Effectiveness of Colonoscopy: The Issue of Detecting Right- Versus Left-Sided Lesions
Linda Rabeneck, MD, MPH Professor of Medicine, University of Toronto, Ontario, Canada |
Given the central role of colonoscopy in colorectal cancer (CRC) screening, diagnostic work-up and surveillance, it is important that the procedure be as accurate as possible. The objective here is to summarize our current understanding of colonoscopy effectiveness.
Colonoscopy is less effective in the right colon
Colonoscopy is less effective for lesions in the proximal colon. An Ontario case-control study of 10,942 persons with CRC and 51,460 controls reported that colonoscopy was associated with decreased overall CRC mortality, but this association was primarily due to lower mortality from left-sided cancers.1 A statewide German study of 3,287 persons who underwent screening colonoscopy reported that the prevalence of left-sided but not right-sided advanced neoplasia was reduced within a 10-year period after colonoscopy.2 More recently, a population-based case-control study from Germany compared the rates of colonoscopy in the preceding 10 years among persons with CRC and community controls. The adjusted odds ratios for any CRC, right-sided CRC and left-sided CRC were 0.23 (95 percent confidence interval, 0.19-0.27), 0.44 (0.35-0.55) and 0.16 (0.12-0.20), respectively.3 Taken together, these studies show that colonoscopy is less effective for lesions in the proximal colon in usual clinical practice.
Reasons for these findings
The lesion was not seen
Given our current technology, there are several reasons why a lesion might not be seen at colonoscopy. First, it may not be reached by the colonoscope during the procedure. Reaching the cecum is more challenging technically than reaching the distal colon.
Second, a lesion may not be seen because the bowel preparation was not adequate and the mucosa was not fully visualized. The right colon is more difficult to clean. The use of split-dose prep — in which half of the preparation is taken on the evening before colonoscopy and the other half on the day of the procedure — provides superior mucosal visualization. This is likely because the quality of bowel preparation varies inversely with the duration of the interval between the last dose of the agent and the start of colonoscopy.
Third, the lesion may not be seen because of inadequate technique. Studies of CT colonography followed by colonoscopy have shown that most adenomas greater than or equal to 6 mm that are missed at colonoscopy are located on the proximal side of a fold or near the anal verge. Adenoma detection rate (ADR) is an important indicator of colonoscopy quality. A Polish cohort study of 45,026 persons who underwent screening colonoscopy reported an association between ADR and risk of subsequent CRC.4 A recent Ontario cohort study of 14,064 persons with CRC reported that those who underwent colonoscopy by an endoscopist with a polypectomy rate of greater than or equal to 30 percent were less likely to be diagnosed with an incident CRC following the procedure compared with those in whom the colonoscopy was done by an endoscopist with a less than 10 percent polypectomy rate.5 Thus, polypectomy rate — arguably easier to measure than ADR — is an important indicator of colonoscopy quality.
Recent attention has focused on the role of the serrated neoplasia pathway, which depicts the progression of serrated colorectal polyps, including traditional serrated adenomas (TSAs) and sessile serrated adenomas (SSAs), to CRC. SSAs may be flat or slightly elevated, covered by a mucus cap and occur more commonly in the proximal colon (see photo). There may be a lack of knowledge of this appearance. SSAs may be more challenging to detect. A recent U.S. study of 6,681 screening colonoscopies performed by 15 gastroenterologists reported that in 13 percent of colonoscopies, one or more proximal serrated polyps (hyperplastic, SSA or TSA) were detected. In addition, variation in the detection of proximal serrated polyps was observed, as well as an association between proximal serrated polyp detection rates and ADRs. A U.S. study of 7,192 average-risk persons who underwent colonoscopy at an urban medical center reported an increase in the detection of SSAs from 2006 to 2008, possibly due to increased awareness and improved technique.
Prior incomplete polypectomy
Polypectomy is the key to reducing CRC incidence following colonoscopy. Despite the central role of polypectomy, there is variation in technique coupled with clear evidence that incomplete polypectomy may lead to missed CRC. The serrated neoplasia pathway may contribute to prior incomplete polypectomy, as these polyps may be more challenging to remove completely. In addition, in the past decade, prior to the focus on the serrated neoplasia pathway, a gastroenterologist faced with a biopsy report of a hyperplastic polyp may not have considered it important to ensure complete removal of the lesion.
Rapid tumor growth
In some cases, a rapidly progressing CRC may not have been present at colonoscopy, which may have been truly negative. The polyp cancer hypothesis describes the slow evolution of CRC from adenomatous polyps. However, CRCs that arise from adenomas in Lynch syndrome, which are more commonly right-sided, progress more rapidly and show microsatellite instability (MSI). A U.S. case-control study reported that incident CRCs following colonoscopy were more likely to be MSI positive, located in the proximal colon and smaller than detected CRCs.
What does this mean for clinical practice?
Colonoscopy fails to detect a small but clinically important percentage of lesions, and this lack of effectiveness is more pronounced in the right colon. Future research should be directed at disentangling the relative contributions of tumor biology and colonoscopy quality in explaining this result. Whether routine use of enhanced colonoscopy techniques will result in improved colonoscopy effectiveness in clinical practice remains an open question.
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Endoscopic photograph of sessile serrated adenoma at the hepatic flexure. |
In the meantime, when consent is obtained for a colonoscopy, patients must be informed of the small risk that a cancer or polyp may not be detected. The use of split-dose prep and/or ensuring that the time interval between ingestion of the last dose of agent and colonoscopy is less than 14 hours is advised.
Careful technique is paramount. The results of more recent studies are promising.
With heightened awareness of the reduced effectiveness of colonoscopy in the right colon — and the reasons for this — coupled with meticulous colonoscopy technique, we should be able to close this key performance gap.
Dr. Rabeneck had no conflicts to disclose
References
1. Baxter NN, Goldwasser MA, Paszat LF, Saskin R, Urbach DR, Rabeneck L. Association of colonoscopy and death from colorectal cancer: A population-based, case-control study. Ann Intern Med 2009;150:1-8.
2. Brenner H, Hoffmeister M, Arndt V, Stegmaier C, Altenhofen L, Haug U. Protection from right- and left-sided colorectal neoplasms after colonoscopy: Population-based study. J Natl Cancer Inst 2010;102:89-95.
3. Brenner H, Change-Claude J, Seiler CM, Rickert A. Hoffmeister M. Protection from colorectal cancer after colonoscopy: A population-based, case-control study. Ann Intern Med 2011;154:22-30.
4. Kaminski MF, Regula J, Kraszewska E, Polkowski M, Wojciechowska U, Didkowska J, Zwierko M, Rupinski M, Nowacki MP, Butruk E. Quality indicators for colonoscopy and the risk of interval cancer. New Engl J Med 2010;362:1795-803.
5. Baxter NN, Sutradhar R, Forbes SS, Paszat LF, Saskin R, Rabeneck L. Analysis of administrative data finds endoscopist quality measures associated with postcolonoscopy colorectal cancer. Gastroenterology 2011;140:65-72.
