Background & aims: The incidence of colorectal cancer (CRC) overall is reduced for up to 10 years after a negative colonoscopy. The objective of this research was to determine the incidence of proximal and distal CRC after a negative complete colonoscopy.
Methods: A cohort of Ontario residents aged 50 to 80 years who had a negative complete colonoscopy between January 1, 1992, and December 31, 1997, was identified by using linked administrative databases. Cohort members had no prior history of CRC, inflammatory bowel disease, or recent colonic resection. Each individual was followed up through December 31, 2005, and the relative rate (RR) of overall CRC, distal CRC, and proximal CRC was compared with the remaining Ontario population.
Results: A cohort of 110,402 individuals with a negative complete colonoscopy was identified. The RR of CRC overall and the RR of distal CRC remained significantly lower than the Ontario population. For example, at year 14 the RR of distal CRC was 0.21 (95% confidence interval, 0.05-0.36). The RR of proximal CRC was significantly lower than the Ontario population in half of the follow-up years, mainly after 7 years of follow-up.
Conclusions: Over a 14-year follow-up period, negative complete colonoscopy was associated with a subsequent reduced incidence of CRC overall, and of incident CRC in the distal colon. However, the reduction in incidence of proximal CRC differed in magnitude and timing, and occurred in half the follow-up years, mainly after 7 years of follow-up. These results highlight an important limitation of colonoscopy in usual clinical practice.