Introduction Colonoscopy has been shown to be effective in reducing the incidence of colorectal cancer (CRC), presumably resulting from the removal of premalignant adenomas. There are data, however, to suggest that colonoscopy is less effective at preventing malignancy in the right colon, when compared to the left colon. A proposed explanation for this observation is that right-sided adenomas may be missed at colonoscopy, either due to inadequate bowel preparation or, alternatively, due to the presence of serrated adenomas that are more difficult to visualise. Within the Bowel Cancer Screening Programme (BCSP) patients found to have adenomas are entered into a surveillance programme, based on predefined guidelines. This study compares the findings at surveillance colonoscopy with the index colonoscopy in these individuals.

Methods All patients having surveillance colonoscopies at the West London Bowel Cancer Screening Centre between 1^st January 2009 and 28^th February 2013 were included in the study. The results of the initial index procedure and subsequent surveillance procedures were retrieved from the endoscopy reporting system (Scorpio) and the histology of all polyps resected and retrieved was obtained from the hospital pathology system. The site of all adenomas removed for all procedures was recorded and the distribution of the adenomas found in the left and right colon were compared for the index and surveillance procedures (Chi squared).

Results 242 patients were included in the study. In total 848 adenomas were found during the index colonoscopies and 379 adenomas were found during the surveillance procedures. 143 (59.1%) surveillance colonoscopies were performed at 1 year and 99 (40.9%) were performed at 3 years. The table below shows the number and percentage of adenomas found in the left colon (up to and including the splenic flexure) and the right colon (proximal to the splenic flexure) in both the index and surveillance procedures.

Significantly more adenomas were identified within the right colon at the surveillance procedures than were in the index examinations (p = 0.0001).

Conclusion This study suggests that more proximal adenomas are found during surveillance colonoscopies than on initial screening colonoscopies. As approximately 60% of the surveillance examinations were at one year it is likely that these lesions were missed at the initial examination. Greater care needs to be taken at initial colonoscopy to visualise the proximal bowel and clear it of neoplasia, particularly in those individuals with multiple polyps.

Disclosure of Interest None Declared.