WASHINGTON - HAVING a colonoscopy, the procedure where a doctor looks for potentially cancerous growths in the large intestine, helps reduce the risk of death from colorectal cancer but only if the tumor is on the left side of the colon, a study found on Tuesday.

The authors of the study reviewed the health records of 10,292 patients who died of colorectal cancer and, for each of them, five controls who did not, and found that the odds of dying from left-sided colorectal cancer was greatly reduced in patients who had a complete colonoscopy.

But they also found that colonoscopy provided almost no protection for those with right-sided colorectal cancer.

The left side is responsible for storage and evacuation of stool.

'This study demonstrates that colonoscopy is an effective procedure for the prevention of death from colorectal cancer; it just may not be quite as effective as we've thought in the past,' said the lead author on the study, colorectal surgeon Nancy Baxter of St Michael's Hospital in Toronto.

'The study should caution physicians about saying that colonoscopy will reduce the risk of dying from colorectal cancer by 90 per cent. A 60- to 70-per cent risk reduction rate seems more reasonable,' said Dr David Ransohoff, a medical doctor at the University of North Carolina.

Among possible reasons why colonoscopy is less effective in detecting right-side cancers, the researchers hypothesised that polyps and cancers in the right and left sides of the colon may differ biologically, that right-sided polyps could be harder to detect, or that the right side of the colon might not be completely examined during a colonoscopy.

Colorectal cancer is the second-leading cause of cancer death in North America, said the study, which appears on the website of the Annals of Internal Medicine. -- AFP