Sigmoidoscopy, Colonoscopy and Colorectal Cancer Incidence
Sigmoidoscopy, Colonoscopy and Colorectal Cancer Incidence
We carried out a systematic literature review and meta-analysis according to a predefined protocol. Reporting follows the PRISMA and MOOSE statements.
We searched PubMed, Web of Science, and Embase for eligible studies from inception to 7 November 2013 (for Embase until 2 August 2012 only, owing to termination of our institutional licence; until then, no additional relevant article had been retrieved through Embase). The combinations of keywords used were (“sigmoidoscopy” or "colonoscopy" or "endoscopy" or "polypectomy") and ("colorectal" or "colon" or "large bowel") and ("cancer") and ("relative risk" or "relative risks" or "ratio" or "ratios" or "rate" or "rates") and ("cohort" or "case control" or "trial" or "intervention" or "randomized" or "follow up"). We searched the reference lists of identified sources for additional relevant studies.
Published randomised controlled trials and observational studies were eligible for inclusion if they assessed the effects of screening sigmoidoscopy or screening colonoscopy versus no endoscopy on colorectal cancer incidence or mortality, or both in the general population at average risk for colorectal cancer. The review was restricted to original articles published in English. We excluded studies published as abstracts only as we considered the information to be unsufficient for our assessment. Two reviewers (CS, MH) independently performed study selection according to eligibility criteria. Disagreements were resolved by discussion or a third reviewer (HB).
Two reviewers (HB, MH) independently extracted relevant information from both types of studies into a standardised form. Information extracted from randomised controlled trials was first author, year of publication, country, number and age range of participants, type of intervention, years of enrolment, and median follow-up time. In addition we extracted participation rates in the intervention group and contamination proportions (that is, the proportions undergoing lower gastrointestinal endoscopy in the control group) where reported. Information extracted from observational studies was first author, year of publication, country, years of colorectal cancer diagnosis or death, type of lower gastrointestinal endoscopy (sigmoidoscopy, colonoscopy), time frame of endoscopies, and covariates considered.
For both randomised controlled trials and observational studies we extracted estimates of relative risk along with 95% confidence intervals according to site of colorectal cancer (any, proximal, distal) and outcome (incidence, mortality) as far as reported. For randomised controlled trials we extracted the results from both the intention to screen and the per protocol analyses.
The two reviewers resolved disagreements in data extracted by further review and discussion.
A quality assessment of included studies was conducted for descriptive purposes and to evaluate potential differences in results according to quality criteria (see Supplementary Table 1 for details of the quality indicators). Two reviewers (HB, MH) independently assessed study quality and resolved disagreements by further review and discussion.
We combined the studies in a narrative synthesis, focusing on differences in effect estimates according to study design, type of endoscopy, and cancer site. To simplify terminology, we uniformly refer to the effect estimates of relative incidence or mortality from randomised controlled trials and cohort studies and of odds ratios from case-control studies as estimates of “relative risk.” We calculated pooled effect sizes together with 95% confidence intervals using random effects models, stratified by study design (randomised controlled trials or observational) and type of endoscopy (screening sigmoidoscopy or screening colonoscopy). From reported confidence intervals we calculated standard errors using the delta method. Subgroup analyses were conducted according to cancer site and type of analysis (intention to screen, per protocol; randomised controlled trials only). We assessed heterogeneity in effect estimates using I, τ, and Cochran's Q statistic. Publication bias was assessed by funnel plots. The Bucher method was applied to indirectly compare the effectiveness of sigmoidoscopy and colonoscopy for colorectal cancer screening.
We used the "meta" package in R version 3.0.2 (R Foundation for Statistical Computing, Vienna, Austria) to perform meta-analyses.
Methods
We carried out a systematic literature review and meta-analysis according to a predefined protocol. Reporting follows the PRISMA and MOOSE statements.
Data Sources and Searches
We searched PubMed, Web of Science, and Embase for eligible studies from inception to 7 November 2013 (for Embase until 2 August 2012 only, owing to termination of our institutional licence; until then, no additional relevant article had been retrieved through Embase). The combinations of keywords used were (“sigmoidoscopy” or "colonoscopy" or "endoscopy" or "polypectomy") and ("colorectal" or "colon" or "large bowel") and ("cancer") and ("relative risk" or "relative risks" or "ratio" or "ratios" or "rate" or "rates") and ("cohort" or "case control" or "trial" or "intervention" or "randomized" or "follow up"). We searched the reference lists of identified sources for additional relevant studies.
Study Selection
Published randomised controlled trials and observational studies were eligible for inclusion if they assessed the effects of screening sigmoidoscopy or screening colonoscopy versus no endoscopy on colorectal cancer incidence or mortality, or both in the general population at average risk for colorectal cancer. The review was restricted to original articles published in English. We excluded studies published as abstracts only as we considered the information to be unsufficient for our assessment. Two reviewers (CS, MH) independently performed study selection according to eligibility criteria. Disagreements were resolved by discussion or a third reviewer (HB).
Data Extraction and Quality Assessment
Two reviewers (HB, MH) independently extracted relevant information from both types of studies into a standardised form. Information extracted from randomised controlled trials was first author, year of publication, country, number and age range of participants, type of intervention, years of enrolment, and median follow-up time. In addition we extracted participation rates in the intervention group and contamination proportions (that is, the proportions undergoing lower gastrointestinal endoscopy in the control group) where reported. Information extracted from observational studies was first author, year of publication, country, years of colorectal cancer diagnosis or death, type of lower gastrointestinal endoscopy (sigmoidoscopy, colonoscopy), time frame of endoscopies, and covariates considered.
For both randomised controlled trials and observational studies we extracted estimates of relative risk along with 95% confidence intervals according to site of colorectal cancer (any, proximal, distal) and outcome (incidence, mortality) as far as reported. For randomised controlled trials we extracted the results from both the intention to screen and the per protocol analyses.
The two reviewers resolved disagreements in data extracted by further review and discussion.
A quality assessment of included studies was conducted for descriptive purposes and to evaluate potential differences in results according to quality criteria (see Supplementary Table 1 for details of the quality indicators). Two reviewers (HB, MH) independently assessed study quality and resolved disagreements by further review and discussion.
Data Synthesis and Analysis
We combined the studies in a narrative synthesis, focusing on differences in effect estimates according to study design, type of endoscopy, and cancer site. To simplify terminology, we uniformly refer to the effect estimates of relative incidence or mortality from randomised controlled trials and cohort studies and of odds ratios from case-control studies as estimates of “relative risk.” We calculated pooled effect sizes together with 95% confidence intervals using random effects models, stratified by study design (randomised controlled trials or observational) and type of endoscopy (screening sigmoidoscopy or screening colonoscopy). From reported confidence intervals we calculated standard errors using the delta method. Subgroup analyses were conducted according to cancer site and type of analysis (intention to screen, per protocol; randomised controlled trials only). We assessed heterogeneity in effect estimates using I, τ, and Cochran's Q statistic. Publication bias was assessed by funnel plots. The Bucher method was applied to indirectly compare the effectiveness of sigmoidoscopy and colonoscopy for colorectal cancer screening.
We used the "meta" package in R version 3.0.2 (R Foundation for Statistical Computing, Vienna, Austria) to perform meta-analyses.