Colonoscopies miss many cancers, study
A Canadian study, published this
month in the journal Annals of Internal Medicine, found that
colonoscopies, while still widely recommended, are much less accurate
than previously thought.
In the new study, the test missed just about every cancer in the right
side of the colon, where cancers are harder to detect but about 40
percent arise. And it also missed roughly a third of cancers in the left
side of the colon.
Instead of preventing 90 percent of cancers, as some doctors have told
patients, colonoscopies might actually prevent more like 60 percent to
“This is a really dramatic result,” Dr. 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?’
Dr. Ransohoff and other screening experts say patients should continue
to have the test, because it is still highly effective. But they also
recommend that patients seek the best colonoscopists by, for example,
asking pointed questions about how many polyps they find and remove.
They also say patients should be scrupulous in the unpleasant bowel
cleansing that precedes the test, and promptly report symptoms like
bleeding even if they occur soon after a colonoscopy.
The American Cancer Society says that even if the test is less effective
than many had believed, it has no plans to change its recommended
intervals between screenings – the test still prevents most cancers, but
the expense and risk of the test argue against doing it more often.
The cancer society and the Centers for Disease Control and Prevention
also are focusing on developing measurements of quality so that doctors
who do colonoscopies can assess themselves and improve.
But gastroenterologists say that, if nothing else, the study points up
the paucity of evidence for the common suggestion that anyone who had a
clean bill of health from a colonoscopy is almost totally protected for
at least a decade.
“We have to not overpromise,” said Dr. Ransohoff, who wrote an editorial
accompanying the colonoscopy paper. “We need to look at the evidence,
and we shouldn’t go beyond it.”
The new 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
The results were “a shock,” said Dr. Nancy N. Baxter, the lead author of
the paper and a colorectal surgeon at the University of Toronto. When
she saw them, she said, “I asked my analyst to rerun the data.”
Now, researchers say, the challenge is to find out why the test missed
so many cancers, in particular, those on the right side of the colon,
and whether the problem can be fixed.
About 148,000 people will learn they have colon cancer this year, the
American Cancer Society reports, and nearly 50,000 will die of it.
It might be that Canadian doctors were not sufficiently skilled. About a
third of the colonoscopies were done by general internists and family
practitioners who might not have had the experience to do the test well.
But, said Dr. Douglas K. Rex, director of endoscopy and professor of
medicine at Indiana University, that cannot be the entire explanation
because at least one study, as yet unpublished, involving California
Medi-Cal patients also found the test missed many cancers on the right
side of the colon.
That leaves several other possibilities.
Perhaps patients did not sufficiently cleanse their bowels of fecal
material, a particular problem for the right side of the colon.
“After the prep has been completed, mucus and intestinal secretions
start rolling out of the small intestine and colon,” Dr. Rex explained.
The secretions, he added, pour from the base of the appendix into the
right side of the colon and are “very sticky” and can obscure polyps.
One solution, supported by six studies, is to be sure there is just a
short time between when patients finish taking the strong laxative that
cleanses their bowel and the colonoscopy, Dr. Rex said. That usually
means taking half of the laxative the night before the screening test
and the rest in the morning, something that often is not done, he added,
but that he and others recommend.
Cancer may also be different in the right colon, researchers said.
Flat and indented polyps tend to cluster in the right colon. And so do
another kind, serrated lesions, which, some studies indicate, might turn
into cancer much more quickly than typical polyps.
“It’s possible that we will never get as good a result” in the right
colon, said Robert Smith, director of screening for the American Cancer
Still, he said, that does not mean that patients should have more
frequent colonoscopies. The tests are “hugely expensive,” he said, and
insurers may not pay for more frequent colonoscopies. The test also
carries a small risk of perforating the bowel. Even if colonoscopies
miss some cancers, colon cancer remains a rare disease and, after a
colonoscopy, “the likelihood that you have cancer is very, very low,”
Dr. Smith said.
Dr. Harold C. Sox, editor of the Annals of Internal Medicine, is
choosing another option. He is having a stool test, the fecal occult
blood test, between colonoscopies. It looks for blood in the stool,
which can arise from colon cancer.
Dr. Smith does not advocate that strategy, saying that the stool test
can have false positives from things like red meat or broccoli that have
nothing to do with colon cancer. He worries that frequent stool tests
will lead to frequent false alarms and frequent colonoscopies without
making much of a dent in the colon cancer death rate.
CT colonoscopies, so-called virtual colonoscopies, are not a solution,
some screening experts said.
“The issues are prep quality, flat lesions, serrated lesions and people
not being careful enough in the inspection process,” Dr. Rex said. There
is no evidence, he added, that a virtual colonoscopy will help with the
inspection process. And, he said, “it almost certainly is not as
effective a technology as colonoscopy for flat and serrated lesions.”
Instead, patients should be compulsive about their bowel prep and be
sure the test is done by one of the best colonoscopists in their area,
gastroenterologists said. Doctors should find polyps in at least 25
percent of men and 15 percent of women. They should take at least eight
minutes to withdraw an endoscope from the colon. And they should do a
high volume of screening. Dr. Smith said a high volume was at least
three or four colonoscopies a day.
After the test, patients can ask whether the doctor got to the right
side of the colon and how that was documented.
Colon cancer experts said people should realize that even if
colonoscopies prevent just 60 percent of colon cancer deaths, that still
is a lot. Mammograms, for example, prevent 25 percent of breast cancer
deaths, and the PSA test for men has not been shown to prevent prostate
“If I was to provide one main message, it would be that colonoscopies
are the way that colon cancer mortality gets reduced,” Dr. Ransohoff
is a good test, but it isn’t completely effective. And you know what? We
ought to be happy with that.”