Smoking and Risk of Colorectal Cancer may differ by Anatomical Subsite and Sex.

The purpose of this study was to examine if the increased risk of colorectal cancer due to cigarette smoking differed by anatomical subsite and sex. We analyzed data from 188,052 participants (45% men, aged 45-75 years), who were enrolled in the Multiethnic Cohort Study in 1993-1996. During a mean follow-up of 16.7 years, we identified 4,879 incident cases of invasive colorectal adenocarcinoma. In multivariate Cox regression models, compared with never smokers of the same sex, male ever smokers had a 39% [hazard ratio (HR) =1.39; 95% Confidence interval (CI): 1.16,1.67] higher risk of left, but not of right (HR=1.03; 95% CI: 0.89,1.18) colon cancer, while female ever smokers, had a 20% (HR=1.20; 95% CI: 1.06,1.36) higher risk of right, but not of left (HR=0.96; 95% CI: 0.80,1.15) colon cancer. Compared with male smokers, female smokers had a greater increase in risk of rectal cancer with number of pack-years (Pheterogeneity = 0.03). Our results suggest that male smokers are at increased risk of left colon cancer and female smokers at increased risk of right colon cancer. Our study also suggests that females who smoke may have a higher risk of rectal cancer due to smoking than their male counterparts.