Study provides evidence supporting recommended 10 year interval for colonoscopies for most patients

Patients with a negative colonoscopy examination have a reduced risk of developing colorectal cancer for more than 10 years, compared to the general population, according to a study in the May 24/31 issue of JAMA: The Journal of the American Medical Association.

Colorectal cancer is the third most commonly diagnosed cancer and the second leading cause of cancer deaths in North America. Screening for CRC and its precursor lesions has become an increasingly prevalent practice. Colonoscopy has been recommended as the preferred initial screening test by several medical organizations and is being widely performed in the United States for screening among average-risk individuals. Colonoscopy allows for removal of most precancerous polyps at the time of detection. A screening interval of 10 years after a normal colonoscopy has been adopted based on the estimate of the time it takes for an adenomatous (benign tumor) polyp to transform into carcinoma. However, the duration over which the risk of CRC remains decreased following the performance of a normal colonoscopy has been unknown.

Harminder Singh, M.D., of the University of Manitoba, Canada, and colleagues analyzed data from individuals who underwent a colonoscopic evaluation that did not result in the diagnosis of colorectal neoplasia to determine the magnitude and duration of their lowered risk of developing CRC. The patients (n = 35,975), who had been evaluated between April 1989 and December 2003, were identified using Manitoba Health's physician billing claims database. Standardized incidence ratios (SIRs) were calculated to compare colorectal cancer incidence in this group with colorectal cancer incidence in the provincial population. The patients were followed up from the time of the colonoscopy until diagnosis of colorectal cancer, death, moving from Manitoba, or end of the study period on December 31, 2003.

The researchers found that a negative colonoscopy was associated with SIRs of 0.69 (31 percent lower incidence of CRC compared to general population) at 6 months, 0.66 (34 percent lower incidence) at 1 year, 0.59 (41 percent lower incidence) at 2 years, 0.55 (45 percent lower incidence) at 5 years, and 0.28 (72 percent lower incidence) at 10 years. The proportion of colorectal cancer located in the right side of the colon was significantly higher in the colonoscopy group than the rate in the Manitoba population (47 percent vs. 28 percent).

"This study demonstrates that following a negative result from a colonoscopy performed in the usual clinical practice, the risk of developing CRC is at most 60 percent to 70 percent of the risk of developing CRC in the general population and the duration of the interval of decreased CRC risk persists for more than 10 years. Furthermore, if an individual undergoes a single negative colonoscopy, excepting any follow-up endoscopies at which CRC is diagnosed, the risk of developing CRC is even lower and the duration of the interval of decreased risk again exceeds the 10-year interval currently recommended between screening colonoscopies. Our findings suggest that screening colonoscopies do not need to be performed at intervals shorter than 10 years," the authors conclude.