What Causes Breast Cancer?

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What Causes Breast Cancer?

Certain changes in DNA can cause normal breast cells to become cancerous. DNA is the chemical in each of our cells that makes up our genes -- the instructions for how our cells work. Some inherited DNA changes can increase the risk for developing cancer and are responsible for the cancers that run in some families. But most breast cancer DNA changes happen in single breast cells during a woman's life rather than having been inherited. These are called acquired changes, and most breast cancers have several of these acquired gene mutations. But so far, the causes of most acquired mutations that could lead to breast cancer remain unknown.

While we do not yet know exactly what causes breast cancer, we do know that certain risk factors are linked to the disease. A risk factor is anything that affects a person's chance of getting a disease such as cancer. Different cancers have different risk factors. Some risk factors, such as smoking, drinking, and diet are linked to things a person does. Others, like a person's age, race, or family history, can't be changed. But risk factors don't tell us everything. Having a risk factor, or even several, doesn't mean that a person will get the disease. Some women who have one or more risk factors never get breast cancer. And most women who do get breast cancer don't have any risk factors. While all women are at risk for breast cancer, the factors listed below can increase a woman's chances of having the disease.

Although many risk factors may increase your chance of developing breast cancer, it is not yet known exactly how some of these risk factors cause cells to become cancerous. Hormones seem to play a role in many cases of breast cancer, but just how this happens is not fully understood.

Risk factors you cannot change

Gender: Simply being a woman is the main risk for breast cancer. While men also get the disease, it is about 100 times more common in women than in men.

Age: The chance of getting breast cancer goes up as a woman gets older. About 2 out of 3 women with invasive breast cancer are age 55 or older when the cancer is found.

Genetic risk factors: About 5% to 10% of breast cancers are thought to be linked to inherited changes (mutations) in certain genes. The most common gene changes are those of the BRCA1 and BRCA2 genes. Women with these gene changes have up to an 80% chance of getting breast cancer during their lifetimes. Other gene changes may raise breast cancer risk as well.

Family history: Breast cancer risk is higher among women whose close blood relatives have this disease. The relatives can be from either the mother's or father's side of the family. Having a mother, sister, or daughter with breast cancer about doubles a woman's risk. (It's important to note that 70% to 80% of 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 greater chance of getting a new cancer in the other breast or in another part of the same breast. This is different from a return of the first cancer (which is called recurrence).

Race: White women are slightly more likely to get breast cancer than are African-American women. But African American women are more likely to die of this cancer. At least part of the reason seems to be because African-American women have faster growing tumors. Asian, Hispanic, and American Indian women have a lower risk of getting breast cancer.

Dense breast tissue: Dense breast tissue means there is more glandular tissue and less fatty tissue. Women with denser breast tissue have a higher risk of breast cancer. Dense breast tissue can also make it harder for doctors to spot problems on mammograms.

Menstrual periods: Women who began having periods early (before age 12) or who went through the change of life (menopause) after the age of 55 have a slightly increased risk of breast cancer. They have had more menstrual periods and as a result have been exposed to more of the hormones estrogen and progesterone.

Earlier breast radiation: Women who have had radiation treatment to the chest area (as treatment for another cancer) earlier in life have a greatly increased risk of breast cancer.

Treatment with DES: In the past, some pregnant women were given the drug DES (diethylstilbestrol) because it was thought to lower their chances of losing the baby (miscarriage). Recent studies have shown that these women (and their daughters who were exposed to DES while in the womb), have a slightly increased risk of getting breast cancer. For more information on DES see our document, DES Exposure: Questions and Answers.

Breast cancer risk and lifestyle choices

Not having children or having them later in life: Women who have had not had children, or who had their first child after age 30, have a slightly higher risk of breast cancer. Being pregnant more than once and at an early age reduces breast cancer risk. Pregnancy reduces a woman's total number of lifetime menstrual cycles, which may be the reason for this effect.

Recent use of birth control pills: Studies have found that women who are using birth control pills have a slightly greater risk of breast cancer than women who have never used them. Women who stopped using the pill more than 10 years ago do not seem to have any increased risk. It's a good idea to talk to your doctor about the risks and benefits of birth control pills.

Postmenopausal hormone therapy (PHT): Postmenopausal hormone therapy (also known as hormone replacement therapy or HRT), has been used for many years to help relieve symptoms of menopause and to help prevent thinning of the bones (osteoporosis). There are 2 main types of PHT. For women who still have a womb (uterus), doctors generally prescribe estrogen and progesterone (known as combined PHT). Estrogen alone can increase the risk of cancer of the uterus, so progesterone is added to help prevent this. 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).

Combined PHT: It has become clear that long-term use (several years or more) of combined PHT increases the risk of breast cancer and may increase the chances of dying of breast cancer. The breast cancer may also be found at a more advanced stage, perhaps because PHT seems to reduce the effectiveness of mammograms. Five years after stopping PHT, the breast cancer risk seems to drop back to normal.

ERT: The use of estrogen alone does not seem to increase the risk of developing breast cancer much, if at all. But when used long-term (for more than 10 years), some studies have found that ERT increases the risk of ovarian and breast cancer.

At this time, there are few strong reasons to use PHT, other than for short-term relief of menopausal symptoms. Because there are other factors to think about, you should talk with your doctor about the pros and cons of using PHT. If a woman and her doctor decide to try PHT for symptoms of menopause, it is usually best to use it at the lowest dose that works for her and for as short a time as possible.

Not breast-feeding: Some studies have shown that breast-feeding slightly lowers breast cancer risk, especially if the breast-feeding lasts 1½ to 2 years. This could be because breast-feeding lowers a woman's total number of menstrual periods, as does pregnancy

Alcohol: Use of alcohol is clearly linked to an increased risk of getting breast cancer. Women who have one drink a day have a very small increased risk. Those who have 2 to 5 drinks daily have about 1½ times the risk of women who drink no alcohol. The American Cancer Society suggests limiting the amount you drink to one drink a day.

Being overweight or obese: Being overweight or obese is linked to a higher risk of breast cancer, especially for women after change of life and if the weight gain took place during adulthood. Also, the risk seems to be higher if the extra fat is in the waist area. But the link between weight and breast cancer risk is complex, and studies of fat in the diet as it relates to breast cancer risk have often given conflicting results. The American Cancer Society recommends you maintain a healthy weight throughout your life and avoid gaining too much weight.

Lack of exercise: Studies show that exercise reduces breast cancer risk. The only question is how much exercise is needed. One study found that as little as 1 hour and 15 minutes to 2½ hours of brisk walking per week reduced the risk by 18%. Walking 10 hours a week reduced the risk a little more. The American Cancer Society suggests that you exercise for 45 to 60 minutes 5 or more days a week.

Uncertain risk factors

High fat diets: Studies of fat in the diet have not clearly shown that this is a breast cancer risk factor. Most studies found that breast cancer is less common in countries where the typical diet is low in fat. On the other hand, many studies of women in the United States have not found breast cancer risk to be linked to how much fat they ate. Researchers are still not sure how to explain this difference. More research is needed to better understand the effect of the types of fat eaten and body weight on breast cancer risk.

The American Cancer Society recommends eating a healthy diet that includes 5 or more servings of vegetables and fruits each day, choosing whole grains over processed (refined) grains, and limiting the amount of processed and red meats.

Antiperspirants and bras: Internet e-mail rumors have suggested that underarm antiperspirants can cause breast cancer. There is very little evidence to support this idea. Also, there is no evidence to support the idea that under wire bras cause breast cancer.

Abortions: Several studies show that induced abortions do not increase the risk of breast cancer. Also, there is no evidence to show a direct link between miscarriages and breast cancer. For more detailed information, see our document, Can Having an Abortion Cause or Contribute to Breast Cancer?

Breast implants: Silicone breast implants can cause scar tissue to form in the breast. But several studies have found that this does not increase breast cancer risk. If you have breast implants, you might need special x-ray pictures during mammograms.

Pollution: A lot of research is being done to learn how the environment might affect breast cancer risk. At this time, research does not show a clear link between breast cancer risk and environmental pollutants such as pesticides and PCBs.

Tobacco Smoke: Most studies have found no link between active cigarette smoking and breast cancer. An issue that continues to be a focus of research is whether secondhand smoke (smoke from another person's cigarette) may increase the risk of breast cancer. But the evidence about secondhand smoke and breast cancer risk in human studies is not clear. In any case, a possible link to breast cancer is yet another reason to avoid being around secondhand smoke.

Night Work: A few studies have suggested that women who work at night (nurses on the night shift, for example) have a higher risk of breast cancer. This is a fairly recent finding, and more studies are being done to look at this issue.