Evidence backing colorectal cancer screening mounts

Two new studies published this week in The New England Journal of Medicine further solidified the importance and effectiveness of CT colonography and repeat screening for colorectal cancers.

In recent years, endoscopic colonography has established itself as the primary screening method for the detection of colorectal cancers and precancerous polyps. However, the overall invasiveness, inconvenience and high cost of the procedure make alternatives or improvements much sought after.

CT colonography is one alternative to traditional colonoscopy that is being researched. This procedure is considered to be faster, less invasive and carries a lower risk of complications. In order to determine the accuracy of CT colonography as a screening method, The National CT Colonography Trial conducted a prospective study of 2,531 asymptomatic participants aged 50 years or older from 15 clinical sites. All participants underwent CT colonography followed by traditional colonography.

CT colonography was able to detect lesions measuring 10 mm or more in 90% of patients. However, the sensitivity of the procedure decreased with decreasing size of the lesions.

Robert Fletcher, MD, a professor emeritus from Harvard Medical School, noted in an accompanying editorial that use of CT colonography may result in some false positives. Of the 17% of patients who appeared to have one or more polyp 1 cm or larger, only one in four actually had the polyp, he wrote.

The second study examined evidence to provide some support for the currently recommended rescreening interval for traditional colonoscopy, required in those patients who have no precancerous polyps. Current guidelines from the U.S. Multisociety Task Force on Colorectal Cancer and the American Cancer Society recommend colonoscopy every 10 years in people with normal examinations. But according to the study, there is little evidence to support this recommendation.

Researchers from several U.S. institutions enrolled 2,436 people aged 50 or older to undergo baseline screening colonoscopy; 51.6% were rescreened after five years. No cancers were found at the rescreening and only 16% of people had one or more adenomas. Nineteen advanced adenomas were identified at rescreening, of which a little over half were distal to the splenic flexure. These data led the researchers to conclude that five-year risk of colorectal cancer is “extremely low” in people previously screened with colonoscopy.

“These two studies have strengthened the evidence base for practices already recommended by expert groups,” Fletcher wrote. “Now we can be more confident that judgments made by expert groups, on the basis of lesser evidence, were correct.” – by Leah Lawrence

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N Engl J Med. 2008;359:1218-1224.

Both studies are interesting and have implications in terms of practice and in terms of what tool is used to survey patients for polyps and at what frequency it should be performed. The virtual colonoscopy is an evolving tool and the attractions are obvious in that it potentially avoids some of the discomforts of a colonoscopy. It did appear to detect the majority of significant findings, but as was pointed out in the paper and the editorial, there are some concerns: the fact that there were about 10% of polyps missed, the issue related to radiation exposure, and the false-positive rate, which is not a trivial issue. For about one in six of patients there were findings outside the bowel that were picked up that required some degree of further follow-up. For many this was probably nothing, but they did further evaluation. Whether this should become a preferred standard in terms of screening — for the average-risk person — that discussion is still ongoing.

Also, there has been no prospective randomized trial that has indicated what should be the optimal timing in terms of frequency for colonoscopy for the average person. We sort of inferred from previous studies that once every 10 years may be sufficient (American Cancer Society recommendation) or arguably once only at the age of 50. This study is reassuring in that it tells us that at five years there appears to be a very low pick-up (1% to 2%) for people who are average risk who had a full screening and had no concerns on their initial colonoscopy.

– Eileen O’Reilly, MD

Associate Member of the Gastrointestinal Oncology Department at Memorial Sloan Kettering
Associate Professor at Weill Medical College of Cornell University