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Consensus on Colorectal Screening Still Difficult to Achieve

Mike Miller

Recent scientific articles about the efficacy of colonoscopy for screening and detection of colorectal cancer have complicated an already intense debate about what is the best early detection strategy for colorectal cancer.

Given the current $6 billion dollar cost of treating colorectal cancers, Richard Klausner, M.D., director of the National Cancer Institute, said NCI has to be particularly attuned to which "organizations say what types of things about colorectal screening." He noted that while 80% of American women are now screened for breast cancer with mammograms, in 1998 use of fecal occult blood tests was around 40% and use of sigmoidoscopy was only about 25%.

Several cancer organizations met in March at the NCI-sponsored Colorectal Cancer Screening Workshop for Persons at Average Risk to sort through the latest evidence. All of the current screening practices—fecal occult blood test, double contrast barium enema, flexible sigmoidoscopy, FOBT with flexible sigmoidoscopy, colonoscopy, virtual colonoscopy, and DNA protein stool tests—have strengths and weaknesses, and proponents and opponents.

Which Test?

FOBT, for example, has proven effective in several randomized trials. "In trials we’ve conducted in Minnesota, as well as in three European trials, reductions in mortality due to FOBT were between 18 and 45%," said John H. Bond, M.D., V.A. Medical Center, Minneapolis, Minn.

But it is not without its critics. The two biggest complaints about FOBT by many of Bond’s colleagues have been its inability to detect precancerous polyps and its production of a large number of false positive results. But Bond said that there are new test designs, such as the Hemoccult II Sensa test (quicker and more reliable than earlier FOBTs), immunochemical testing, and tandem testing, all of which should increase the reliability of FOBT and hence decrease the need for colonoscopy.

Flexible sigmoidoscopy has also shown promising results; a recent 13-year study with 800 patients showed that flexible sigmoidoscopy reduced incidence by 80% but not mortality. Many expert groups recommend flexible sigmoidoscopy and FOBT together because the combination, according to Robert H. Fletcher, M.D., Harvard Medical School, Boston, Mass., "detects more neoplasms and reduces mortality." The disadvantages of combination testing are increased logistics, inconvenience, higher false positive rates, greater cost, and less patient compliance.

There are already several very large studies looking at the efficacy of FOBT and flexible sigmoidoscopy for screening. A multicenter trial in the United Kingdom, headed up by Wendy Atkin, Ph.D., St. Mark’s Hospital, Harrow, England, is looking at once-only flexible sigmoidoscopy at age 60 to see how many patients will need to go on to colonoscopy if large polyps or adenomas are found. A study of FOBT, which surveyed 2 million people, will wrap up next year and will evaluate FOBT for cost, number of false positives, and symptomatic care.

In 1994, the NCI began enrolling patients in its multicenter Prostate, Lung, Colorectal, and Ovarian Cancer Screening Trial. But the trial has a 10- to 14-year follow-up design, and enrollment is closing this year, so findings from the PLCO may not be known for several years.

In the colorectal component of the trial, enrollees were initially screened with flexible sigmoidoscopy every 3 years, but PLCO review committee members decided that a 5-year follow up was becoming a more commonly accepted standard and switched over to that interval.

There are other unknowns in the discussion as well: what stage of disease to target (current practice focuses on small adenomas or polyps, but some researchers suggest advanced lesions might be a better target), how to detect flat adenomas, and how to best study proximal (farthest from the rectum and highest up in the GI tract) adenomas.

What About Colonoscopy?

But according to many researchers, "the big elephant in the room" these days when it comes to colorectal cancer screening is colonoscopy. "Evidence for benefit is decidedly indirect, and we don’t know what compliance rates, accuracy, and complication rates would be if the procedure were done on a widespread basis," said David Lieberman, M.D., Oregon Health Sciences University, Portland. The current debate focuses primarily on whether to establish randomized clinical trials to test the efficacy of colonoscopy as a screening tool (its superiority in the diagnostic realm is not in question), and if trials are established, how large and at what cost.

Lieberman insists that "we really need to know the benefit-to-harm ratio of this procedure before advocating widespread screening, which is why we need a clinical trial."

But whether clinical trials are launched may depend on many factors, including the court of public opinion and available research dollars.

"Our perception is that you have to disprove the effectiveness of colonoscopy," said Kevin Lewis, of the Colon Cancer Alliance, New York, N.Y. "If you can make sigmoidoscopy cheap and safe, you can do the same with colonoscopy."



             
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