Background: Patients with multiple or large adenomas are considered as high-risk for metachronous colorectal cancer.

Objective: Evaluate the risks of detecting colorectal cancer, advanced adenoma, and advanced serrated polyps at one-year surveillance colonoscopy in patients with > 5 adenomas or adenomas > 20 mm.

Design: Descriptive, retrospective, multicentric, cohort study. We calculated the absolute risk of developing colorectal cancer, advanced adenomas, and advanced serrated polyps at the one-year surveillance colonoscopy. Potential risk factors for advanced neoplasia at follow-up were evaluated with univariable and multivariable logistic regression analyses.

Settings: This study included data from a multicenter cohort colorectal cancer screening program, conducted from January 2014 to December 2015, based on fecal immunochemical tests in Spain.

Patients: We included 2119 participants with at least one adenoma ≥20 mm or ≥5 adenomas of any size.

Main outcome measures: We calculated the absolute risk of developing colorectal cancer, advanced adenomas, and advanced serrated polyps at the one-year surveillance colonoscopy. Potential risk factors for advanced neoplasia at follow-up were evaluated with univariable and multivariable logistic regression analyses.

Results: At one year, participants displayed 6 colorectal cancers (0.3%), 228 advanced adenomas (10.5%), and 58 advanced serrated polyps (2.7%). The adjusted analysis identified two factors associated with advanced neoplasia: >5 adenomas (odds ratio 1.53; 95% CI: 1.15-2.03; p=0.004) and polyps in a proximal location (odds ratio 1.52; 95% CI: 1.15-2.02; p=0.004).

Limitations: First, the sample size was relatively small compared to other studies with similar aims. Another limitation was the lack of a comparison group, which could have provided more practical results, in terms of surveillance recommendations.

Conclusion: The colorectal cancer detection rate at a one-year colonoscopy surveillance was low among patients classified at high risk of advanced neoplasia. The risk factors for advanced neoplasia were ≥5 adenomas and proximal polyps at baseline.