Pre-diagnostic colonoscopies reduce cancer mortality - Results from linked population-based data in South Australia

Background: To investigate the association between pre-diagnostic colonoscopy and colorectal cancer mortality in South Australia. Methods: Colonoscopy histories were obtained for colorectal cancer patients diagnosed in 2003-2013 using linked Medical Benefits Schedule (MBS) claims, hospital-inpatient and cancer-registry data. Colonoscopy histories included the year of colonoscopy, numbers of examinations, and the time from first colonoscopy to diagnosis. Histories of multiple exposures to colonoscopies, and exposures of greater than a year from initial colonoscopy to diagnosis, were regarded as indicators of screening or surveillance activity. Colonoscopies occurring within one year of diagnosis were regarded as more likely to be a response to cancer symptoms than those occurring > 1 year before diagnosis. Associations between colonoscopy history and post-diagnostic survival were analysed using sub-hazard ratios (SHRs) from competing risk regression adjusted for socio-demographic and cancer characteristics. Results: Having pre-diagnostic colonoscopy was associated with an unadjusted reduction in risk of colorectal cancer death of 17% (SHR: 0.83, 95% CI 0.78-0.89). After adjusting for time period and sociodemographic characteristics, the risk of colorectal cancer death reduced by 17% for one pre-diagnostic colonoscopy examination; 27% for two pre-diagnostic colonoscopy examinations; and 45% for three or more pre-diagnostic colonoscopy examinations. Those with a time of over one year from first colonoscopy in the study window to diagnosis, when compared with less than one year, had a 17% lower risk of colorectal cancer death in this adjusted analysis. These reductions were substantially reduced or eliminated when also adjusting for less advanced stage. Conclusions: Pre-diagnostic colonoscopy, and more so, multiple colonoscopies and first colonoscopy occurring over one year from initial colonoscopy to diagnosis, were associated with longer survival post diagnosis. This was largely explained by less advanced cancer stage at the time of diagnosis.