Breast Cancer Risk factors: Aside from being female, age is the most important risk factor for breast cancer. Potentially modifiable risk factors include weight gain after age 18, being overweight or obese (for postmenopausal breast cancer), use of combined estrogen and progestin MHT, physical inactivity, and consumption of one or more alcoholic beverages per day. Medical findings that predict higher risk include high breast tissue density (a mammographic measure of the amount of glandular tissue relative to fatty tissue in the breast), high bone mineral density (routinely measured to identify women at increased risk for osteoporosis), and biopsy-confirmed hyperplasia (especially atypical hyperplasia). High-dose radiation to the chest, typically related to cancer treatment, also increases risk. Reproductive factors that increase risk include a long menstrual history (menstrual periods that start early and/or end late in life), recent use of oral contraceptives, never having children, and having one’s first child after age 30.

Risk is also increased by a personal or family history of breast cancer and inherited genetic mutations in the breast cancer susceptibility genes BRCA1 and BRCA2. Although these mutations account for approximately 5%-10% of all breast cancer cases, they are very rare in the general population (less than 1%), so widespread genetic testing is not recommended. Some population groups, such as individuals of Ashkenazi Jewish descent, have an increased prevalence of BRCA1 and BRCA2 mutation carriers. Women with a strong family history of breast and/or ovarian cancer should be offered counseling to determine if genetic testing is appropriate. Studies suggest that prophylactic removal of the ovaries and/or breasts in BRCA1 and BRCA2 mutation carriers decreases the risk of breast cancer considerably, although not all women who choose this surgery would have developed breast cancer. Women who consider these options should undergo counseling before reaching a decision.

Men with family members who are BRCA gene mutation carriers are also at risk for these mutations, and male BRCA 2 mutation carriers are at particularly increased risk for breast cancer.

Modifiable factors that are associated with a lower risk of breast cancer include breastfeeding, moderate or vigorous physical activity, and maintaining a healthy body weight. Two medications, tamoxifen and raloxifene, have been approved to reduce breast cancer risk in women at high risk. Raloxifene appears to have a lower risk of side effects, such as uterine cancer and blood clots. In women with estrogen-receptor positive breast cancer, additional treatment with tamoxifen reduces the risk of second breast cancers by about half.

Research is ongoing to identify additional modifiable risk factors for breast cancer. The International Agency for Research on Cancer recently concluded that there is limited evidence that tobacco smoking causes breast cancer. There is also some evidence that shift work, particularly at night, is associated with an increased risk of breast cancer.