environmental risk factors are encountered; optimal approaches to assessing exposures, designing and analyzing epidemiologic studies, and integrating analysis of genetic and environmental influences; the possible combined effects of a multitude of low-level chemical exposures; and interpretation of findings from studies in animals and in vitro systems.

Although many questions remain regarding the contributions of environmental factors to breast cancer risk, evidence suggests that women may have some opportunities to reduce their risk of breast cancer through personal actions: avoiding unnecessary medical radiation throughout life, avoiding use of some forms of postmenopausal hormone therapy, avoiding smoking, limiting alcohol consumption, increasing physical activity, and, for postmenopausal breast cancer, minimizing weight gain. The potential risk reductions for any individual woman will vary and may be modest, but the impact of these actions could be important at a population level. In many cases, however, lack of robust data on environmental agents’ effects on human breast cancer risk, especially during different life stages, and some sense of the trade-offs involved, are major challenges for identifying evidence-based actions that could be taken at the individual or societal level to reduce breast cancer risk.

Recommendations for research include applying a life course perspective and a transdisciplinary approach to studies of breast cancer, developing new and better tools for epidemiologic research and carcinogenicity testing of chemicals and other substances, developing effective preventive interventions, developing better approaches to modeling breast cancer risks, and improving communication about breast cancer risks to health care providers, policy makers, and the public.

Breast cancer has long been the most common invasive noncutaneous cancer among women in the United States, accounting for an estimated 230,480 new cases in 2011.1 After lung cancer, it is the second most common cause of women’s cancer mortality, with about 39,520 deaths expected in 2011. In 2011, there were also approximately 2,140 new cases of breast cancer and 450 breast cancer deaths among men in the United States.

Knowledge about the complexity of breast cancer continues to grow: the characterization of multiple tumor subtypes; the likelihood that critical events in the origins of breast cancer can occur very early in life; the variety of pathways through which breast cancer risks may be shaped; the likely contribution to breast cancer of some fundamental biological processes;

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1Approximately 57,650 noninvasive (in situ) breast tumors will also have been diagnosed in 2011.



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