 years), ET has been found to increase the
risk of ovarian cancer in some studies.
At this time there appear to be few strong reasons to use
post-menopausal hormone therapy (either combined HT or ET), other than
possibly for the short-term relief of menopausal symptoms. Along with
the increased risk of breast cancer, combined HT also appears to
increase the risk of heart disease, blood clots, and strokes. It does
lower the risk of colorectal cancer and osteoporosis, but this must be
weighed against possible harm, especially since there are other
effective ways to prevent and treat osteoporosis.
Although ET does not seem to increase breast cancer risk, it does increase the risk of blood clots and stroke.
The decision to use hormone therapy after menopause should be made by
a woman and her doctor after weighing the possible risks and benefits,
based on the severity of her menopausal symptoms and the woman’s other
risk factors for heart disease, breast cancer, and osteoporosis. If a
woman and her doctor decide to try hormones for symptoms of menopause,
it is usually best to use it at the lowest dose needed to control
symptoms and for as short a time as possible.
Breastfeeding
Some studies suggest that breastfeeding may slightly lower breast
cancer risk, especially if it is continued for 1½ to 2 years. But this
has been a difficult area to study, especially in countries such as the
United States, where breastfeeding for this long is uncommon.
One explanation for this possible effect may be that breastfeeding
reduces a woman’s total number of lifetime menstrual cycles (similar to
starting menstrual periods at a later age or going through early
menopause).
Drinking alcohol
The use of alcohol is clearly linked to an increased risk of
developing breast cancer. The risk increases with the amount of alcohol
consumed. Compared with non-drinkers, women who consume 1 alcoholic
drink a day have a very small increase in risk. Those who have 2 to 5
drinks daily have about 1½ times the risk of women who don’t drink
alcohol. Excessive alcohol consumption is also known to increase the
risk of developing several other types of cancer.
Being overweight or obese
Being overweight or obese after menopause increases breast cancer
risk. Before menopause your ovaries produce most of your estrogen, and
fat tissue produces a small amount of estrogen. After menopause (when
the ovaries stop making estrogen), most of a woman’s estrogen comes from
fat tissue. Having more fat tissue after menopause can increase your
chance of getting breast cancer by raising estrogen levels. Also, women
who are overweight tend to have higher blood insulin levels. Higher
insulin levels have also been linked to some cancers, including breast
cancer.
But the connection between weight and breast cancer risk is complex.
For example, the risk appears to be increased for women who gained
weight as an adult but may not be increased among those who have been
overweight since childhood. Also, excess fat in the waist area may
affect risk more than the same amount of fat in the hips and thighs.
Researchers believe that fat cells in various parts of the body have
subtle differences that may explain this.
Physical activity
Evidence is growing that physical activity in the form of exercise
reduces breast cancer risk. The main question is how much exercise is
needed. In one study from the Women’s Health Initiative, as little as
1.25 to 2.5 hours per week of brisk walking reduced a woman’s risk by
18%. Walking 10 hours a week reduced the risk a little more.
Unclear factors
Diet and vitamin intake
Many studies have looked for a link between what women eat and breast
cancer risk, but so far the results have been conflicting. Some studies
have indicated that diet may play a role, while others found no
evidence that diet influences breast cancer risk. Studies have looked at
the amount of fat in the diet, intake of fruits and vegetables, and
intake of meat. No clear link to breast cancer risk was found.
Studies have also looked at vitamin levels, again with inconsistent
results. Some studies actually found an increased risk of breast cancer
in women with higher levels of certain nutrients. So far, no study has
shown that taking vitamins reduces breast cancer risk. This is not to
say that there is no point in eating a healthy diet. A diet low in fat,
low in red meat and processed meat, and high in fruits and vegetables
might have other health benefits.
Most studies have found that breast cancer is less common in
countries where the typical diet is low in total fat, low in
polyunsaturated fat, and low in saturated fat. But many studies of women
in the United States have not linked breast cancer risk to dietary fat
intake. Researchers are still not sure how to explain this apparent
disagreement. It may be at least partly due to the effect of diet on
body weight (see below). Also, studies comparing diet and breast cancer
risk in different countries are complicated by other differences (like
activity level, intake of other nutrients, and genetic factors) that
might also affect breast cancer risk.
More research is needed to understand the effect of the types of fat
eaten on breast cancer risk. But it is clear that calories do count, and
fat is a major source of calories. High-fat diets can lead to being
overweight or obese, which is a breast cancer risk factor. A diet high
in fat has also been shown to influence the risk of developing several
other types of cancer, and intake of certain types of fat is clearly
related to heart disease risk.
Chemicals in the environment
A great deal of research has been reported and more is being done to
understand possible environmental influences on breast cancer risk.
Compounds in the environment that have estrogen-like properties are
of special interest. For example, substances found in some plastics,
certain cosmetics and personal care products, pesticides (such as DDE),
and PCBs (polychlorinated biphenyls) seem to have such properties. These
could in theory affect breast cancer risk.
This issue understandably invokes a great deal of public concern, but
at this time research does not show a clear link between breast cancer
risk and exposure to these substances. Unfortunately, studying such
effects in humans is difficult. More research is needed to better define
the possible health effects of these and similar substances.
Tobacco smoke
For a long time, studies found no link between cigarette smoking and
breast cancer. In recent years though, more studies have found that
long-term heavy smoking is linked to a higher risk of breast cancer.
Some studies have found that the risk is highest in certain groups, such
as women who started smoking when they were young. In 2009, the
International Agency for Research on Cancer concluded that there is
limited evidence that tobacco smoking causes breast cancer.
An active focus of research is whether secondhand smoke increases the
risk of breast cancer. Both mainstream and secondhand smoke contain
chemicals that, in high concentrations, cause breast cancer in rodents.
Chemicals in tobacco smoke reach breast tissue and are found in breast
milk.
The evidence on secondhand smoke and breast cancer risk in human
studies is controversial, at least in part because the link between
smoking and breast cancer hasn’t been clear. One possible explanation
for this is that tobacco smoke may have different effects on breast
cancer risk in smokers and in those who are just exposed to smoke.
A report from the California Environmental Protection Agency in 2005
concluded that the evidence about secondhand smoke and breast cancer is
“consistent with a causal association” in younger, mainly premenopausal
women. The 2006 US Surgeon General’s report,
The Health Consequences of Involuntary Exposure to Tobacco Smoke,
concluded that there is “suggestive but not sufficient” evidence of a
link at this point. In any case, this possible link to breast cancer is
yet another reason to avoid secondhand smoke.
Night work
Several studies have suggested that women who work at night—for
example, nurses on a night shift—may have an increased risk of
developing breast cancer. This is a fairly recent finding, and more
studies are looking at this issue. Some researchers think the effect may
be due to changes in levels of melatonin, a hormone whose production is
affected by the body’s exposure to light, but other hormones are also
being studied.
Controversial factors
Antiperspirants
Internet e-mail rumors have suggested that chemicals in underarm
antiperspirants are absorbed through the skin, interfere with lymph
circulation, cause toxins to build up in the breast, and eventually lead
to breast cancer.
Based on the available evidence (including what we know about how the
body works), there is little if any reason to believe that
antiperspirants increase the risk of breast cancer. For more information
about this, see our document
Antiperspirants and Breast Cancer Risk.
Bras
Internet e-mail rumors and at least one book have suggested that bras
cause breast cancer by obstructing lymph flow. There is no good
scientific or clinical basis for this claim. Women who do not wear bras
regularly are more likely to be thinner or have less dense breasts,
which would probably contribute to any perceived difference in risk.
Induced abortion
Several studies have provided very strong data that neither induced
abortions nor spontaneous abortions (miscarriages) have an overall
effect on the risk of breast cancer. For more detailed information, see
our document,
Is Abortion Linked to Breast Cancer?
Breast implants
Several studies have found that breast implants do not increase the
risk of breast cancer, although silicone breast implants can cause scar
tissue to form in the breast. Implants make it harder to see breast
tissue on standard mammograms, but additional x-ray pictures called
implant displacement views can be used to examine the breast tissue more completely.
Breast implants may be linked to a rare type of lymphoma called
anaplastic large cell lymphoma.
This lymphoma has rarely been found in the breast tissue around the
implants. So far, though, there are too few cases to know if the risk of
this lymphoma is really higher in women that have implants.
Carrots
Papaya
Tomato
Kiwi
Green leafy vegetables
Strawberries
Red bell peppers
Green tea
Yellow bell peppers
Soy products
Broccoli
Fish
