The association between smoking and bladder cancer risk appears to be stronger now than it was 30 years ago, particularly among women, according to a report in the Aug. 17 issue of JAMA.
Smoking now accounts for 50% of the population-attributable risk for bladder cancer in men and 52% in women, whereas it accounted for approximately 50% of the risk in men but only 20%-30% of the risk in women in the 1980s, said Neal D. Freedman, Ph.D., of the National Cancer Institute, and his associates.
"This is the first article to our knowledge to demonstrate that ... the population-attributable risks for smoking and bladder cancer are now similar in U.S. men and women," they noted.
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Smoking now accounts for roughly half of the population-attributable risk for bladder cancer in men and women.
An epidemiologic study cannot establish causality, but it appears likely that these findings stem from two trends that occurred during the same time frame: The prevalence of smoking has increased among women, and the composition of cigarette smoke has changed, with higher concentrations of tobacco-specific nitrosamines and of beta-naphthylamine, a known bladder carcinogen.
Dr. Freedman and his colleagues examined the link between smoking and bladder cancer in part because rates of bladder cancer have remained stable over the past 30 years, even though the prevalence of smoking has substantially declined during that period. They used data from the prospective National Institutes of Health-AARP (NIH-AARP) Diet and Health Study, which assessed smoking status and followed men and women residing in eight states from 1995 through 2006.
For this analysis, the investigators focused on 281,394 men and 186,134 women who were aged 50-71 years and were cancer-free at baseline. They linked the study information with data in cancer registries in the eight states, and found that 3,896 of the men and 627 of the women developed incident bladder cancer.
Compared with subjects who had never smoked, those who were current smokers had an adjusted hazard ratio of 3.89 and those who were former smokers had an adjusted HR of 2.14. The adjusted hazard ratio for currently smoking men and women combined was 4.06 (JAMA 2011;306:737-45).
Dr. Freedman and his associates then performed a meta-analysis of seven U.S. prospective cohort studies of smoking and bladder cancer that were conducted in 1963-1987, to compare how risk estimates have changed over time. They found that the summary hazard ratio was 2.94 for men and women combined, "which was significantly lower than that observed in our study."
"These results support the hypothesis that the risk of bladder cancer associated with cigarette smoking has increased with time in the U.S., perhaps a reflection of changing cigarette composition," the researchers said.
The findings further suggest that the stronger association between smoking and bladder cancer may have offset the decreased prevalence of smoking in the population, which might explain why the prevalence of bladder cancer has not declined.
"These observations parallel those previously reported for lung cancer, in which changes in cigarette design have been linked to stronger" associations between smoking and that malignancy, they noted.
This study was limited in that the data did not quantify smoking, so smoking duration or pack-years of exposure could not be accounted for, they added.
This study was supported by the National Institutes of Health, the National Cancer Institute, the AARP, and eight state cancer registries. No other relevant financial disclosures were reported.