Cigarette smoking and risk of colorectal cancer among Norwegian women

OBJECTIVE: The association between cigarette smoking and colorectal cancer (CRC) is still not established. In 2002, Norwegian women had the second highest incidence of CRC in the world. A large proportion of Norwegian women are ever smokers. We examined the association between cigarette smoking and CRC incidence among Norwegian women. METHODS: We followed 68,160 women, aged 30–69 years, from the Norwegian Women and Cancer Study who completed a questionnaire in 1996 or 1998 by linkages to national registers through 31 December 2005. Rate ratios (RRs) and 95% confidence intervals (CIs) were estimated by fitting Cox proportional hazard models. Subsequently, we estimated the population attributable fraction. RESULTS: Altogether, 425 incident cases of primary, invasive CRC were identified. Ever smokers had a 20% increased risk of CRC (RR = 1.2; 95% CI = 1.0–1.5), a 30% increased risk of colon (RR = 1.3; 95% CI = 1.0–1.7), and a 10% increased risk of rectal (RR = 1.1; 95% CI = 0.7–1.5) cancer compared to never smokers. The population attributable fraction was estimated to be 12% which indicated that approximately one in eight of the CRC cases could have been prevented at a population level. CONCLUSION: Our results support the hypothesis that cigarette smoking is a preventable cause of CRC among women.