Early-detection and prevention effects of screening sigmoidoscopy: evidence from randomized trials revisited

Large-scale randomized controlled trials (RCTs) have established compelling evidence that screening by flexible sigmoidoscopy reduces colorectal cancer (CRC) incidence. Reported incidence results include cancers that were already prevalent and yet undiagnosed, but no longer preventable at screening. We aimed to derive, disentangle, and fully disclose early-detection and long-term prevention effects of screening sigmoidoscopy from published trial results.

We used data from 3 large-scale RCTs from the United Kingdom (UKFSST), Italy (SCORE), and the United States (PLCO), which included a total number of 359 198 participants. For each trial and each length of follow-up, we derived the numbers and proportions of CRC cases that were either early detected or prevented among screening attenders.

In the UKFSST, which reported the longest follow-up data, screening sigmoidoscopy prevented 64% (95% CI = 59% to 69%) of incident distal CRC that would have been expected in the absence of screening during a median of 21.3 years. Within follow-up periods between 10 and 12 years, the proportions of distal CRC cases that were either early detected or prevented among screening users ranged between 67% (95% CI = 61% to 72%) in the PLCO and 80% (95% CI = 68% to 89%) in the SCORE trial, with approximately equal shares of early-detected and prevented cases in the SCORE and the PLCO trials, and a higher share of prevented cases in the UKFSST.

A single screening sigmoidoscopy prevents 2 out of 3 incident cancers in the distal colon and rectum over a period of more than 20 years, on top of early-detecting prevalent cases at screening.