Reduced Smoking Called Key to Male Drop in Cancer Mortality

ATLANTA, Oct. 4 -- Smoking cessation accounts for about 40% of the decrease in cancer deaths seen among U.S. men since 1991, according to a report by American Cancer Society researchers.

From 1991 to 2003, reductions in tobacco smoking have prevented at least 146,000 cancer deaths among men, reported epidemiologists Michael Thun, M.D., and Ahmedin Jemal, Ph.D., in the October issue of Tobacco Control.

The decrease in lung cancer and overall cancer mortality among men began approximately 30 years after the downturn in the smoking rate of U.S. men, the investigators pointed out.

But no decrease in lung cancer mortality has yet occurred among U.S. women, who began regular smoking some 20 to 30 years after men and have been slower to give it up, the epidemiologists said.

From 1991, when the cancer death rate peaked, to 2003, the overall age-adjusted cancer death rate in the U.S. dropped by 16.1% in men and by 8.4% in women.

During the same period, the lung-cancer death rate (a proxy for the total impact of cigarette smoking on all cancer mortality) decreased by 20% in men but increased by 9.6% in women, the researchers reported.

Overall, the researchers reported that on the basis of the absolute change in death rates, the decrease in lung cancer death rates in men accounted for 40% of the decrease in their overall cancer death rates.

In contrast, none of the 8.4% decrease in the all-cancer death rate for women was accounted for by lung cancer. In fact, lung-cancer deaths went in the opposite direction (9.6% increase) of their overall death rates. The decrease in overall cancer mortality in women would have been larger had there not been an increase in lung cancer mortality, the investigators said.

Explaining the use of lung-cancer deaths as a proxy in this study, Drs. Thun and Jemal noted that although active smoking causes a long list of other cancers, lung cancer deaths make up 80% of all smoking-attributable cancer deaths in the U.S. Cigarette smoking, they said, accounts for a larger fraction of lung cancer deaths in men (88%) and women (72%) than any other smoking-related cancer except laryngeal cancer.

Discussing their methodology, the researchers wrote that while it seemed prudent to base the estimates only on observed data and only during the period when cancer deaths actually decreased, this approach greatly underestimated the full impact of reduction in smoking on cancer death rates over the preceding three decades.

A second method was to take into account what might have happened to cancer death rates over the last half century had cigarette smoking not been discouraged. Projecting the increase in lung cancer death rates that would have occurred with increases at the previous rates, they found that without the drop in smoking that began in the 1960s, there would essentially be no decrease in cancer death rates today.

Furthermore, they said, the predicted lung cancer death rates would have increased from 1991 in both sexes, and the increase would have been sufficient to cancel out nearly all of the decrease in overall cancer mortality from 1991 to 2003. This scenario, although hypothetical, they said "provides an alternative, and perhaps more realistic view of what the actual benefits of past reductions in smoking have been with respect to cancer mortality."

In a third approach, the researchers considered the number of cancer deaths avoided, rather than the reduction in death rates, and concluded that about 146,000 lung cancer deaths were prevented or postponed by the decrease in the age-specific lung-cancer death rates in men between 1991 and 2003.

Even though this method considered only the observed change in lung cancer rates during this period, it illustrates that a large number of deaths from lung cancer were avoided because of the decrease in the lung cancer death rates in men during this period.

In turn, Drs. Thun and Jemal said, these findings reflect the reduction in cigarette smoking that occurred because of anti-smoking and other tobacco-control measures implemented since the 1950s.

The researchers mentioned several caveats regarding these estimates. Restricting analyses to lung cancer may underestimate the contribution of smoking reduction by up to 20%. Offsetting this underestimate, at least partly, they said, is the fact that they ignored other risk factors, such as exposure to carcinogens in occupational settings that have also changed since the 1950s. Despite these limitations, the researchers said they believe that the true contribution of smoking reduction is within the range of these estimates.

"The payoff from past investment in tobacco control has only just begun," Drs.Thun and Jemal wrote. "The aging of birth cohorts with lower smoking initiation rates and the anticipated future decrease in lung cancer mortality in women will help to sustain progress." What is necessary, they added, "is that sustained progress in tobacco control is essential if we are to continue to make progress against cancer."