Risk of proximal and distal colorectal cancer following flexible sigmoidoscopy: a population-based cohort study

Little is known about the risk of proximal and distal colorectal cancer (CRC) following flexible sigmoidoscopy (FS) in usual clinical practice. Our objective was to estimate the annual incidence of CRC within 7 yr following FS and to identify factors associated with incident CRC in those with a negative FS.

In this population-based retrospective cohort study, we included men and women 50-80 yr of age who had a negative or positive FS during 1996-1998 in Ontario. We followed each individual through December 31, 2005 and calculated the age- and sex-standardized incidence rates (SIRs) and 95% confidence intervals (CIs) for distal and proximal CRC. We compared the relative rate (RR) and 95% CIs of incident CRC between the negative and positive FS cohorts and the remaining Ontario population. Cox models were used to evaluate factors associated with incident cancers.

The RR for distal CRC was significantly lower among those with a negative FS than in the Ontario population during each year of follow-up except the first year. For example, at 7 yr, the SIR for distal CRC following negative FS was 0.74 cancers/1,000 persons (95% CI 0.46-1.13) compared with 1.07/1,000 (95% CI 1.02-1.11) in the Ontario population (RR = 0.69, 95% CI 0.40-0.99). The RR for proximal CRC, except for year 2, did not differ between the negative FS cohort and the Ontario population during follow-up. In the positive FS cohort, the RR for distal CRC was significantly higher in the first year of follow-up compared with the Ontario population, but not thereafter. The results were similar for the RR for proximal CRC in the positive FS cohort. Only age was significantly associated with incident CRC following negative FS.

Following negative FS, the incidence of distal but not proximal CRC was reduced for up to 7 yr. Following a positive FS, the incidence of distal and proximal cancer after the first year of follow-up did not differ from the Ontario population. The benefit of FS was confined to the distal colon, emphasizing the potential limitation of FS in practice.