Faculty Peer Reviewed
The patient is a 52 year-old white male who presented to his internist for a routine examination. He has no family history of colorectal cancer. He has not previously been screened. He recently heard about CT colonography and would like to know how it compares to conventional colonoscopy. Colorectal cancer (CRC) is the third most common cancer and the second most common cause of cancer death in the United States(1). Multiple studies have demonstrated that most colorectal cancers develop from smaller adenomas(2). These adenomas, or polyps, are presymptomatic lesions that can be identified and removed before malignant transformation occurs. Current CRC screening techniques, which include fecal occult blood testing, sigmoidoscopy, double-contrast barium enema examination, and colonoscopy, have led to decreases in morbidity and mortality(2). However, only 42% of Americans age 50 years and older have undergone any type of screening(3). Lack of awareness, socioeconomic causes, limited availability, and patient discomfort are some of the possible reasons for this sub-optimal level of screening.
The American College of Radiology recently endorsed the use of computed tomographic colonography (CTC) for colon cancer screening(1). CTC, also known as “virtual colonoscopy,” was first described in 1994 as a method for evaluating the colonic lumen by Vining et al(1). CTC is a non-invasive imaging technique that uses two- and three-dimensional images to evaluate the colon.
Much like a conventional optical colonoscopy (OC), patient preparation for CTC includes laxative purgation followed by colonic insufflation with carbon dioxide. The goal needs to have a well cleansed and well distended colon to facilitate polyp detection. Image acquisitions are then performed with the patient in the supine and prone positions. Whereas OC generally requires conscious sedation and a day off from work, CTC can be performed without sedation, and therefore patients can return to work on the same day(4).
Apart from the patient convenience it affords, CTC is theoretically superior to OC for evaluation of bowel proximal to areas of stenosis or obstruction. Additionally, it may be more easily performed in elderly or frail patients in whom mobility or sedation is difficult and in patients with allergies to medications commonly used for sedation.
The potential advantages of CTC are many, but the question yet to be answered-at least in any reproducible, large-scale clinical study-is how does CTC perform when compared to the “gold standard,” OC? Johnson et al. attempted to answer this question with the American College of Radiology Imaging Network (ACRIN) National CT Colonography Trial. Their results were recently published in the New England Journal of Medicine.(5) This multi-center study was conducted at 15 sites and compared the polyp detection rate of CTC to that of OC. The study recruited 2,600 asymptomatic adults aged 50 years and older. The majority of the study participants (89%) had no known risk factors for CRC. Radiologists were instructed to identify all polyps measuring greater than 5 mm on CTC. The gold standard in this study was an OC immediately following each CTC. The primary end point was detection by CTC of large adenomas and adenocarcinomas (10 mm or larger in diameter) that had been detected by colonoscopy. For polyps > 10 mm, CTC had a mean sensitivity of 90% and a mean specificity of 86%. The positive predictive value (PPV) and negative predictive value were 23% and 99%, respectively. The relatively low PPV is attributed to the low prevalence of adenomas in the study population. This study will likely be cited by many as evidence that CTC should be considered a clinically viable screening tool for CRC along with OC, which historically has been shown to miss up to 5% of polyps 1 cm or greater in size(6).
Opponents of CTC cite concerns regarding appropriate thresholds for referral for conventional colonoscopy. The current recommendation is that all patients with lesions > 6 mm identified on CTC be referred for colonoscopy for polyp confirmation and resection(7). Based on the results of the ACRIN study, this translates into 12% of patients screened with CTC requiring follow-up OC. Optimal management of extracolonic findings, appropriate intervals between studies, and long-term effects of increased radiation exposure are other concerns related to CTC.
Future directions of CTC include development of “prepless” CTC, which involves fluid and fecal tagging with iodine or barium. This is achieved by having the patient ingest small amounts of barium or iodine with meals prior to imaging. This high-attenuation contrast material is then incorporated into residual fecal matter present at the time of examination, facilitating differentiation from polyps and thereby potentially obviating the need for colon prep before CTC. Because most patients find the colon prep the most onerous part of the experience, such prepless CT could lead to significantly more people being screened. Research is also being devoted to computer-assisted efforts to enhance polyp visualization. Software programs have been designed to detect colorectal lesions with morphologies suggestive of polyps(8). Such programs will no doubt increase the sensitivity and specificity of CTC.
In their first-ever joint consensus statement for CRC, the American Cancer Society and the Multi-Society Task Force on Colorectal Cancer included CTC as one of several possible CRC screening options(1). In spite of these endorsements, however, Medicare announced in May that it would not cover CTC, citing insufficient evidence of improved healthcare outcomes. The agency stated further that with the higher prevalence of polyps among Medicare beneficiaries, many patients would still need follow-up colonoscopies(9).
CTC has been shown to detect clinically significant polyps with sensitivity approaching OC, is less invasive, and would almost certainly be utilized by some patients not currently being screened for CRC. What impact Medicare’s decision will have on the more widespread adoption of CTC is uncertain; one thing, however, is clear: CTC is a continuously evolving clinical tool that holds much promise for future prevention of CRC.
Ely Felker is a fourth year medical student at NYU School of Medicine. Faculty peer reviewed.
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4. Boodman SG. Inside out. The Washington Post. April 28, 2009
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8. Yoshida H, Nappi J, MacEneaney P, Rubin DT, Dachman AH. Computer-aided diagnosis scheme for detection of polyps at CT colonography. Radiographics 2002;22:963-979.
9. Rubenstein S. Medicare: not enough evidence to pay for virtual colonoscopies. The Wall Street Journal. May 13, 2009.