Indications for colonoscopy in this series showed that all patients were at increased risk of colorectal cancer. They were referred because of a detected or strongly suspected neoplastic lesion on double contrast colonographies, previous polypectomies, colorectal cancer operations, recent rectal bleeding, or change in bowel habit (table 2).
Our observations support previous reports that flat or depressed tumours occur in Western populations. Due to differences in the classification of flat adenomas between Western and Japanese pathologists, comparison of series from Japan and Europe is difficult.15,27
Adenomas, which show high grade dysplasia according to Western trained pathologists, are regarded as early carcinomas in Japan and grouped together with cancers invading the submucosa. Furthermore, discrepancy exists even among Japanese pathologists in differentiating high grade dysplasia from early invasive carcinoma.28
Flat colorectal tumours were observed in 6.0% of patients in Malmö (52/866 patients), excluding those with familiar polyposis, hereditary non-polypoid colon cancer, and inflammatory bowel disease. A retrospective study from Canada reported flat adenomas in 8.6% of 210 patients19
whereas flat colorectal tumours occurred in 12.2% in Omaha.20
The other Swedish material from Stockholm cannot be considered in this respect because patients were randomly chosen in that study.6
A considerably higher incidence of flat adenomas (22.7%) was found in the prospective study performed in Texas, USA.23
The incidence of both endoscopically and histologically proved flat epithelial neoplasms among all colorectal epithelial tumours was 6.8% in the present material, compatible with 8.5% in the Vancouver report.19
These values however are less than those observed in Mainz (19.2%24
) and considerably less than those published from Stockholm (42%6
), Leeds (41%21
), and Texas 44.4%23
(table 1). These latter values are closer to those of the Japanese (38.7% (210/542 non-invasive epithelial tumours)29
and 42.9% (2498/5816)30
). The difference between the two Swedish studies can be explained by the different approaches (randomly chosen versus prospective) whereas the reason for the different values between Leeds and Malmö is unclear. The methods were similar, conducted under the guidance of experienced Japanese endoscopists at both centres.
The female to male ratios in patients with flat adenomas and early carcinomas (1.4) differed significantly from that of protruding tumours (0.9) in our series. In Stockholm there was also a slight female preponderance (1.3).6
These observations are in sharp contrast with data from both Canada and Japan with a male predominance of 1.6 in Vancouver,19
4.8 in the original report by Muto and colleagues,7
and 3.4 in a second Japanese study.8
The Texan study did not analyse the sex distribution of flat tumours.23
We have no explanation for the different ratios.
Several series have reported a preponderance of flat adenomas in the right hemi colon: 56% in our material, 64% reported by Jaramillo and colleagues,6
and 30.9% by Adachi and colleagues.8
In contrast, the Canadian study demonstrated an equal distribution of flat adenomas in the large bowel.19
The fact that flat adenomas are more frequent in the right hemi colon is important because it implies that the whole colon should be investigated in the search for these tumours.
Flat adenomas are regarded as having a higher malignant potential than protruding adenomas. The frequency with which high grade dysplasia is found varies however, both in the West and in Japan. In Western countries high grade dysplasia was seen in 14% in Britain,22
in 12% and 14.7% in Stockholm,6,9,18
in 18.9% in our series, and in 41% in Canada.19
In Japan, high grade dysplasia has been reported in 8%,8
and 61% from the Cancer Institute in Tokyo (including intramucosal carcinoma which is classified as high grade dysplasia in the West) classified by a Western pathologist.9
A possible explanation for these differences might be selection of patients. Nevertheless, there is agreement between authors that flat tumours with high grade dysplasia or invasive carcinoma are smaller in size compared with protruding tumours.6,23,30–32
These observations are corroborated by the present material.
Colonoscopic observation of a central depression in a flat adenoma is an important warning sign that may predict invasion. Thus Kudo and colleagues30
reported that 14.6% of 130 depressed lesions invaded the submucosa compared with 1.1% of 2431 non-depressed flat lesions. The corresponding values presented from Sweden were 43% versus 7%.6
A similar observation was reported from Leeds with only two depressed tumours, one of which showed high grade dysplasia and the other was invading the submucosa.21
Our observations are in agreement with these data, with high grade dysplasia found in five of 14 IIa+IIc or IIc/IIc+IIa adenomas (35.7%) versus five of 47 (7.1%) in non-depressed flat adenomas. Furthermore, four of the five flat early adenocarcinomas were IIa+IIc or IIc/IIc+IIa. Thus it seems justified to support the observation made earlier by others8,22,30,33,34
that a central depression is an ominous endoscopic sign of high risk of malignant transformation. Chromoendoscopy makes it easier to discover the central depression, as was also shown by recent studies.24,35
In general, endoscopic discovery of flat and depressed lesions is more reliable after the use of indigocarmine, as was shown in the Texan study: 62% of lesions were found following dye spraying.23
In conclusion, we found flat colorectal neoplasms in a Western population, their incidence being 6.8% in the present material. They were more often located in the right colon and smaller than their protruding counterparts. High grade dysplasia was revealed both more frequently (18.0% v 7.3%) and in smaller (mean 8 mm v 23 mm) superficial type II than in protruding type I adenomas. Depression should be regarded as an important endoscopic sign predicting an increased risk of invasive carcinoma. Knowledge that flat type II colorectal adenomas occur in Western patients is important and should alert endoscopists to search more vigorously for them so that afflicted patient can be treated and cured. Video colonoscopy combined with dye spraying chromoendoscopy seems to be helpful in this respect.