Cigarette smoking and colorectal cancer mortality among 602,242 Norwegian males and females

Background: Colorectal cancer (CRC) is one of the main cancer types, with high incidence and mortality in Norway. We examined the association between different measures of smoking exposure and CRC mortality overall and by subsite in a large Norwegian cohort. Methods: We followed 602,242 participants from four Norwegian health surveys, aged 19-67 years at enrollment between 1972 and 2003 by linkage to the national registries through December 2007. We used Cox proportional hazard models to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) by smoking categories for different CRC endpoints. Results: During a mean follow-up of 14 years, 2,333 Norwegian males and females died of CRC (60% men). Male and female ever smokers had a 20% (HR 1.23, CI 1.08-1.40 and HR 1.22, 95% CI 1.06-1.40, respectively) increased risk of death from CRC compared with sex-specific never smokers. For proximal colon cancer mortality, female ever smokers had a 50% (HR 1.49, 95% CI 1.20-1.87) increased risk compared with female never smokers. The increased risk of rectal cancer mortality was about 40% higher for male ever smokers (HR 1.43, 95% CI 1.14-1.81) compared with male never smokers. A test for heterogeneity by sex showed an increased risk of rectal cancer mortality among men which was significant for former smokers (Wald chi(2)=0.02) and an increased risk of proximal colon cancer mortality among women which was significant for ever and former smokers (Wald chi(2)=0.02 and chi(2)=0.04, respectively). Conclusion: Smoking is associated with increased CRC mortality in both sexes. The risk of rectal and proximal colon cancer mortality was most pronounced among male and female smokers respectively.