Ethnic or environmental differences in disparate geographic regions may influence the histology of flat colorectal neoplasias.

Histologic sections of endoscopically flat colorectal polyps removed in Tokyo and Stockholm were reviewed. A total of 178 flat colorectal neoplasias (88 from the Tokyo Medical College Hospital, Tokyo and 90 from the Karolinska Hospital, Stockholm) were classified following strict histologic criteria by two different pathologists (one Swedish and the other Japanese). The number of polyps with high grade dysplasia, with intramucosal carcinoma and with invasive carcinoma were significantly higher (p < 0.001) in Tokyo (61.4% or 54/88) than in Stockholm (15.0% or 14/90). The present results suggest that flat colorectal neoplasic polyps may be histologically more "severe" and more "aggressive" in Japanese than in Swedish patients. The possibility that more "advanced" lesions had been inadvertently removed in Tokyo was discounted as Japanese endoscopists were also instrumental in excising many of the flat colorectal polyps in Stockholm. Ethnic and/or environmental differences seem to play a crucial role in the evolution of flat colorectal neoplasic polyps from LGD and HGD, to intramucosal and to invasive carcinoma.