The colonoscopy, the standard test for colorectal cancer reluctantly taken by mllions of baby boomers, isn't as perfect as some doctors thought.
Instead of preventing 90 percent of colorectal cancers, the invasive procedure might actually spot only six or seven cases in 10 , according to a Canadian study published in the Annals of Internal Medicine. Getting any precancerous polyps removed after the test often meant almost never getting colon cancer.
The results may make anyone who has had such a test in the past 10 years - the amount of time anyone with a clean bill of health from a colonoscopy is almost totally protected for - to think about having another test done. About 148,000 people will learn they have colon cancer this year, and nearly 50,000 will die of it.
The problem may be more serious for African-Americans. American Cancer Society recently reported research that more blacks than whites are dying of colon cancer, possibly due to blacks' lower screening rates and poor access to quality care.
The new research on the efficacy of the test found that it missed just about every cancer in the right side of the colon, where cancers are harder to detect but where about 40 percent arise. And it missed roughly a third of cancers in the left side of the colon.
"This is a really dramatic result," David F. Ransohoff, a gasteroenterologist at the University of North Carolina, told the New York Times. "It makes you step back and worry, 'What do we really know?'"
Colonoscopies are still effective and patients should still have the tests, experts say. Compared to other cancer tests, colonoscopies still prevent a lot of colon cancer deaths at 60 percent, compared to 25 percent of breast cancer deaths being prevented by mammograms. The PSA test for men hasn't been shown to prevent prostate cancer deaths.
"If I was to provide one main message, it would be that colonoscopies are the way that colon cancer mortality gets reduced," Ransohoff said. "Colonoscopy is a good test, but it isn't completely effective. And you know what? We ought to be happy with that."
In its editorial on the issue, the Annals of Internal Medicine worried about doctors promising too much. It warned such statements could be "viewed as a mistake that we must explain to a patient and perhaps to a lawyer. A goal of avoiding all deaths from colon cancer may be admirable, but we do not have evidence that we can achieve it.
"A mindset that 'no CRC (colorectal cancer) can occur on our watch' may already contribute to overutilization of colonoscopy in postpolypectomy surveilance...," the journal said. "Although colonoscopy is genereally safe, it is still an invasive procedure with a 0.2% rate of serious complications - 10 times higher than for any other commonly used, cancer screening test."
The American Cancer Society doesn't plan to change its recommended intervals between screenings. The tests are expensive and insurers may not pay for more frequent ones. The test also carries a small risk of perforating the bowel.
The study matched each of 10,292 people who died of colon cancer to five people who lived in the same area and were of the same age, sex and socioeconomic status. The researchers asked how many patients and control subjects had had colonoscopies and whether the doctors had removed polyps. Then the researchers compared the groups and asked how much the colon cancer death rate had declined in people who had had the screening test.
Even with the tests being less reliable than originally thought, colon cancer death rates have hit a racial divide between whites and blacks, with a 48 percent difference - about 25 deaths per 100,000 people among blacks and 17 per 100,000 in whites. The rate of diagnoses in blacks was about 19 percent higher than it was for whites in 2005, the most recent year for which statistics are available.
The colon cancer screening rate is 50 percent for whites and 40 percent for blacks.