Evidence of Benefit
In the Telemark Study in Norway, 400 people aged 50 to 59 years were offered FS in 1983 and, if polyps were found, a colonoscopy was available to them at that time, and again 2 years later, and also 6 years later (the intervention was more aggressive than the one-time sigmoidoscopy currently being tested in the United Kingdom). A nonrandomized control group was selected that did not have a baseline examination. In 1996, all persons were invited to have a colonoscopy. The result was that ten persons in the control group and two in the screening group developed CRC (RR = 0.2; 95% CI, 0.03-0.95). The screening group, however, had a higher overall mortality compared with the control group.
A RCT of sigmoidoscopy screening in the United Kingdom suggests that the impact of endoscopic screening, at least on the left side of the colon, is substantial and prolonged. In this RCT, 170,000 persons were randomly assigned to one-time sigmoidoscopy versus usual care. At sigmoidoscopy, polyps were removed and patients with cancer were referred for treatment. Based on sigmoidoscopy findings, persons were considered to be low risk if they had normal exams or only one or two small (<1 cm) tubular adenomas; such persons were not referred for either colonoscopy workup or colonoscopic surveillance. In a follow-up of 10 years, the left-sided CRC incidence in the low-risk group (about 95% of attendees were low risk) was 0.02% to 0.04% per year-a very low risk of CRC compared with average risk. The cause of reduced risk-whether due to detection and removal of large or small polyps, or selection of individuals at lower risk-is yet unclear, but may be assessed in further analysis. The natural history of large polyps is not well known, but some evidence suggests that such lesions become clinical CRC at a rate of approximately 1% per year. Evidence from multiple studies has raised questions about the ability of endoscopy to reduce CRC mortality in the right colon.[45,46,47] Thus, it is unclear what the overall impact of endoscopy (e.g., colonoscopy screening) is, and whether there may be a large difference in impact on the left side of the colon compared with the right side.
Two case-control studies have been reported that evaluate the efficacy of screening sigmoidoscopy in preventing CRC mortality;[48,49] one study used rigid sigmoidoscopy, and the other used rigid and FS. Both studies were conducted in prepaid health plans and suggested a significantly decreased risk (70%-90%) of fatal cancer of the distal colon or rectum among individuals with a history of one or more sigmoidoscopic examinations compared with nonscreened patients.
There are no strong direct data to determine frequency of screening tests in programs of screening.