A colonoscopy will not detect colorectal cancer in every circumstance, a study in Ontario suggests.

About two to six per cent of cancers are missed during the procedure, depending on the site of the cancer, says a study in the January issue of the journal Gastroenterology.

Researchers at the Institute for Clinical Evaluative Sciences studied more than 12,000 patients in Ontario with colorectal cancer to identify risk factors that could cause the cancer to be missed during a colonoscopy.

"Patients should be made aware of the small two to six per cent risk that a cancer may be missed or may not be detected, because some cancers are rapidly progressing and may not be present,” said Dr. Linda Rabeneck, a senior author of the study and a gastroenterologist at Sunnybrook Hospital in Toronto.

Those linked to a higher risk of the new or missed colorectal cancers include:

• Women
• Older individuals
• Those with right-sided colorectal cancers.
• Those who had a colonoscopy done by an internist or family physician.
• Those who had a colonoscopy done in a private office or clinic rather than at a hospital.

People with diverticular disease, where small sacs or pockets develop in the wall of the colon, also showed an increased risk.

"This information is crucial to help increase the accuracy of these procedures and catch and treat the cancer in its early stages," Rabeneck said.

The study was based on the health records of patients from 1997 to 2002. The patients had undergone colonoscopy to detect possible cancers in the three years before diagnosis.

The researchers do not know if the findings would apply to the rest of Canada. In Ontario, the delivery of colonoscopies in private clinics is not regulated, while there are strict standards for performing the procedure in hospitals, Rabeneck said.

Widespread screening for colorectal cancer could improve survival rates for the disease, which is the second leading cause of cancer-related deaths in Canada, accounting for 10 per for cancer deaths.