Clinical characteristics of colorectal flat adenoma and the risk factors associated with missed flat adenoma: A retrospective cohort study.

Colorectal flat adenoma is particularly closely linked to interval cancer of the proximal colon, and therefore its clinical importance has been increasing. However, there is a lack of research on colorectal flat adenoma. This study aims to examine the clinical characteristics of colorectal flat adenoma and the risk factors related to missed flat adenoma. This retrospective cohort study was conducted on the medical records of patients who had colonoscopies performed at this hospital (single-center, Korea) from January 2018 to December 2023. The colorectal adenomas were categorized into flat and protruding adenoma depending on their morphology, and the clinical characteristics of each group were examined. The risk factors related to the flat adenoma miss rate were analyzed for patients who had received more than 2 colonoscopies during the target research period. Out of the 4859 patients who received index colonoscopies, 9358 cases of colorectal adenoma were discovered, of which 1734 (18.5%) were flat adenoma and 7624 (81.5%) were protruding adenoma. Multivariate analysis confirmed that males above 65 years old with a familial history of colorectal cancer and proximal location were the factors related to flat adenoma. Out of the 1304 patients who had received 2 or more colonoscopies, the 415 cases of missed flat adenoma discovered in 369 (28.3%) patients were analyzed. The flat adenoma showed a statistically significantly higher adenoma miss rate than that of the protruding adenoma group. (odds ratio 1.417; 95% confidence interval, 1.299-1.667, P < .001). It was also confirmed that the risk factors of missed flat adenoma were proximal location, inadequate bowel preparation, shorter withdrawal time, and concomitant protruding adenoma. The location, bowel preparation status, withdrawal time, and presence of concomitant protruding adenoma were the factors related to missed flat adenoma. A more careful examination must be conducted when performing colonoscopies on patients who have the above risk factors.