David A. Johnson, MD, MACG, FASGE, chief of the division of gastroenterology at Eastern Virginia Medical School in Norfolk discusses how — even as additional screening options are introduced to patients — colonoscopy remains the present and future of colorectal cancer prevention.
Q: The U.S. Preventive Services Task Force recently assigned an "A" grade to alternative col
orectal cancer screening techniques, like CT colonography. What are your thoughts on the recommendation?
Dr. David A. Johnson: For their guidelines, the U.S. Preventive Services Task Force did some modeling that suggested both CTC and colonoscopy were able to reduce colon cancers; however, it was built on this strategy of detection of large polyps. This strategy misses the mark, because big polyps are easy to find with colonoscopy, less so with CTC, but the real emerging crisis for gastroenterology is the detection of flat lesions and sessile serrated polyps that have a high proclivity for developing into colon cancers.
The nail in the coffin, for me, was a recently published post-hoc analysis by researchers in Amsterdam who compared CTC and colonoscopy in a randomized trial, evaluating detection for high-risk SSPs — those larger or equal to 10 millimeters or any SSP with dysplasia. These SSPs are flat lesions, the detection of which has always been a concern in CTC because it is easy to see why the radiologist likely would not see these lesions, which do not raise up into the lumen of the colon. However, in colonoscopy, there are several findings that can highlight attention to areas of likely SSPs. These include a "mucous cap," alteration in colonic vascular pattern, a "rim" of debris or bubbles or alteration of a contour of a fold. As these are subtle findings, these would be impossible to see with CTC, but an astute, quality colonoscopist should detect them. This study found that 4.3 percent of individuals who received colonoscopies were diagnosed with at least one high-risk SSP, while only 0.8 percent of individuals who received CTC were diagnosed with them — these lesions are five-and-a-half times more likely to be picked up in colonoscopy. Furthermore, colonoscopy, compared to CTC, had higher detection of flat high-risk SSPs (17 versus 0), high-risk SSPs in the proximal colon (32 versus 1) and SSPs with dysplasia (30 versus 1). Hence, my "nail in the coffin" statement on CRC screening by CTC.
If you apply these findings back to the U.S. Preventative Services Task Force recommendations, it really unravels their analysis in a profound way, because their modeling strategy didn’t include this type of analysis for flat lesions and SSPs. This is important, as 10 to 20 percent of CRC goes through the serrated pathway, and these lesions account for more than one third of interval CRC, typically due to missed lesions or incompletely resected lesions on the initial exam.
Q: What do you think about the recent push for CMS to cover CTC, in addition to optical colonoscopy?
DJ: The question is whether CTC is covered as a screening test. I divide tests into "cancer detection tests" and "cancer prevention tests" — cancer prevention tests detect polyps before they become cancerous. The ideal, perfect goal for screening is to never get to colon cancer, but to prevent colon cancer by removing the things that can lead to CRC. As some studies have suggested that there may be a relatively acceptable level of detection for elevated larger polyps, there has been a significant effort from the American College of Radiology Imaging Network to position CTC as a comparable CRC screening to colonoscopy. Colonoscopy, however, clearly remains the demonstrated best option, the preferred gold standard option, because it not only detects, but also can prevent colon cancer by identification and resection of precancerous polyps.
Q: How has increased use of CTC affected gastroenterology practices and patients?
DJ: There are very few areas where CTC is used on a regular basis. It is a wonderful test for patients who, for example, have an obstructing colon lesion, and the physician needs to get some determination of the proximal colon before going into surgery. That’s been the primary role of CTC within gastroenterology practices so far, as a better test than barium enema. There is, however, growing concern about the increased exposure to radiation with CTC used as a screening test. There is clear increased recognition regarding the potentially harmful consequences of exposure to ionizing radiation. The incremental risk of developing abdominal cancer when you’ve been exposed to radiation is based on the overall baseline radiation of approximately five milisieverts. To put this in perspective, the typical abdominal CT scan is about 10 to 20 milisieverts, and if you're obese, the number goes up. If you start to look at radiation exposure over a lifetime, it becomes a real risk when you're talking about screening test selection.
Q: What do you think the future of CRC screening will look like?
DJ: Clearly, for the present and the foreseeable future, colonoscopy will remain the gold standard. The emphasis will be on high quality performance with comparative benchmarking to nationally-established thresholds for specific measurements. Individual technicians need to be held accountable to quality measures, e.g. meeting — or hopefully surpassing — targeted baseline thresholds for screening colonoscopy adenoma detection rates, as well as overall successful cecal intubation, adequate colonoscopy preparation and compliance with national guideline recommendations for repeat intervals for colonoscopy screening or polyp surveillance. This, for my mind, remains the future: prevention of colon cancer through high-quality colonoscopy screening.
There are other tests that are emerging, but again, most of these tests to date — the stool-based test, the blood-based test — have been for detection of cancer, not prevention. Any modality that gets more people who refuse screening — despite being well-informed by their care provider on the values of colonoscopy screening — to increase compliance with CRC screening is better than no screening. Care providers, however, have the responsibility to explain the values or limitations for sensitivity and specificity of these tests and to clearly define the role of cancer detection versus cancer prevention, the latter being the ultimate goal of a valuable screening program. Healthcare providers need to really emphasize that colonoscopy remains the "gold standard." We know that when patients have options, they're more likely to comply, but it needs to be done in the context of being far less effective secondary options to colonoscopy.