In Western countries, most colorectal cancers are believed to arise from adenomatous polyps. Screening and early intervention programs have targeted the removal of polyps from the colon. Less pathologic significance has been attributed to flat or depressed lesions noted on colonoscopy or sigmoidoscopy. Conversely, Japanese researchers estimate that 12 to 40 percent of early malignancies appear as flat or depressed lesions rather than as polyps. Small, preliminary studies in European populations indicated that flat or depressed lesions could account for up to two thirds of adenomas. Rembacken and colleagues conducted a prospective study to assess the prevalence and distribution of flat or depressed lesions in patients referred for routine colonoscopy.

The recommendations of the Japanese Research Society for Cancer of Colon and Rectum were followed for detection and classification of lesions. Dyes were used to assist in identification of lesions. Of the 1,000 patients who underwent colonoscopy, 321 adenomas were identified in 225 patients, six Duke's A carcinomas were identified in six patients and in 24 patients, 25 more advanced carcinomas were identified. Of the 321 adenomas identified, 202 (63 percent) were polypoid, 117 (36 percent) were flat and two (zero to 6 percent) appeared to be depressed. Of the 321 adenomas identified, 31 (10 percent) were severely dysplastic, 15 of which were flat, 15 were polypoid and one appeared to be depressed. Of the six Duke's A carcinomas, two were polypoid, two were flat and two were depressed lesions.

The authors conclude that flat adenomas have a risk of progressing to malignancy similar to that of protruded lesions, when the lesions are smaller than 10 mm in diameter. Larger flat lesions and depressed lesions, however, are almost twice as likely to contain areas of severe dysplasia or foci of invasive carcinoma as protruded lesions (of similar size). Many potentially preventable colon cancers are currently missed because physicians focus on polypoid and raised lesions. The authors urge the training of physicians in the Japanese techniques.

editor's note: Some studies just make things fall into place. We all recall patients who developed colon cancer after apparently successful removal of polyps. Nationally, results from the Veterans' Affairs study reveal that endoscopy prevented only about one half of subsequent colorectal cancers. Taken with the Japanese experience, this study suggests that in avid “polyp hunting” we tend to overlook significant flat or depressed lesions. Indeed, the study suggests that these lesions may have an even greater potential for malignant change than the raised lesions. Are all our concepts and practices in colorectal cancer screening about to change?—a.d.w.