Introduction: Flat lesions of the colorectal mucosa have been a topic of debate since they were first described. Japanese series suggest that flat adenomas represent up to 25 percent of all colorectal adenomas and that they are a particularly advanced form of neoplasia for their size. Series published by Western endoscopists show a lower incidence and a lower degree of dysplasia. We performed this study to see whether American colonoscopists interpret the shape of lesions of the colorectal mucosa differently from their Japanese colleagues.
Methods: A book of endoscopic photographs of 50 small lesions of the colorectal mucosa was shown to 11 American and 8 Japanese colonoscopists. The colonoscopists were asked to decide whether each lesion was sessile, flat, depressed, pedunculated, or that there was no polyp present. Results were tabulated by lesion and by endoscopist.
Results: Although the average number of lesions deemed sessile was similar between groups (American, 33.8 +/- 2.1 95 percent confidence interval; Japanese, 29.4 +/- 2.2) there were marked differences in determination of flat lesions (American, 5.3 +/- 1.5; Japanese, 14.5 +/- 2.2), depressed lesions (American, 0.8 +/- 0.4; Japanese, 2.4 +/- 0.7) and those deemed "no polyp" (American, 5.1 +/- 0.9; Japanese 0). At least six of the eight Japanese colonoscopists agreed that a particular lesion was flat on 12 occasions, but American colonoscopists agreed on only 1 of these. Six of eight Japanese colonoscopists agreed on lesions as sessile in 27 cases, with Americans agreeing in 25 of these.
Conclusions: There are significant differences in interpretation of the shape of lesions of the colorectal mucosa between Japanese colonoscopists and their American counterparts. These differences are likely to account for some of the apparent differences in incidence in series of flat colorectal adenomas reported from Japan and Western countries.