Smoking increases rectal cancer risk to the same extent in women as in men: results from a Norwegian cohort study

BACKGROUND: Smoking has recently been established as a risk factor for rectal cancer. We examined whether the smoking-related increase in rectal cancer differed by gender. METHODS: We followed 602,242 participants (49% men), aged 19 to 67 years at enrollment from four Norwegian health surveys carried out between 1972 and 2003, by linkage to Norwegian national registries through December 2007. Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated by fitting Cox proportional hazard models and adjusting for relevant confounders. Heterogeneity by gender in the effect of smoking and risk of rectal cancer was tested with Wald χ(2). RESULTS: During a mean follow-up of 14 years, 1,336 men and 840 women developed invasive rectal cancer. Ever smokers had a significantly increased risk of rectal cancer of more than 25% for both men (HR = 1.27, 95% CI = 1.11-1.45) and women (HR = 1.28, 95% CI = 1.11-1.48) compared with gender-specific never smokers. Men smoking ≥20 pack-years had a significantly increased risk of rectal cancer of 35% (HR = 1.35, 95% CI = 1.14-1.58), whereas for women, it was 47% (HR = 1.47, 95% CI = 1.13-1.91) compared with gender-specific never smokers. For both men and women, we observed significant dose–response associations between the risk of rectal cancer for four variables [Age at smoking initiation in years (both p(trend) <0.05), number of cigarettes smoked per day (both p(trend) <0.0001), smoking duration in years (p(trend) <0.05, <0.0001) and number of pack-years smoked (both p(trend) <0.0001)]. The test for heterogeneity by gender was not significant between smoking status and the risk of rectal cancer (Wald χ(2), p -value; current smokers = 0.85; former smokers = 0.87; ever smokers = 1.00). CONCLUSIONS: Smoking increases the risk of rectal cancer to the same extent in women as in men.