Not all women have the same risk for developing breast cancer over a lifetime. Certain factors increase a woman’s risk, and some have a bigger impact on risk than others. However, having several risk factors doesn’t mean you’ll inevitably develop breast cancer. Likewise, having few risk factors doesn’t mean that you’ll never develop it.
Many risk factors, such as age and gender, are not within our control. Others, especially those related to personal behaviors, can be modified.
It’s important to know about risk factors because they can help identify women who might benefit from certain preventive strategies. A woman should work with her clinician to determine her personal risk for breast cancer and plan screenings, office visits, and preventive care accordingly. Online resources that can help in determining risk include the Breast Cancer Risk Assessment Tool (www.cancer.gov/bcrisktool) and Harvard School of Public Health’s “Disease Risk Index” (www.diseaseriskindex.harvard.edu).
Known risk factors for breast cancer include the following:
Gender. This is the strongest risk factor for breast cancer. Men can develop breast cancer, but it’s 100 times more common in women than men, mostly because women’s breast tissue is far more exposed to hormones such as estrogen that promote abnormal cell growth.
Age. This is one of the strongest risk factors for breast cancer. About 85% of cases occur in women ages 50 and over, while 5% occur in women under age 40.
Family history. Women who have two or more first-degree relatives (mother, daughter, sister) with breast or ovarian cancer have a greater than 50% chance of developing breast cancer. One of the main reasons for this elevated risk is an inherited mutation in one of two genes, BRCA1 and BRCA2. Other gene mutations can also lead to inherited breast cancer, but these are rarer and don’t affect risk as much BRCA mutations. Inherited risk accounts for 5% to 10% of breast cancers.
Previous breast cancer. If you’ve had cancer in one breast, you have a three- to four-fold increased risk of developing a new cancer in the other breast or another part of the same breast. (This is not the same as a recurrence of the original cancer.)
Breast density. Women with denser breast tissue, as seen on a mammogram, have a higher risk for breast cancer than women whose breasts are relatively more fatty. The greater the proportion of dense breast tissue on a mammogram, the higher the risk.
Certain benign breast conditions. Women who’ve had biopsies that show an overgrowth of cells (hyperplasia) in the ducts or lobules have an increased risk for breast cancer, particularly if the cells appear abnormal (a condition called atypical hyperplasia). Benign breast conditions that don’t cause this type of overgrowth do not appear to increase risk; these include fibrocystic disease, simple fibroadenoma, mastitis, fat necrosis, and benign tumors.
Lobular carcinoma in situ (LCIS). Women with LCIS are at increased risk of developing breast cancer in either breast.
Exposure to radiation. Women who’ve had high-dose radiation to the chest as part of treatment for another cancer (such as Hodgkin’s disease) have an increased risk of developing breast cancer, especially if they underwent radiation during adolescence. Radiation after age 40 does not appear to increase breast cancer risk.
Race. White women are slightly more likely to develop breast cancer than African-American women, but African-American women are more likely to die of the disease. Asian, Hispanic, and Native American women appear to have a lower risk than white or African-American women of developing and dying from breast cancer.
Estrogen exposure. The longer a woman is exposed to estrogen, the greater her risk for breast cancer. Multiple pregnancies and pregnancy at a young age reduces a woman’s total number of menstrual cycles and thus her risk of breast cancer, though this is not as strong a risk factor as many others. Women who menstruate early, before age 12, and/or go through menopause late (after age 55) have a slightly higher risk of breast cancer, possibly because of the increased lifetime exposure to estrogen. Current oral contraceptive use slightly increases breast cancer risk, but it returns to normal once the pills are stopped. Current or recent use of postmenopausal hormone therapy with estrogen plus a progestin increases breast cancer risk within as few as two years; risk returns to normal within five years of stopping. (Postmenopausal hormone therapy with estrogen alone does not appear to increase breast cancer risk.)
Diethylstilbestrol (DES) exposure. Women who took DES — a drug used from the 1940s through the 1960s to prevent miscarriage and since banned — have a slightly higher risk of developing breast cancer. Their daughters may also.
Weight. Being overweight or obese has been linked to breast cancer risk, especially for women after menopause, but the relationship is complicated. It may be that risk is increased in women who gain weight in adulthood but not in those who’ve been overweight since childhood.
Alcohol. Women who drink alcohol have an increased risk of breast cancer, compared with women who don’t drink, and the risk rises with the number of drinks consumed.
Other cancers. Women who’ve been diagnosed with cancer of the ovary, colon, or endometrium are more likely to develop breast cancer than women who don’t have these cancers.
Originally published: October 2011
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