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 for Breast Cancer that 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%-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%-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.
- 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 2-5 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 the 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 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 1¼ 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 45-60 minutes 5 or more days a week.
Uncertain Breast Cancer 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 email 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 underwire 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.
- 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 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.
In summary: Breast cancer DNA changes can cause normal breast cells to become cancerous, the most common of which are in the BRCA1 and BRC2 genes.