While no one questions the desirability of colorectal screening for cancer, tests that are useful for screening continue to be evaluated by experts. Recent interest has focused on CT colonography, a procedure by which the colon is evaluated by radiologists with methods that resemble CT scans and that provide views of the inside of the colon similar to those seen with regular colonoscopy. Debate continues to rage over whether it is ready to be added to the panel of tests suitable for regular screening by average-risk adults. We present two articles on the pro and con side regarding this issue.
THE ARGUMENT AGAINST CT COLONOGRAPHY
Douglas K. Rex, MD
Distinguished Professor of Medicine
Indiana University School of Medicine
Director of Endoscopy, Indiana University Hospital
Indianapolis, IN
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| Douglas K. Rex, MD | |
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There is abundant enthusiasm that CT colonography (also known as virtual colonoscopy) will improve adherence and compliance with colorectal cancer screening recommendations, but that is no substitute for evidence.
Unfortunately we still do not have a shred of scientific evidence that CT colonography (CTC) will substantially improve adherence to colorectal cancer screening. In the only randomized controlled trial on the topic,(1) patients offered a choice of CTC or colonoscopy were no more likely to get screened than patients offered only one of the tests. If CTC does improve adherence, we would expect polypectomy rates for large polyps to increase and cancer incidence to fall. Conversely, if CTC were primarily used in patients who would otherwise be willing to undergo colonoscopy, polypectomy rates could fall and cancer incidence rates could rise, along with costs! The benefit of screening for polyps is only realized through polypectomy. Until the issue of adherence is resolved, screening CTC should be confined to patients who are clearly unwilling to undergo colonoscopy, especially if they have a high pre-test probability of polyps.
Other major problems with CTC are small polyp management, radiation risk, and extracolonic findings. CTC experts have developed reporting criteria that recommend ignoring polyps < 5 mm in size regardless of their number and the confidence with which they are interpreted.(2) It is certainly true that these polyps pose less risk to patients than larger polyps. Nonetheless, ignoring these polyps entirely is a remarkable recommendation if one considers the detailed reporting of incidental findings used for every other aspect of CT scans, and the lack of knowledge regarding the natural history of these polyps. Multiple adenomas at colonoscopy is among the most consistent predictors of the emergence of advanced colorectal neoplasia.(3) Patients have a right to know the results of their tests, especially since some patients with multiple small polyps would choose (probably wisely) to undergo polypectomy. Finally, if CTC is as good as colonoscopy at detecting large polyps and small polyps don’t matter, why is CTC recommended at 5-year intervals and colonoscopy at 10 year intervals? This can only reflect uncertainty regarding the importance of small polyps.
As for the risks of radiation exposure from CTC, CTC experts remain in denial while others continue to raise concerns.(4,5) Ethical practice requires that substantial real or potential risks be discussed with asymptomatic patients who have alternative options for screening. However, radiologists continue to ignore their obligations to patients in this regard.
As for extracolonic findings, the US Preventive Services Task Force concluded that they will likely result in net harm to patients.(6) There is no question that CTC is a better test than double contrast barium enema and that CTC has the potential to help prevent colorectal cancer. However, CTC use will be costly and we should understand its influence on adherence and polypectomy rates before pushing ahead on a broad front.
In my opinion, the best use of CTC at this time is in patients who decline colonoscopy. These patients should understand the potential risks of radiation exposure, particularly if they are young and asymptomatic. The best way to be protected against colorectal cancer is for a well-informed patient(7) to identify an experienced colonoscopist who has demonstrated his/her commitment to quality.(8) No other screening test can approach colonoscopy in its capacity to identify and clear colorectal neoplasia when it is performed by such an operator.(9,10)
References
- Scott RG, Edwards JT, Fritschi L, Foster NM, Mendelson RM, Forbes GM. 2004. Community-based screening by colonoscopy or computed tomographic colonography in asymptomatic average‐risk subjects. Am J Gastroenterol; 99:1145-51.
- Zalis ME, Barish MA, Choi JR, Dachman AH, Fenlon HM, Ferrucci JT, Glick SN, Laghi A, Macari M, McFarland EG, Morrin MM, Pickhardt PJ, Soto J, Yee J. 2005. CT colonography reporting and data system: a consensus proposal. Radiology; 236:3-9.
- Martinez ME, Baron JA, Lieberman DA, Schatzkin A, Lanza E, Winawer SJ, Zauber AG, Jiang R, Ahnen DJ, Bond JH, Church TR, Robertson DJ, Smith‐Warner SA, Jacobs ET, Alberts DS, Greenberg ER. 2009. A pooled analysis of advanced colorectal neoplasia diagnoses after colonoscopic polypectomy. Gastroenterology; 136:832-41.
- Brenner DJ, Hall EJ. 2007. Computed tomography‐‐an increasing source of radiation exposure. N Engl J Med; 357:2277-84.
- Einstein AJ. 2009. Radiation protection of patients undergoing cardiac computed tomographic angiography. JAMA; 301:545-7.
- Screening for colorectal cancer: U.S. Preventive Services Task Force recommendation statement Ann Intern Med 2008;149:627-37.
- Rex D. 2009. 10 questions you need to ask about colonoscopy. New York Times, March 16. http://health.nytimes.com/health/guides/test/colonoscopy/overview.html?WT.z.
- Rex DK, Petrini JL, Baron TH, Chak A, Cohen J, Deal SE, Hoffman B, Jacobson BC, Mergener K, Petersen BT, Safdi MA, Faigel DO, Pike IM. 2006. Quality indicators for colonoscopy. Gastrointest Endosc; 63:S16-28.
