With many countries initiating population colorectal cancer (CRC) screening, including Hong Kong, more robust guidance for surveillance interval is required to maximize subject's benefit with optimal use of resources. Surveillance interval after removal of advanced adenoma at screening colonoscopy remains unclear. The current recommendation of 3-year is based on data collected before widespread implementation of population screening programs and quality metrics in colonoscopy. These high-risk subjects are those most likely to benefit from surveillance and represent the majority of the demand in surveillance colonoscopies.

The aim of the study is to determine whether the risk of metachronous advanced neoplasia increases if surveillance interval was beyond the current recommendation of 3 years for high-risk subjects with advanced adenoma polyp at screening colonoscopy.

With many countries initiating population colorectal cancer (CRC) screening, including Hong Kong, more robust guidance for surveillance interval is required to maximize subject's benefit with optimal use of resources. Surveillance interval after removal of advanced adenoma at screening colonoscopy remains unclear. The current recommendation of 3-year is based on data collected before widespread implementation of population screening programs and quality metrics in colonoscopy. These high-risk subjects are those most likely to benefit from surveillance and represent the majority of the demand in surveillance colonoscopies.

The investigators hypothesize that the risk of metachronous advanced neoplasia significantly increases if surveillance interval was prolonged beyond 3 years for high-risk subjects. If such is true, our study's findings will provide definitive evidence to existing guidelines and the future Hong Kong population CRC screening programme of setting surveillance interval at 3-year. Conversely, if our study shows that there is no significant increase in risk beyond 3-year surveillance interval, an extended interval of 5-year is justified.

Inclusion Criteria:

1. Advanced adenoma at screening colonoscopy
2. ≥3 adenomas at screening colonoscopy
3. Cecal intubation at screening colonoscopy (preferably documented by images/video of the apendiceal orifice and the ileocecal valve; but not required)
4. Complete excision of all polyps at screening colonoscopy findings (after review of endoscopy reports and pathological specimens)
5. Eligible for surveillance in out-patient setting

Exclusion Criteria:

1. Lack of consent
2. Incomplete screening colonoscopy
3. Incomplete endoscopic excision of polyps at screening colonoscopy
4. CRC at screening colonoscopy
5. Polyps requiring Endoscopic Submucosal Dissection at screening colonoscopy
6. Serrated polyps ≥ 10 mm in diameter at any colorectal location or ≥ 5 mm if located proximal to the splenic flexure on screening colonoscopy
7. Genetic cancer syndrome (adenomatous or serrated polyposis syndrome; Lynch or Lynch-like syndrome)
8. Inflammatory bowel disease
9. History of surgical colon resection for any reason
10. Severe co-morbidity with reduced life expectancy (NYHA 3-4)