These systematic reviews found that a combination of risk factors with breast density was the best approach to estimating a woman's risk of breast cancer. We also found that both tamoxifen and raloxifene reduced the risk of invasive breast cancer. Thus, the evidence supports systematic assessment of women's risk of breast cancer and the recommendation that women at high risk consider chemoprevention to reduce that risk. An additional finding of our review was that most studies suggest that exercise, weight reduction, low-fat diet, and reduced alcohol intake may reduce a woman's risk of breast cancer, supporting that recommendations for lifestyle changes should be part of programs for primary prevention of breast cancer.
) published in 2002 and focused on selecting patients for chemoprevention of breast cancer with tamoxifen recommended that physicians consider prescribing tamoxifen for women who have a high risk for breast cancer and who are at low risk of adverse effects. There have been several developments in prevention of breast cancer since then, including new risk models using assessment of breast density and approval by the Food and Drug Administration of raloxifene for the prevention of breast cancer. The guidelines did not consider sex hormone levels for assessing risk or lifestyle changes for prevention of breast cancer.
Our meta-analysis found that breast density, as determined by either qualitative BI-RADS or quantitative methods, is a strong risk factor for breast cancer. The combination of breast density by either method with risk factors provides the best estimates of breast cancer risk.
Although estradiol and testosterone levels are associated with the risk of developing estrogen receptor–positive breast cancer (36
), these measurements are not yet ready for routine clinical use because commonly used methods are expensive and only moderately correlated with each other (95
). There is a need for a standardized and inexpensive method for measuring sex hormone levels with established value for improving estimates of the risk of breast cancer based on assessments of risk factors and breast density.
The US Food and Drug Administration has approved tamoxifen and raloxifene for prevention of breast cancer in high-risk women. Although our meta-analysis suggested that raloxifene may have a somewhat greater benefit than tamoxifen, the Study of Tamoxifen and Raloxifene, a randomized trial that directly compared these two agents, found that the two drugs had essentially identical effects on the risk of invasive breast cancer (96
). Therefore, the choice of agent depends on consideration of the risk profile for potential adverse effects of an individual patient (46
). The level of risk for breast cancer at which chemoprevention should be considered depends on the balance of benefits and costs of therapy. Analyses to define that level of risk are underway.
Our systematic reviews support recommending exercise, weight management, and reducing alcohol intake to lessen breast cancer risk in postmenopausal women. In contrast, we found that increased intake of fruits and vegetables was not associated with a decreased risk of breast cancer. The results of our reviews of studies published up to 2008 agree with a systematic review by Michels et al. (100
) of studies published up to 2005. They found associations between body mass index, weight gain, or alcohol intake and increased risk of postmenopausal breast cancer and no association between fruit and vegetable intake and risk of invasive breast cancer. They also found no association between dietary intake of antioxidant vitamins A, C, and E and carotenoids, and they observed inconsistent or no associations with blood levels of antioxidant vitamins.
Our review and the studies on which it is based have limitations. Most of the evidence that we found about assessing and reducing risk of breast cancer involved postmenopausal women. There is less evidence that combinations of risk factors and breast density also identify high-risk premenopausal women who may consider ways to reduce their risk. Models that combine breast density and risk factors for breast cancer still have modest predictive accuracy for breast cancer (c-statistics range = 0.63 to 0.67); thus, there is a need for new markers that are strongly associated with risk of breast cancer. Although breast density is a strong risk factor for breast cancer, BI-RADS grading that could be widely used has only modest reproducibility and more reproducible quantitative approaches are not yet validated or feasible for clinical use; thus, our estimate of increased predictive accuracy may not be applicable to clinical practice at the current time. Absolute rates of benefits and harms provide clinically meaningful estimates of the value of risk markers and treatments to reduce risk. However, the studies in our reviews had diverse designs, populations with different degrees of risk, and methods of analyzing and expressing data that precluded reporting the results about benefits and harms as absolute rates.
A further limitation is that evidence about lifestyle changes to reduce breast cancer risk is generally based on observational studies. Components of food intake are complex and difficult to ascertain by questionnaire, and self-report has limited accuracy that tends to attenuate associations. There may be interactions or associations between intake of nutrients, such as dietary fat, and total energy intakes or between intakes and personal characteristics, such as body fat, estrogen levels, intake of alcohol, or use of medications. This complexity underscores the uncertainties inherent in observational studies of diet and risk of breast cancer and the necessity of large randomized trials to quantify the effects of specific dietary changes on the risk of breast cancer. The strength of associations between lifestyle changes and risk of breast cancer is modest. Nevertheless, because these lifestyle changes are safe, they can be recommended to all women regardless of breast cancer risk.
Several practical issues must be addressed before systematic assessment for risk of breast cancer is implemented widely. Physicians and patients will need to be educated about breast cancer risk and ways to reduce risk. Systems to routinely assess risk factors and breast density, and to report the patient's risk of breast cancer to her and to her physician, must be developed. Risk estimates would need to be communicated to women in ways that minimize inappropriate worry and support well-informed decisions (101
). Assessing risk would also identify women who have a strong family history of breast cancer that may warrant genetic testing, so reports will need to encourage appropriate referral for genetic counseling (103
). Estimation of breast cancer risk may also be useful for deciding whether to refer a woman for additional assessment with magnetic resonance imaging (104
In conclusion, evidence from these reviews supports systematic assessment of postmenopausal women for breast cancer risk with risk factors and assessment of breast density. Chemoprevention should be considered for those at high risk; however, cost–benefit analyses are needed to provide specific recommendations about who should be offered chemoprevention. Several lifestyle changes can be recommended to postmenopausal women, regardless of their estimated risk category.