Flexible sigmoidoscopy is an endoscopic examination of the distal colon and rectum with a flexible endoscope: either a sigmoidoscope (60 cm long) or a colonoscope (130/160 cm long). Flexible sigmoidoscopy is performed after cleansing the distal colorectum using an enema, usually administered about 30–60 min before the examination. The procedure is restricted by the length of the endoscope, the extent of bowel cleansing and patient tolerance and is often regarded as successful if the rectum and sigmoid colon have been adequately examined. Flexible sigmoidoscopy is usually performed without sedation.
The use of flexible sigmoidoscopy in CRC screening is based on the observation that a majority of adenomas and cancers are located in the distal colon (within reach of flexible sigmoidoscopy) [28, 29]. Flexible sigmoidoscopy has a high sensitivity and specificity for both adenomas and CRC within this area of examination. The sensitivity for advanced adenomas (those with a high malignant potential) and CRC in the whole colon is about 60–70% of that achieved by colonoscopy, depending on the distribution of adenomas and CRC in the colon, the prevalence of lesions in the distal colon as precursors for proximal lesions and the threshold for follow-up colonoscopy after flexible sigmoidoscopy screening  (Table 1). For the distal colon, sensitivity and specificity of the tool are comparable to those achieved by colonoscopy provided adequate bowel cleansing and performance of the technique by an experienced endoscopist.
Considerable variation in the quality of flexible sigmoidoscopy has been reported between endoscopists with regard to detection of colorectal polyps and adenomas [31–33]. Therefore, as for other operator-dependent CRC screening tools, a high level of competency and rigorous training and quality assurance policies are necessary to achieve good-quality CRC screening programmes involving flexible sigmoidoscopy .
Efficacy and effectiveness
Three randomized controlled trials have been conducted to evaluate the effect of flexible sigmoidoscopy screening on CRC incidence and mortality [34–36]. The most recent, the UK flexible sigmoidoscopy screening trial, is the largest of these studies and provides the most comprehensive data . The trial enroled more than 170 000 people aged 55–64 who were randomly assigned to flexible sigmoidoscopy screening or no screening. Screening was performed between 1994 and 1999, with an attendance rate of 71%. After 11 years of follow-up, CRC incidence and mortality were significantly lower in individuals invited to screening compared with the control group: absolute difference in incidence 35 CRC cases, risk ratio 0.77, 95% CI 0.70–0.84; absolute difference in mortality 14 CRC deaths, risk ratio 0.69, 95% CI 0.59–0.82 . In people who actually attended the screening, CRC incidence was reduced by 33% (risk ratio 0.67, 95% CI 0.60–0.76) and CRC mortality by 43% (risk ratio 0.57, 95% CI 0.45–0.72). The effects were similar in men and women.
The Norwegian Colorectal Cancer Prevention (NORCCAP) trial has reported interim results after a significantly shorter follow-up time (6 years for CRC mortality, 7 years for CRC incidence) compared with the UK trial (11 years for both end-points) . In the intention-to-screen analysis, CRC mortality was reduced by 27% (risk ratio 0.73, 95% CI 0.47–1.13). No apparent effect of screening was seen for CRC incidence, with similar incidence rates in the screening and control groups. The most obvious reason for this is the short follow-up time in this study. A follow-up time of at least 10 years is required before any effect of flexible sigmoidoscopy screening can be expected, as shown in previous statistical modelling studies . This is as a result of the findings of screen-detected cancers at the screening intervention, which lead to an early peak of incidence of CRC in screened individuals and the long time interval for the progression from adenoma to CRC. For individuals who attended screening, CRC mortality was significantly reduced by 59% (hazard ratio 0.41, 95% CI 0.21–0.82) . CRC incidence of rectosigmoid CRC was reduced from 79 individuals per 100 000 person-years in the control group to 58 per 100 000 person-years in screening attendees (P = 0.1) .
A small-scale, single-centre, randomized study from Telemark County, Norway, was the first randomized controlled trial to evaluate flexible sigmoidiscopy. These trials started in 1983, when 400 individuals were randomly assigned to screening and 399 to a no-screening control group . After 13 years of follow-up, the trial reported an 80% risk reduction in CRC incidence in the screening group compared with the control group in an intention-to-screen analysis. There are two possible explanations for the good efficacy results of the intervention in this study compared with the results of later studies. First, the attendance rate for screening was very high (81%), and secondly, all individuals with polyps of any kind at screening were offered colonoscopy both after screening and again after 2 and 6 years .
The two Norwegian studies are population based, as they recruited individuals directly from the population registry by random allocation to either the screening or control group. Thus, observations regarding efficacy are directly transferable to effectiveness. By contrast, the UK trial used a two-step invitation procedure, in which only people who actively expressed an interest in participating were enroled in the trial. Although CRC incidence in the trial control group was similar to the incidence expected in the general population , it may not be straightforward to apply the results of the UK trial to clinical practice. The UK trial provides the most reliable and valid estimates of the efficacy of flexible sigmoidoscopy screening, but the effectiveness of flexible sigmoidoscopy screening in the general population is still uncertain .
Two other large randomized trials are currently ongoing. The Prostate, Lung, Colorectal and Ovarian (PLCO) cancer screening trial in the USA is the only trial that includes a repetitive flexible sigmoidoscopy screening intervention. In total, 77 465 men and women aged 55–74 were randomly assigned to screening and 77 477 individuals to the no-screening control group . The Italian Screening COlon REtto (SCORE) trial largely follows the UK flexible sigmoidoscopy screening trial design but is smaller in size, with 34 292 individuals randomly assigned to either the screening or control group . The results of these trials will enable the efficacy of flexible sigmoidoscopy screening to be further quantified.