Colonoscopy vs Sigmoidoscopy Screening

Abstract

THE WORLD OF CANCER SCREENING HAS BEEN ROCKED recently by controversy. Long-standing recommendations on screening for breast, cervical, and prostate cancer have all been questioned based on either new data or reanalyses of older data. A similar controversy is also emerging for colorectal cancer screening. In the 1980s, sigmoidoscopy screening was common despite lack of evidence for its efficacy. In ensuing decades, case-control studies established that sigmoidoscopy was associated with reduced incidence and mortality from leftsided but not right-sided colon malignancies. The results of a randomized trial of sigmoidoscopy screening reported by Atkin et al established the efficacy of sigmoidoscopy in reducing mortality due to colorectal cancer. As in the casecontrol studies, the randomized trial found that this effect was limited to the distal colon. In the mid to late 1980s, colonoscopy was still relatively new and primarily was used for evaluation of gastrointestinal symptoms or for screening in high-risk patients, such as those with strong family histories of colon cancer. Sigmoidoscopy was widespread and apparently efficacious; out of this environment emerged the opinion, first articulated in 1988, that colonoscopy should also be considered as a potential tool for screening average-risk adults for colorectal cancer. This was eventually incorporated into screening guidelines, and reimbursement was initiated by Medicare in 2001. Even though colonoscopy and sigmoidoscopy are equally acceptable options in guidelines from the US Preventive Services Task Force and the US Multisociety Task Force, colonoscopy was granted preferred status in guidelines published by the American College of Gastroenterology. A recent colon screening program by the New York City Department of Health highlighted these changes by encouraging colonoscopy screening to the exclusion of other modes and has been successful in stimulating such screening. In contrast, sigmoidoscopy screening has declined in the United States such that it is difficult to obtain in many urban areas today, and lower reimbursement has accelerated its obsolescence. Even though colonoscopy has achieved a predominant role in colon cancer screening, the logic and justification for its use remains largely theoretical based on its extended range within the colorectum and consequent increased yield in the detection of adenomatous polyps. Despite the increased adverse effect profile of colonoscopy, including higher perforation rate, need for sedation, time and bother commitment of the patient (who must adhere to an arduous bowel preparation and miss a day of work), and increased cost, the presumed mortality benefit of colonoscopy has been used as a justification to outweigh these negatives. Thus, it is disconcerting to recognize that even today limited evidence demonstrates reduced mortality for those who undergo colonoscopy vs sigmoidoscopy. In a case-control study from Canada, colonoscopy was associated with a reduction in colorectal cancer mortality (odds ratio [OR], 0.63; 95% confidence interval [CI], 0.57-0.69), but this reduction was limited to left-sided cancers (OR, 0.33; 95% CI, 0.280.39) with no reduction in right-sided cancers (OR, 0.99; 95% CI, 0.86-1.14). In 2 subsequent observational studies, the association between colonoscopy and reduced colorectal cancer risk was limited to the distal colon. Instead of meeting its expectations, colonoscopy has not yet proven to be more effective than sigmoidoscopy (TABLE). A third approved screening mode for colorectal cancer is the fecal occult blood test (FOBT), which has had efficacy proven in randomized trials. In trials reporting site-specific mortality, less than 5% of patients had a positive FOBT result and underwent diagnostic colonoscopy, and mortality benefits were similar for rightand left-sided colon lesions. Whether sigmoidoscopy plus FOBT is a superior screening strategy vs sigmoidoscopy alone remains a question. There has been considerable speculation as to the reason(s) for the apparent lack of efficacy of colonoscopy on right-sided colon neoplasia. Explanations include molecular aberrations associated with more aggressive tumorigenesis, difficulty of achieving adequate bowel preparation in the proximal colon, preponderance of flat (and difficult-toidentify) lesions in the proximal colon, and technical difficulty of reaching the cecum. Such concern may be particularly salient in the context of recent studies, in which a significant proportion of colonoscopies were performed by nongastroenterologists.