Patients with ≥10 adenomas are considered a high-risk group, yet evidence guiding optimal surveillance intervals remains limited. This study evaluated the risk of metachronous advanced neoplasia (AN) according to the presence of advanced adenoma (AA) at index colonoscopy.

In this multicenter retrospective study, patients with ≥10 conventional adenomas who underwent surveillance colonoscopy within 6 months to 3 years were analyzed. Individuals with colorectal cancer, inflammatory bowel disease, hereditary polyposis syndromes, inadequate bowel preparation, or technically unresectable lesions requiring surgery were excluded. The primary outcome was the incidence of metachronous AN, compared between baseline AA and non-AA groups. Kaplan–Meier and Cox proportional hazards analyses were performed.

Among 447 patients, 313 (70.0%) had baseline AA and 134 (30.0%) did not. Metachronous AN occurred in 23.6% of the AA group and 7.5% of the non-AA group. The cumulative AN risk reached 7.5% at 1 year and 44.9% at 3 years overall, with significantly higher risk in patients with baseline AA (1-year: 9.7% vs. 2.6%; 3-year: 51.6% vs. 30.2%). In the multivariable model, baseline AA remained strongly associated with metachronous AN (HR, 3.83; 95% CI, 1.96–7.49). Across AA subtypes, 3-year AN risk exceeded 50%.

This high-burden population demonstrates a substantial risk of early metachronous AN, particularly among those with baseline AA. These findings support a 1-year surveillance interval for the overall high-burden group, with even more intensive follow-up warranted for patients presenting with baseline AA.