The effectiveness of colonscopy in identifying some colon cancers
Physicians should advise patients of colonscopy test limitations in identifying some colon cancers.
Colonoscopy, a key screening test for colon cancer, appears to be less effective in reducing patients’ risk of dying from cancers that originate on the right side of the colon. The groundbreaking research led by St. Michael’s Hospital and the Institute for Clinical and Evaluative Sciences (ICES) was published today on the Annals of Internal Medicine website (www.annals.org) and will be printed in the January 6, 2009 issue.
“While colonoscopy remains the gold standard for evaluation of the colon, our study sheds light on some of the real-world limitations of this practice for screening and prevention,” said Dr. Nancy Baxter, a colorectal surgeon and researcher at St. Michael’s Hospital and scientist at ICES. “At the same time, the findings suggest there may also be opportunities to improve the quality of the test.”
Researchers reviewed the health records of more than 10,000 people aged 52-90 years who received a colorectal cancer diagnosis between 1996 and 2001 and who had died of the disease prior to 2003. These patients were compared to a control group from across Ontario who did not die of colorectal cancer during the study period. The health records of both groups were reviewed to determine how many people had undergone either a screening or diagnostic colonoscopy.
According to the researchers, previous colonoscopy was strongly associated with fewer deaths from left-sided colorectal cancer. However, the data showed that colonoscopy seemed to have almost no benefit in preventing deaths from right-sided colorectal cancer.
“People should still consider colonoscopy a good screening tool but it’s not going to give them 100 per cent protection,” Baxter said. “What’s important for doctors is that we start to understand the limitations of colonoscopy, that we take steps to improve how colonoscopy works in the right colon, and that we start to explore what the differences are in cancer development between the right and left colon.”
The researchers suggest several reasons why colonoscopy may be less effective in preventing deaths from right-sided colorectal cancer:
* Some colonoscopies which are considered "complete" may not have visualized the full length of the colon. This means polyps or suspicious lesions in the furthest portion of the bowel (the right side) might have been missed.
* Bowel preparation procedures, which involve using laxatives to purge the colon prior to screening, may be less effective at clearing the right side of the bowel. If so, existing polyps might be obscured by stool remaining in this portion of the colon.
* It is also possible that right and left colonic cancers and polyps may differ biologically. Right-sided growths may be less likely to have a fleshy stalk and are occasionally flat, which makes them harder to identify and remove, or they may grow more rapidly.
“Colonoscopy is an effective intervention,” said Dr. David F. Ransohoff, author of an accompanying editorial. “The study results, however, should caution physicians about saying that colonoscopy will reduce the risk of dying from colorectal cancer by 90 percent. A 60 to 70 percent risk-reduction rate seems more reasonable.”
Colorectal cancer is currently the second-leading cause of cancer death in North America. A complete colonoscopy is a procedure where a physician inserts a long, flexible tube called a colonoscope through the patient’s rectum and along the length of the bowel. The goal is to scan the entire colon for potentially cancerous or pre-cancerous growths. If such a polyp or lesion is detected, it can often be removed during the colonoscopy so that no additional procedures or surgery are needed.