Smoking status before and after colorectal cancer diagnosis and mortality in Korean men: A population‐based cohort study

Abstract Background Smoking is a well‐known risk factor for colorectal cancer incidence; however, the effect of smoking before and after cancer diagnosis on mortality has not been addressed well. Thus, we aimed to evaluate the association of prediagnosis and postdiagnosis smoking status and mortality among colorectal cancer patients. Methods A retrospective cohort consisted of 37,079 male colorectal cancer patients. Smoking status was defined from information within 2 years of colorectal cancer diagnosis for prediagnosis and at least 1 year later for postdiagnosis. The prediagnostic and postdiagnostic smoking status were categorized into four groups (nonsmoker/nonsmoker, nonsmoker/smoker, smoker/nonsmoker, and smoker/smoker). Hazard ratios (HRs) and 95% confidence intervals (CIs) were estimated using the Cox proportional hazard model. Results During a median of 6.3 years of follow‐up, a total of 3980 deaths and 2137 deaths from colorectal cancer occurred. The number of prediagnosis smokers were 11,100 and 62.4% of them quitted smoking after the diagnosis. Significantly elevated mortality rate in prediagnosis smokers was observed regardless of postdiagnosis smoking status (smoker/nonsmoker [HR, 1.30; 95% CI, 1.20–1.41] and smoker/smoker [HR, 1.21; 95% CI, 1.09–1.34]). Among patients treated with surgical operation only, those who quit smoking after diagnosis showed lower mortality rates compared to continual smokers (HR, 0.80; 95% CI, 0.67–0.96). Conclusions Smoking before cancer diagnosis rather than postdiagnosis has stronger impact on prognosis colorectal cancer patients, and quitting smoking may improve survival, especially among early stage colorectal cancer patients.