Background and study aims: current guidelines recommend genetic counseling and intensive colonoscopy surveillance for patients with ≥10 colorectal adenomas based on scarce data. We investigated the prevalence of this condition in a FIT (fecal immunochemical test)-based colorectal (CRC) screening program and the incidence of metachronous lesions during follow-up.

Patients and methods: we retrospectively included all FIT-positive participants with ≥10 adenomas at index colonoscopy between 2010 and 2018. Surveillance colonoscopies (SVC) were collected until 2019. Patients with inherited syndromes, serrated polyposis syndrome, total colectomy or lacking surveillance data, were excluded. Cumulative incidence of CRC and advanced neoplasia (AN) were analyzed by Kaplan-Meyer analysis. Risk factors of metachronous AN were investigated by multivariable logistic-regression analysis.

Results: 215/9,582 (2.2%) participants had ≥10 adenomas. Germline genetic testing was performed in 92% of patients with ≥20 adenomas identifying 2 (3.3%) inherited syndromes. 3-year cumulative incidence of CRC and AN was 1%, and 16%, respectively. In 39 (24.2%) patients no polyps were found at first SVC. The presence of advanced adenoma was independently associated with a higher risk of AN at first SVC (OR: 3.91, 95% confident interval : 1.12-13.62; p=0.03). Beyond the first SVC, the risk of metachronous AN was lower.

Conclusions: the prevalence of ≥10 adenomas in a FIT-based CRC screening program is 2.2% and a small proportion of inherited syndromes are detected even amongst those with ≥ 20 adenomas. Low rate of post-colonoscopy CRC is observed and the risk of AN beyond the first SVC tends to progressively decrease throughout successive follow-up.