Risk of colorectal cancer associated with active smoking among female teachers

The objective of this study is to examine the risk of colorectal cancer associated with active smoking among members of the California Teachers Study (CTS), a large cohort of female public school employees for whom highly detailed smoking information is available. The analysis was conducted among the 122,264 CTS participants who lived in California at cohort entry in 1995, had no prior history of colorectal cancer, and provided detailed smoking information. Cases of invasive colorectal cancer diagnosed 1995-2009 were identified from the California Cancer Registry. 1,205 cases were identified, including 650 in the proximal colon, 267 in the distal colon and 288 in the rectum. Hazard ratios and 95% confidence intervals were estimated using Cox proportional hazards models, stratified by age at cohort entry and adjusted for race/ethnicity. Compared to never smokers, current smokers had an approximately 30% increased risk of colorectal cancer. This risk increased with greater intensity and duration of smoking. A slightly elevated risk was also noted for former smokers, but was only statistically significant for those who had quit within 5 years prior to cohort entry. Preliminary stratified analyses suggested consistent results across cancers of the colon and rectum and within subsites of the colon. If these relationships persist when we further adjust for other risk factors, our results would provide convincing evidence for the importance of active smoking as a risk factor for colorectal cancer. Such evidence could provide the impetus for updating screening guidelines to include targeting people with long active smoking histories.