A colonoscopy, the procedure in which a doctor looks for dangerous growths in the large intestine, may not be as great a protection from death from colorectal cancer as once thought, a study found Tuesday.
In fact, a colonoscopy was associated with "essentially no reduction" in the risk of dying of cancer if the lesion that caused the disease was located in the right side of the colon, the part that plays a major role in absorbing water and electrolytes during digestion, according to the study.
"The study shows that colonoscopy is not a perfect test for preventing death from colon cancer. It supports recommendations to do colonoscopy to screen for colon cancer, but it also raises concerns that colonoscopy may be less effective for cancers arising in the right colon," a summary of the study said.
Colon cancer is the second most deadly cancer in North America after lung cancer, the study, which appears on the website of the Annals of Internal Medicine, said.
Colonoscopy is the most effective, but also the most costly and risky means of screening for potentially cancerous growths in the large intestine, which is why researchers undertook the study. It sought to determine whether having a colonoscopy in fact reduced the risk of death from colon cancer.
For the study, the researchers looked at the health records of more than 10,000 people in Canada who were diagnosed with and died of colon cancer between 1996 and 2003, and the records of around five times as many people who had not died of colon cancer during the same period.
The researchers measured how often each patient had had a colonoscopy and then compared the chance of dying of colon cancer after having the procedure with the chance of dying of the illness if no colonoscopy had been done.
The calculations were also done separately for cancers in the left and right colon.
What they found was a "large mortality reduction" for cancers on the left side of the colon, but essentially no reduction in deaths for lesions on the right side.
"The study 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," said David Ransohoff, a medical doctor and professor of medicine at the University of North Carolina.
Among possible reasons why colonoscopy is less effective in detecting right-side cancers, the researchers hypothesized 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.
Most cancers of the colon begin as small, slow-developing growths called polyps.
The purpose of colon cancer screening is to detect and remove growths before they spread.