Women with a family history of breast cancer are at higher than average risk for developing the disease.
Approximately 25% of women with breast cancer have a family member with the disease. Some of these women carry a BRCA mutation, but most do not. Why these other families are at higher risk is not yet understood. There are probably other inherited genetic mutations that we haven't yet discovered that are increasing breast cancer risk. It is also possible that there are specific traits or environmental exposures shared by family members that can increase breast cancer risk.
Your risk of developing breast cancer is increased if:
- You have two or more relatives with breast or ovarian cancer.
- Breast cancer occurs before age 50 in your mother or sister, or a relative (grandmother or aunt) on either side of the family. The risk is higher if your mother or sister has a history of breast cancer.
- You have relatives with both breast and ovarian cancer.
- You have one or more relatives with two cancers (breast and ovarian, or two different breast cancers).
- You have one or more male relatives with breast cancer.
- You have a family history of breast or ovarian cancer and Ashkenazi Jewish heritage.
- Your family history includes a history of diseases associated with hereditary breast cancer such as Li-Fraumeni or Cowden syndromes.
Women who have a family history of breast cancer that includes relatives who were diagnosed at a young age are at higher risk for breast cancer than are women whose relatives were diagnosed when they were 50 or older. That's because women who get breast cancer when they are older are less likely to have it occur as a result of an inherited genetic mutation. Instead, they are more likely to have had their cancer triggered by environmental factors. (This is what triggers breast cancer in most women.) By environmental factors, we don’t necessarily mean pollution, but any molecule in the cell’s environment. These molecules may already be circulating inside the body, like hormones, or they may have entered the body from outside in substances that are swallowed or inhaled.
Having a family history of breast cancer does not mean that you will develop breast cancer. But it does mean that you are at increased risk. Having one first-degree relative (mother, sister, or daughter) with breast cancer approximately doubles a woman’s risk. Think of it this way: In a group of 100 women who do not have a family history of breast cancer, five would be expected to go on to develop breast cancer. In contrast, in a group of women with one first-degree relative who has the disease, 10 would be expected to go on to develop breast cancer. Having two first-degree relatives increases a woman’s risk five-fold. This means that in a group of 100 women with two first-degree relatives with breast cancer, 25 would be expected to go on to develop breast cancer.