The rate of new or missed colorectal cancer (CRC) after colonoscopy and their risk factors in usual practice are unknown.  The objective of this study was to evaluate the rate and risk factors in a population-based study.

Investigators analyzed data from the Canadian Institute for Health Information, the Ontario Health Insurance Program, and Ontario Cancer Registry for all patients (>20 years of age) with a new diagnosis of right-sided, transverse, splenic flexure/descending, rectal or sigmoid CRC in Ontario from April 1, 1997 to March 31, 2002, who had a colonoscopy within the three years before their diagnosis.  Patients with new or missed cancers were those whose most recent colonoscopy was six to 36 months before diagnosis.  Characteristics that might be risk factors for new or missed CRC were examined.

Investigators identified a diagnosis of CRC in 3,288 (right sided), 777 (transverse), 710 (splenic flexure/descending), and 7,712 (rectal or sigmoid) patients.  The rates of new or missed cancers were 5.9%, 5.5%, 2.1%, and 2.3%, respectively.  Independent risk factors for these cancers in men and women were older age; diverticular disease; right-sided or transverse CRC; colonoscopy by an internist or family physician; and, colonoscopy in an office.

Because having an office colonoscopy and certain patient, procedure, and physician characteristics are independent risk factors for new or missed CRC, physicians must inform patients of the small risk (2% to 6%) of these cancers after colonoscopy.  The influence of type of physician and setting on the accuracy of colonoscopy, potentially modifiable risk factors, warrants further study.