Smoking and colorectal cancer: a 20-year follow-up study of Swedish construction workers.

BACKGROUND: Although cigarette smoking has consistently been shown to be positively related to the risk of adenomatous polyp development (benign neoplastic growth of epithelial tissue in the colon), most studies of cigarette smoking and the risk of colorectal cancer have been negative. However, in two large prospective studies in women and men, a statistically significant association between cigarette smoking and an increased risk of colorectal cancer was found, but only after more than 35 years of smoking.
PURPOSE: To shed further light on the alleged relationship between long-term smoking and colorectal cancer risk, we performed a retrospective cohort study among Swedish construction workers, with many long-term smokers, complete long-term follow-up, and a large number of observed cases.
METHODS: We analyzed the association of smoking with colon cancer and with rectal cancer, using data on a cohort of approximately 135000 male construction workers. High-quality exposure information was collected with the use of a comprehensive questionnaire filled out at the time of enrollment in the cohort, from 1971 through 1975. Complete follow-up was achieved through 1991 and the subjects were observed for an average of 17.6 years, thereby contributing approximately 2375000 person-years of follow-up. We calculated age-adjusted rate ratios (RRs) with the use of Poisson-based multiplicative multivariate models followed by further multivariate modeling that adjusted for other variables.
RESULTS: A total of 713 incident colon cancers and 505 rectal cancers were observed. There was no statistically significant association between current smoking status, number of cigarettes smoked or number of years smoking, and risk of colorectal cancer. The age-adjusted RRs were 0.98 (95% confidence interval [CI] = 0.82-1.17) and 1.16 (95% CI = 0.94-1.44) for colon and rectal cancers, respectively, among current smokers, and 1.07 (95% CI = 0.63-1.82) and 1.08 (95% CI = 0.58-2.03) among smokers of 25 or more cigarettes per day, relative to nonsmokers. Among smokers for more than 30 years at the start of follow-up, the age-adjusted RRs were 1.03 (95% CI = 0.85-1.25) and 1.21 (95% CI = 0.96-1.53) for colon and rectal cancers, respectively, relative to nonsmokers. Heavy smokers of cigars and pipes had a statistically nonsignificant tendency toward excess risk for colon cancer, but there was no clear dose-risk trend.
CONCLUSION: Our large cohort study did not indicate any excess risk of colon cancer in males who were long-term heavy smokers and provided only weak support for an association with rectal cancer. Our data are thus consistent with the majority of previous reports. The reasons for the discrepancies in comparison with recent U.S. data have yet to be identified.