Sigmoidoscopy and Colonoscopy are Inversely Associated with Both Left- and Right-sided Advanced Adenomas

Background: The U.S. Preventive Services Task Force recommends routine colorectal cancer screening starting at age 50; however, evidence is insufficient to recommend one screening method over the other methods. Common colorectal cancer screening methods include fecal occult blood tests (FOBT), sigmoidoscopy, and colonoscopy.

Objective: We examined the association between advanced adenomas, known precursors to colorectal cancer, and history of screening by FOBT, sigmoidoscopy, and colonoscopy.

Methods: We conducted a case-control study of advanced colorectal adenomatous polyps, which included 306 advanced adenoma cases and 2,287 controls without advanced adenomas, aged 24 to 79, who received an index colonoscopy from 1998 to 2007. All participants completed a questionnaire covering screening history and other colorectal cancer risk factors. Participants with polyps underwent a standard pathology review; adenomas ≥10 mm or at with at least 20% villous components were considered advanced adenomas. We used separate logistic regression models to estimate adjusted odds ratios (ORs) and 95% CIs for the associations between advanced adenomas and previous FOBT, sigmoidoscopy, and colonoscopy at least 2 years before the index colonoscopy.

Results: Each screening method was associated with decreased odds of advanced adenomas; however, only associations for sigmoidoscopy and colonoscopy were statistically significant. The OR for advanced adenomas, comparing participants with at least one previous FOBT to those who had never had FOBT, was 0.82 (95% CI: 0.63–1.06); for sigmoidoscopy, OR = 0.59 (95% CI: 0.46–0.77); and for colonoscopy, OR = 0.53 (95% CI: 0.39–0.72). These associations were similar with respect to left- and right-sided advanced adenomas.

Discussion: Our findings suggest colonoscopy and sigmoidoscopy have similar inverse associations with advanced adenomas in both left and right sides of the colon. This is in contrast to studies of colorectal cancer that suggest endoscopy is associated with a decreased risk for tumors of the left but not right side. The discrepancy between our findings and these studies may be explained if right-sided colorectal cancers are more likely to have nonadenoma precursors that are overlooked by screening.