A Cochrane review [Abstract] 1 included four randomised controlled trials and two controlled trials with over 320 000 subjects. Follow-up times ranged from 8 to 18 years. Combined results from the 4 eligible RCTs showed that those allocated to screening had a reduction in colorectal cancer mortality of 16% (RR 0.84, 95% CI 0.78 to 0.90). In the 3 studies that used biennial screening there was a 15% relative risk reduction (RR 0.85, 95% CI 0.78 to 0.92) in colorectal cancer mortality. When adjusted for screening attendance in the individual studies, the mortality reduction was 25% (RR 0.75, 95% CI 0.66 to 0.84). The all-cause mortality from four of the randomised trials combined showed no difference between the screening and control groups (RR 1.00, 95% CI 0.99 to 1.03), but a major limitation of using all-cause mortality as an endpoint in cancer screening trials is that it is poorly powered as the intervention is targeted to a disease that causes only a small proportion of overall deaths.
Harmful effects of screening include the psycho-social consequences of receiving a false-positive result and the potentially significant complications of colonoscopy or a false-negative result, the possibility of overdiagnosis (leading to unnecessary investigations or treatment) and the complications associated with treatment.
Estimated screening benefit for potential screening populations should guide health policy decisions about to whom screening can be offered. For example, in England and Wales in 2004, the cumulative 10 year mortality from colorectal cancer in males for the decades beginning 40, 50 and 60 are respectively 5, 22 and 70 per 10 000 individuals. If offering screening reduced this mortality from colorectal cancer by 16%, then the reduction in CRC deaths over the following 10 years for each of these age groups would be 0.8, 3.5 and 11.2 respectively, per 10 000 invited. If the mortality reduction of 25% estimated for those who regularly attend screening would be used, the reduction in CRC mortality over 10 years for those aged 40, 50 and 60 would be 1.25, 5.5 and 17.5 per 10 000 respectively.
Comment: Although screening benefits are likely to outweigh harms for populations at increased risk of colorectal cancer, we need more information about the harmful effects of screening, the community´s responses to screening and screening costs for different health care systems before widespread screening can be recommended.
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