Colonoscopies have blind side, study says
Popular test to detect signs of cancer only effective on left side of colon, review of 10,000 cases finds
December 16, 2008
Megan Ogilvie (cut and paste URL):
A popular screening test for colon cancer is less accurate and prevents fewer deaths than doctors commonly believe, Toronto researchers say.
Their study, published yesterday, found colonoscopies are only effective at catching cancers that develop on the left side of the colon, and miss almost all cancers that start on the right side.
"All of the benefit of colonoscopy – and we know it reduces colon cancer deaths – is virtually related to the reduction of deaths from cancers arising in the lower, or left side, of the colon," said Dr. Linda Rabeneck of Cancer Care Ontario, the study's senior author.
For patients with cancer that developed on the colon's right side, a colonoscopy only reduced the risk of death by 1 per cent, she said.
Experts say the study, published in the journal Annals of Internal Medicine, will help doctors understand the limitations of colonoscopy and spur efforts to improve the test's monitoring of the right colon. They say it remains a valuable screening tool and stress the study does not mean people should avoid having colonoscopies.
"This is a darn good test, but it's not perfect," said Dr. David Ransohoff, a gastroenterologist and professor of medicine at the University of North Carolina, Chapel Hill.
Doctors often tell patients a colonoscopy can trim the risk of dying from colon cancer by 90 per cent. But Ransohoff said the new data suggests closer to 60 or 70 per cent.
"The findings of zero protection for the right colon is so different than what we expected, it just grabs you by the lapel," he said. "What it means for the profession is we need to step back and think: `What evidence do we have for the effectiveness of colonoscopy?'"
The Ontario-wide study looked at more than 10,000 people diagnosed with colon cancer between 1996 and 2001, and who had died of the disease by 2003. Researchers then matched each patient with five other similar individuals who did not die of colon cancer. By comparing the two groups, they were able to evaluate the effectiveness and limitations of colonoscopy.
A colonoscopy is used to detect and remove polyps, the small growths on the inner surface of the colon which can turn cancerous. Colorectal cancer is the second leading cause of cancer death in Canada, and is 90 per cent curable if detected early enough.
Dr. Nancy Baxter, the study's lead author and scientist at the Li Ka Shing Knowledge Institute at St. Michael's Hospital, said colonoscopy is still one of the most effective cancer detection tests available."
Comment on this article by:
(cut and paste): http://www.thedoctorschannel.com/video/1225.html
"....In an accompanying editorial, Dr. David F. Ransohoff, from the University of North Carolina, Chapel Hill, discusses several possible explanations for the findings -- including the possibility that colonoscopies listed as complete did not actually reach the cecum, or that colon preparation was inadequate, or that right-sided lesions tend to grow more rapidly and arise between screenings.
Overall, Dr. Ransohoff concludes, "Colonoscopy is an effective intervention, but as Baxter and colleagues suggest, we must realize that current evidence is indirect and does not support a claim of 90% effectiveness."
He adds that "until we have better data, we can be grateful and optimistic to have a useful intervention to offer our patients, but we should be realistic and cautious when talking with them about the magnitude of both benefits and risks."
Ann Intern Med 2009;150. ...."