Previous studies that examined whether the presence of an additional observer, more specifically a GI fellow, during colonoscopy can enhance detection of all polyps and adenomas yielded conflicting results. Of note, all of the aforementioned studies were retrospective and robust evidence derived from well-designed randomized controlled trials are lacking.

The study objective is to examine whether fellow participation during screening, surveillance, or diagnostic colonoscopy influence overall, size-specific, or location-specific adenoma or polyp detection rate.

It will be planned to enroll 812 patients (406 per arms) within 1 year. Adenoma detection rate will be the primary outcome.

Background Colonoscopy is instrumental in colon cancer prevention as through polypectomy it may interfere with the adenoma-carcinoma sequence, thus resulting in a clear survival benefit.

However, not all adenomatous polyps are identified during a colonoscopy. The overall false-negative ("miss") rate for colonic adenomas is estimated to be as high as 24%, according to studies of same-day, tandem colonoscopies. In addition, flat and depressed lesions often remain undetected during white-light colonoscopy.

Low-cost optimization of existing resources, such as use of a second observer or water-aided colonoscopy, were recently found to be able to significantly increase colon adenoma detection rate (ADR).

Previous studies that examined whether the presence of an additional observer, more specifically a GI fellow, during colonoscopy can enhance detection of all polyps and adenomas yielded conflicting results. Of note, all of the aforementioned studies were retrospective and robust evidence derived from well-designed randomized controlled trials (RCTs) are lacking.

.

Technical procedure In the control arm all colonoscopy will be performed by full-time board-certified gastroenterologists who have each done more than 5000 colonoscopy examinations.

In the investigation arm colonoscopy will be performed by gastroenterology fellows. The fellows will always start the case and proceed generally until they are unable to make further progress despite "coaching" from the staff attending.

During the procedures with fellows, the staff attending will always actively participate in the entire procedure and assess for the presence of any lesions.

Colonoscopies will be performed by using only high-definition white-light (HDWL) scopes (Olympus 180 series CF H180).

Bowel preparation will be uniform and consist of 4 L of polyethylene glycol. All detected lesions will be endoscopically removed and samples will be sent to pathologists for histological diagnosis.

Treatment strategy Patients complying with the eligibility criteria will be randomized in a 1:1 fashion to undergo colonoscopy performed by a trainee (under attending physician supervision) or colonoscopy performed by an experienced physician.

Sample size calculation and statistical considerations On the basis of previous retrospective comparative reports, the study is designed to detect an increase in ADR by 8%. Therefore, 812 patients (406 per arm) will be required to have a 80% power to detect the target difference at a 0.05 significance level (two sided). The primary endpoint will be ADR. In order to collect the estimated sample size, up to 1 year of recruiting will be needed.

Inclusion Criteria:

• Adult patients undergoing colonoscopy.

Exclusion Criteria:

• Age under 18 years
• Familial history of polyposis syndrome (familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, juvenile polyposis).
• Incomplete colonoscopy
• Inflammatory bowel disease
• Refusal to provide informed consent.