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The last 30 years have seen an explosion of evidence that lifestyle drives cancer risk, and this evidence has also documented, through rigorous studies, an increasing number of effective prevention strategies.1, 2 We have moved from estimating the potential for prevention on the basis of international variation in cancer rates and data on tobacco smoking and lung cancer from the American Cancer Society (ACS) cohorts that Doll and Peto used for their estimation3 to a broader and deeper understanding of lifestyle factors including diet,4 lack of physical activity and obesity,5 drugs, and infections6 as modifiable causes of cancer. The ACS cohorts have contributed substantially to these advances,7 and researchers funded by the ACS continue to make major contributions to cancer prevention. With what we know now, we could already prevent more than half of all cancers if we implemented effective prevention programs.8

In this context, we welcome the article in this issue by Fontham et al9 that provides an overview of ACS priorities in cancer prevention. These priorities are based on the strength of scientific evidence and the potential benefit of a cancer-prevention strategy. The latter is influenced by the magnitude of disease risk from the exposure of concern, the proportion of the population exposed, the time course for reduction in risk after exposure is reduced or eliminated, and information on the effectiveness, feasibility, and potential adverse consequences of the proposed intervention.10 The ACS also considers resources that would be required to implement the intervention and how the ACS's actions can be designed to complement, rather than duplicate, those of other private and governmental collaborators.

The proportional contributions of various environmental factors to cancer risk have been debated for decades.11 Remarkably, scientific consensus generally accepts the original Doll and Peto estimates,3 in which the major known causes include smoking (30%), diet and lack of physical activity (35%), and infections (17% of cancers worldwide), with smaller fractions ascribed to reproductive and sexual behavior, alcohol consumption, medicines and medical procedures, occupation, geophysical factors, and pollution. The Doll and Peto estimates have withstood multiple reanalyses, the only substantive revisions being to increase the percentage attributed to infection and to combine obesity with diet and physical inactivity. Occupational exposures are estimated to account for between 2% and 5% of cancers in the United States. It is important to recognize that none of these categories are mutually exclusive.

Tobacco control has been a top priority in ACS cancer prevention efforts during the last 50 years. ACS has been a leader in efforts to prevent children from starting to smoke and in promoting smoking cessation. Progress in reducing tobacco use has resulted in a 25.5% reduction in male lung cancer mortality from 1990–2006. This success, although still incomplete, illustrates how prevention can be incorporated into practice through multiple avenues:10 health care providers are crucial to motivating smoking cessation. Laws that increase excise taxes reduce consumption and deter initiation by children. Grass-roots efforts have banned smoking in public places in many states and profoundly changed social norms.

The current report was developed, in part, to address public concern about risk from environmental and occupational exposures. Cancer patients andtheir families often ask their physicians about these issues. Concern is amplified by the perception that environmental exposures are involuntary and not subject to individual control. The report in this issue discusses the role of occupational and environmental causes of cancer in the context of the broader cancer prevention agenda.9 Importantly, the authors describe the role of the International Agency for Research on Cancer and the National Toxicology Program in conducting rigorous evaluation and classification of carcinogens. Consensus reports published by these agencies have helped to motivate major reductions in exposure to recognized carcinogens in occupational and environmental settings in the United States and other high-income countries. In discussing the limitations of current systems for carcinogenicity testing, the authors describe new, alternative methods that toxicologists have developed to help prioritize substances and to reduce the large backlog of substances that have never been tested.

For some exposures, effective prevention requires an understanding of how the timing of exposure, as well as the dose, affects cancer risk. In many cases, we know little about how the timing of exposure affects risk. Even in the case of tobacco, researchers have only recently appreciated the rapidity and extent to which risk changes among those who quit smoking compared with those who continue. Tobacco is easier to study than other lifestyle factors because dose can be measured through the packaging and addictive nature of cigarettes, and because parameters, such as age at initiation, amount smoked, and age at cessation, are more clearly recalled than is the case for dietary exposures.12

With regard to other prevention measures, the ACS has made major contributions to our understanding of the role of obesity as a cause of cancer, the role of diet in cancer prevention, and the role of over-the-counter drugs, such as aspirin, in cancer prevention. ACS addresses these issues through a number of policy statements as summarized in the Fontham et al9 report. These important contributions inform the policy statements that are routinely distributed to providers, the public, and to policy makers.

Clinicians are familiar with their role in counseling patients regarding the disease impact of tobacco and its cessation and are growing more comfortable with counseling on diet, weight control, and increasing physical activity. There is evidence that counseling in the context of primary care can influence diet and physical activity, although providing health recommendations alone does not ensure that patients will change health behaviors. One New Zealand study of general practice indicated that the proportion of middle-aged women attaining 150 minutes of moderate activity per week increased from 10% at baseline to 39% at 24 months, quality of life improved, although no significant differences in clinical outcomes were observed over this time interval.13 The role of providers in delivery of interventions such as these will likely grow in the years ahead. Meanwhile, diet and physical activity continue to receive much attention in the popular press, have a growing body of scientific evidence relating them to cancer incidence, and, through appropriate prevention strategies, could also contribute substantially to reduction in the burden of cancer in society.

In contrast, most clinicians have little experience in evaluating or quantifying the potential risks from chemicals released by consumer products or from pollutants in the ambient environment. The effects of these exposures will depend on the dose, duration, and potentially the timing of exposure. Variations in individual susceptibility may also affect risk. The commitment of ACS to explore further how these issues are evaluated and how communication about these issues can be improved is most welcome.

The ACS is to be commended for its leadership in cancer prevention and its commitment to building scientifically based prevention strategies. The ACS has the ability to convene leaders from the academic and public health communities with its own leaders to think through these complex issues and to articulate positions that guide cancer prevention. We look forward to further deliberations from this committee in the years ahead.

References

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  2. References
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    Colditz GA, Beers C. Active cancer prevention. In: ElwoodM, SutcliffeS, eds. Cancer Control. Oxford: Oxford University Press; 2010 (in press).
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    Doll R, Peto R. The Causes of Cancer: Quantitative Estimates of Avoidable Risks of Cancer in the United States Today. New York: Oxford University Press; 1981.
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