Colon Cancer Screening: More Data for the Debate on Colonoscopy
Two new studies have added fuel to the debate over colorectal cancer screening, an ongoing dispute that focuses on how much screening is enough: Does everybody middle-aged and older need at least one colonoscopy? Or can the simpler and less costly tests--fecal occult blood testing and sigmoidoscopy--catch precancerous growths in most people just as effectively as the more invasive procedure?
 Colonoscopy allows a physician to view the entire colon through an endoscope while sigmoidoscopy shows only the more easily reached, sigmoid or left-hand section of the colon.
Each of the new studies, reported in the July 20, 2000, issue of the New England Journal of Medicine, add a large piece of data to support the pro-colonoscopy view. Both show that colonoscopy can detect a substantial number of precancerous polyps that are missed by sigmoidoscopy.
But the debate is not over by any means, say experts. "Most every authority accepts that colonoscopy is a more sensitive technology than either fecal occult blood testing or flexible sigmoidoscopy," said Ernie Hawk, M.D., who leads gastrointestinal research in NCI's Division of Cancer Prevention. "But the key question is whether greater sensitivity translates into clinical benefit."
That overarching issue encompasses some large unanswered questions about colonoscopy, including whether it can reduce deaths, how it compares to the proven benefits of the fecal occult blood test and sigmoidoscopy, and whether its benefits outweigh its risks.
The two new studies don't answer those questions, but they are important additions to the roster of colon cancer screening studies.
Both were large, one involving more than 3,000 people at 13 Veterans Affairs
medical centers. Using colonoscopy to view the entire colon, the investigators, led by David A. Lieberman, M.D. from the VA center in Portland, Oregon, found advanced polyps in the proximal (right) colon in 128 people. Fifty-two percent of these 128 had no polyps in the sigmoid (left) colon, so their cancers would have been missed by sigmoidoscopy.
The other study, led by Thomas Imperiale, M.D. at Indiana University Medical Center, Indianapolis, involved nearly 2,000 persons in an employee health program. In this study, the results were similar: About half of the 50 cases of advanced polyps in the right colon would have been missed if physicians had relied on the simpler procedure.
Pros and cons
In an editorial accompanying the two studies, Daniel K.Podolsky, M.D., Massachusetts General Hospital, comes down firmly on the side of colonoscopy. "Taken together, these studies reinforce the intuitive assumption that flexible sigmoidoscopy frequently fails to detect important colorectal neoplasms," he concludes. "All persons 50 years of age or older who are at average risk for colorectal cancer should undergo comprehensive evaluation of the entire large bowel."
Others take a more cautious view of the new data. "These studies are important," said Barry Kramer, M.D., former deputy director of NCI's Division of Cancer Prevention. "On the other hand, it's important to remember that there are other options for screening and other things to keep in mind regarding colonoscopy."
For example, there is no evidence that universal colonoscopy screening would save lives. In contrast, the common laboratory test, fecal occult blood testing, has been "absolutely proven" to prevent colon cancer deaths in three different randomized clinical trials, said Kramer, who is now chief of NIH's Office of Medical Applications of Research.
There is also evidence that sigmoidoscopy can reduce colon cancer deaths. A large randomized clinical trial is currently under way to find out whether this is so.
Colonoscopy also has some disadvantages. "It is not without risk," Kramer said. The procedure can result in gastrointestinal bleeding severe enough to lead to hospitalization. It can also result in perforation of the colon. In addition, colonoscopy requires extensive preparation and anesthesia, a major deterrent for many patients.
Innovative methods
Even as the debate over colonoscopy continues, newer and more innovative screening methods are working their way through research pipelines. Molecular markers, high-resolution endoscopy, and virtual colonoscopy (also known as computed tomography or CT colonography) are all under study, said Hawk. One example is a study about to be launched by the American College of Radiology Imaging Network which will compare CT colography to colonoscopy in detecting colon cancer.
Another example of what may lay ahead comes from the University of South Carolina, where Robin Bostick, M.D., is studying biomarkers that might signal an early, precancerous condition in the colon. Someday, Bostick said, physicians may be able to test for colon cancer by using a minute piece of tissue snipped from the colon, comparable to the way cervical tissue is now obtained for Pap testing.
The payoff from these and other studies on colorectal screening could be tremendous in terms of lives saved. Colon cancer is the second most common cancer in the United States, taking more than 50,000 lives each year. Yet it is one of the cancers that has the most potential in terms of prevention, Hawk said.
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