Colonoscopy appears to reduce the overall risk of death from colorectal cancer, a new Canadian study suggests, but it is ineffective at preventing death from cancers that originate in the right side of the colon.

Researchers from Toronto's St. Michael's Hospital found that the death reduction from colonoscopy was almost entirely confined to cancers that originated on the left side of the colon.

"Even if colonoscopy is less effective than we thought, it is still the most effective test we have to prevent cancer deaths - more effective than mammography or pap tests," Dr. Nancy Baxter, colorectal surgeon at St Michael's Hospital and lead study author, told CTV News.

"The curve ball that we found, while it seems to be quite effective in preventing left-sided cancer deaths, it does not appear to have much of an impact on right-sided cancers, and that was a real surprise."

The research team compiled the findings during a review of health records of more than 10,000 people between the ages of 53 and 90. They had all been diagnosed with colorectal cancer between 1996 and 2001 and had died of the disease before 2003.

The study is published in the Annals of Internal Medicine.

During a colonoscopy, a flexible tube is inserted into the rectum to scan the colon for potentially cancerous polyps. The growths can often be removed during the procedure so the patient does not have to undergo surgery.

Doctors have said that the test can reduce the risk of dying from colorectal cancer by as much as 90 per cent.

"The study results, however, should caution physicians about saying that colonoscopy will reduce the risk of dying from colorectal cancer by 90 per cent," Dr. David F. Ransohoff, a professor of medicine at the University of North Carolina, wrote in an editorial that accompanied the study. "A 60 to 70 per cent risk-reduction rate seems more reasonable."

The researchers provided several theories for why colonoscopy may be less effective at preventing deaths from right-sided cancers:

  • Some colonoscopies may have failed to visualize the full length of the colon. The scope enters the colon on the left side, and it may be that doctors do not push it all the way to the end, the right side. This means suspicious growths could be missed.
  • Preparation procedures, such as bowel-clearing laxatives, may be not adequately clear the right side of the bowel. This means growths may be obscured by stool.
  • Right and left colon cancer tumours may be biologically different. Right-sided growths may be more flat, making them harder to spot, or may grow more rapidly than left-sided tumours.

"It's not a perfect test, but there may be something different about cancer in the right upper part of the colon," Dr. Linda Rabeneck, of the Odette Cancer Centre at Sunnybrook Health Sciences Centre, told CTV News.

"Either we have challenges in terms of achieving a high-quality colonscopy in that area - and there are reasons for that - or there is something different in the biology of cancers in this region, and that is the big question that these results shine a light on."

Patients should ensure they follow instructions closely when preparing for a colonoscopy, Rabeneck said. As well, they should ask their doctors if they have viewed the entire colon before the test is over.

Colorectal cancer is the second-leading cause of cancer death in North America.

According to the Canadian Cancer Society, 21,500 Canadians will be diagnosed with colorectal cancer in 2008 and about 8,900 will die of the disease.

Risk factors include being 50 years of age or older, being overweight, having a bowel disease such as ulcerative colitis, or having a family history of colorectal cancer.

The disease has few symptoms in its early stages, but symptoms in later stages can include:

  • Blood in the stool
  • Changes in bowel habits
  • Consistently narrow stools
  • Constant fatigue and unexplained anemia

With a report from CTV medical specialist Avis Favaro and producer Elizabeth St. Philip