NEW YORK (Reuters Health) - The rate of new or missed colorectal cancers in patients undergoing colonoscopy is influenced by where the screening procedure takes place and by who performs the exam, Canadian researchers report. Cancers are more likely to be missed when the colonoscopy is performed in an office setting, instead of a hospital, and by an internist or family physician, instead of a gastroenterologist or surgeon.

A colonoscopy is a test that involves insertion of a lighted tube with a small camera into the bowel, which visualized detection of colorectal cancer and the type of colorectal polyps that will eventually become cancerous and should be removed.

Unless there are known risk factors for colorectal cancer, such as a family history, physicians recommend that patients undergo colonoscopy by the time they are 50 years old. If the results are normal, it should be repeated at 5 years.

"For the delivery of colonoscopy services in Ontario, we need to further study the practice of colonoscopy in offices and private clinics," senior investigator Dr. Linda Rabeneck told Reuters Health. "There is something different about the practice of colonoscopy in these settings that gives rise to higher cancer miss rates, a worrisome finding."

Rabeneck, of the University of Western Ontario, London and colleagues report their study of 12,487 colorectal cancer patients in the journal Gastroenterology. They found that 430 patients (3.4 percent) had a new or missed colorectal cancer that was diagnosed within 6 months to 3 years of having a colonoscopy.

Further analysis showed that using a 3-year interval from time of last colonoscopy, patients with diverticular disease had the highest risk of reoccurrence. Right-sided colorectal cancer was the type most likely to be missed and older patients had a greater risk of having a missed cancer.

As mentioned, other independent risk factors included having a colonoscopy performed by a family physician or internal medicine specialist, and having the test in a physician's office.

Specifically, compared with hospital-based colonoscopy, having the procedure in an office tripled the risk of new or missed colorectal cancers in men and doubled the risk in women.

Compared with gastroenterologists, family physicians and internists were 77 percent more likely to miss this cancer in men and 85 percent more likely in women.

"For physicians performing colonoscopy, we need to advise our patients of the small chance that if they have a cancer we might miss it," Rabeneck concluded, adding: "As we move forward with the Ontario colorectal cancer screening program, we will ensure that colonoscopy standards are implemented."