estimated to be less important than lifestyle factors as contributors to cancer in the United States today (Monson and Christiani, 1997). Given the major shift toward the regulation of occupational exposures to known carcinogens and the active surveillance of workers previously exposed to many of these carcinogens, the most modifiable component of cancer risk remains lifestyle factors.
Doll and Peto acknowledged that although their estimates were theoretical maximums, it would take a great amount of time to effect social change and that the large amount of change necessary to decrease the cancer incidence rate 75 to 80 percent was unlikely. Although Doll and Peto’s methods might seem rudimentary by today’s methodological standards, they provided a foundation for later, similar work.
In 1996 Willett and colleagues used an international comparison approach at the ecological level similar to that used by Doll and Peto (1981) to assess the degree to which cancer mortality could be reduced in the United States. They limited their data on rates of mortality from cancer to those for the United States, Japan, and China because they were considered to be the most reliable data available. They used age-adjusted data from the Surveillance, Epidemiology, and End Results (SEER) Program and from Cancer Incidence in Five Continents, an IARC publication (Parkin et al., 1992). The country with the lowest site-specific cancer incidence rate was considered to have the “baseline” rate. The difference between the highest and the baseline incidence rates was calculated to indicate the maximum degree of cancer reduction possible.
To provide information on behavior-specific reductions in the rate of mortality from cancer, the investigators considered data for unique populations in the United States such as those from the Adventist Health Study, which indicated the magnitude of change possible within the context of the current U.S. culture. Neither Willett and colleagues nor Doll and Peto provided a time frame for the reduction of the cancer mortality rate estimated in their analyses; that is, they did not specify the latency of effects.
Willett and colleagues reported the degree of reduction in the incidence of cancer possible by making specific behavioral changes, considering not only the theoretically greatest possible reduction but also more realistic reductions based on the amount of behavioral change that would likely occur in the U.S. population. Considering the downward trend in cigarette smoking, they suggested that a two-thirds reduction in the number of individuals who smoked and, thus, a similar eventual reduction in tobacco-related cancer mortality rates would be possible. If a percentage of those who consumed more than two drinks per day reduced their intakes, they estimated that alcohol-related cancer mortality rates could be reduced by