Purpose: In recent years, flat adenomas of the colorectal mucosa have been intensively investigated by Japanese pathologists. Results of that work indicate that flat adenomas may antedate the development of colorectal carcinomas. Because of differences in the histologic definition of flat adenomas with severe dysplasia and with intramucosal carcinoma within the group, one single observer having both Western and Asian training in pathology reviewed the material.
Methods: A total of 287 flat colorectal lesions were reviewed: 109 from the Karolinska Hospital, Stockholm, 137 from the Tokyo Medical and Dental University (TMDU) (which included 5 cases from the Nagoya City University), and 41 from the Cancer Institute (CI), Tokyo. Lesions were histologically classified following strict histologic criteria. Thus, flat adenomas were divided into those having low-grade dysplasia (LGD; having dysplastic cells in the deeper half of the epithelium), high-grade dysplasia (HGD; dysplastic cells were found even in the superficial half of the epithelium), intramucosal carcinoma (dysplastic glands displayed molding with buddings and often a cribriform pattern), and adenocarcinoma (braking through the muscularis mucosa, with neoplastic cells in the submucosal layer or deeper).
Results: Whereas in Stockholm only 14.7 percent of lesions had HGD, as much as 56.9 percent and 56.1 percent, respectively, had HGD at the two Tokyo Hospitals. Intramucosal carcinomas were not found in the Stockholm material but occurred in 2.2 percent of lesions seen at TMDU and in 4.9 percent of those seen at the CI. Notably, only 2.7 percent of the specimens at Karolinska Hospital had invasive adenocarcinoma, but it was seen in as many as 4.4 percent at TMDU and 21.9 percent at the CI.
Conclusions: This study indicates that there were histologic differences between flat neoplasias of the colorectal mucosa harvested in Stockholm and Tokyo. In Japan lesions were obviously more advanced (in terms of HGD) and more aggressive (in terms of intramucosal and submucosal invasion). The cause for the differences found in those two disparate geographic regions remains poorly understood. The results, however, may help us understand some of the unclear points and discussions appearing in the literature on this subject.