Certain factors can determine breast cancer risk

The National Cancer Institute offers a good compilation of factors that may help in understanding a person's breast cancer risk:

The risk of breast cancer increases gradually as a woman gets older. However, the risk of developing breast cancer is not the same for all women. Research has shown that the following factors increase a woman's chance of developing this disease:

• Personal history of breast cancer -- Women who have had breast cancer are more likely to develop a second breast cancer.
• Family history -- A woman's chance of developing breast cancer increases if her mother, sister, and/or daughter have a history of breast cancer (especially if they were diagnosed before age 50).
• Certain breast changes on biopsy -- A diagnosis of atypical hyperplasia (a non-cancerous condition in which cells have abnormal features and are increased in number) or lobular carcinoma in situ (LCIS) (abnormal cells found in the lobules of the breast) increases a woman's risk of breast cancer. Women who have had two or more breast biopsies for other benign conditions also have an increased chance of developing breast cancer. This increased risk is due to the condition that led to the biopsy, and not to the biopsy itself.
• Genetic alterations (changes) -- Specific alterations in certain genes (BRCA1, BRCA2, and others) increase the risk of breast cancer. These alterations are rare; they are estimated to account for no more than 10 percent of all breast cancers.
• Reproductive and menstrual history -- W omen who began having periods before age 12 or went through menopause after age 55 are at an increased risk of developing breast cancer. Women who have their first child after age 30 or who never have a child are at an increased risk of developing breast cancer.
• Long-term use of menopausal hormone therapy -- Women who use combination estrogen-progestin menopausal hormone therapy for more than 5 years have an increased chance of developing breast cancer.
• Breast density -- Breasts appear dense on a mammogram if they contain many glands and ligaments (called dense tissue), and do not have much fatty tissue. Because breast cancers tend to develop in the dense tissue of the breast (not in the fatty tissue), older women who have mostly dense tissue on a mammogram are at an increased risk of breast cancer. Abnormalities in dense breasts can be more difficult to detect on a mammogram.
• Radiation therapy ("x-ray therapy") -- Women who had radiation therapy to the chest (including the breasts) before age 30 are at an increased risk of developing breast cancer throughout their lives. This includes women treated for Hodgkin's disease. Studies show that the younger a woman was when she received her treatment, the higher her risk of developing breast cancer later in life.
• DES (diethylstilbestrol ) -- The drug DES was given to some pregnant women in the United States between 1940 and 1971. (It is no longer given to pregnant women.) Women who took DES during pregnancy may have a slightly increased risk of breast cancer. The possible effects on their daughters are under study.
• Body weight -- Studies have found that the chance of getting breast cancer after menopause is higher in women who are overweight or obese.
• Physical activity level -- Women who are physically inactive throughout life may have an increased risk of breast cancer. Being active may help reduce risk by preventing weight gain and obesity.
• Alcohol -- Studies suggest that the more alcohol a woman drinks, the greater her risk of breast cancer.