In this study, Dutch gastroenterologists who are certified for performing colonoscopies on FIT-positive patients in the Dutch population screening program are trained in optical diagnosis with validated methods. After this training, an ex- and in-vivo test phase leads to "accreditation" and endoscopists will be observed in their optical diagnosis for 1 year. During this year, half of the endoscopists will be randomized towards 3-monthly feedback and the other half will receive feedback on their results after 1 year. The endoscopic prediction of endoscopists on polyp histology will be compared to histopathological outcome.

Through the recently started nationwide bowel cancer screening programme in the Netherlands, an extra 70.000 colonoscopies are annually performed. In current practice, all resected colonic lesions are histopathologically analysed. Even diminutive polyps, which rarely harbour cancer or advanced histological features. If endoscopists are able to accurately differentiate between neoplastic and non-neoplastic lesions during colonoscopy, practice could become more efficient and costeffective. This strategy is called optical diagnosis and two clinical practice strategies have been proposed by the American Society of Gastroenterologists (ASGE). First, diminutive polyps could be resected and discarded if >90% of the surveillance intervals predicted on optical diagnosis correlate with the surveillance intervals after histopathological validation (if assessed with high confidence). Second, hyperplastic polyps in the rectosigmoid could be left in situ if endoscopists are able to confidently predict neoplastic histology of diminutive colorectal polyps with a negative predictive value (NPV) of ≥90%.

The accuracy of white light colonoscopy is not acceptable for daily practice (59%-84%), but narrow band imaging (NBI) allows higher accuracies up to 98% and it was demonstrated that experienced endoscopists could reach a NPV of ≥90% for diminutive colorectal lesions. However, recent research shows that community gastroenterologists are not able to meet the quality thresholds proposed by the ASGE. Before this strategy could be safely applied in daily practice, community gastroenterologists should be able to meet thresholds as well.

In this study, Dutch gastroenterologists who are certified for performing colonoscopies on FIT-positive patients in the Dutch population screening program, are trained in optical diagnosis with validated methods. After training, an ex- and in-vivo test phase leads to "accreditation" and endoscopists will be observed in their optical diagnosis for 1 year. During this year, half of the endoscopists will be randomized towards 3-monthly feedback and the other half will receive feedback on their results after 1 year. The endoscopic prediction of endoscopists on polyp histology will be compared to histopathological outcome.

Inclusion Criteria:

• Colonoscopies performed in FIT positive patients obtained for the Dutch colorectal cancer screening program.

Exclusion Criteria:

• Colonoscopies in patients with a history of colorectal cancer, inflammatory bowel disease or polyposis syndrome.
• Colonoscopies in patients with bleeding disorders or anticoagulants and therefore cannot undergo polypectomy.

Endoscopists certified for performing Dutch screening colonoscopies. Colonoscopies in patients performed after a positive FIT obtained for the Dutch colorectal cancer screening program.