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 FOR CT COLONOGRAPHY
Michael Macari, MD
Associate Professor of Radiology
Vice Chair of Radiology Operations
New York University School of Medicine
New York, NY
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| Michael Macari, MD | |
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In 2006, there were an estimated 147,000 new cases and 55,000 deaths attributable to colorectal cancer in the United States.(1) This is despite the multiple screening techniques that are currently available to detect precancerous adenomas and early stage cancers. Colon cancer is known to be a preventable cancer if these lesions are detected early and removed.(2) So why is colon cancer the second leading cause of cancer death in the US? Is it that current screening techniques are inadequate?
The answer is probably yes for several reasons. While the fecal occult blood test (FOBT) and sigmoidoscopy have been shown to reduce colon cancer mortality, both tests have poor sensitivity for the detection of advanced neoplasia throughout the entire colon.(3) Optical colonoscopy (OC) is currently the best test available with a high sensitivity when performed with care by trained individuals. However, there are several limitations to colonoscopy. Universal access to screening colonoscopy is limited; there are simply not enough endoscopists to screen all eligible individuals. Moreover, it is not a perfect test and studies have shown that colonoscopy can miss large polyps.(4,5) Endoscopists who have short endoscopic examination times tend to detect fewer polyps; therefore, colonoscopy is a user-dependent examination.(6) Moreover, a recent study showed that while OC does prevent colon cancer, its effectiveness in decreasing the prevalence of right-sided colon cancers is low or non-existent.(7) Finally, there are known risks to colonoscopy, including bleeding, perforation, and very rarely death.(8)
So why should we consider virtual colonoscopy? Virtual colonoscopy (VC), otherwise known as CT colonography (CTC), is a technique that uses a thin section, low radiation dose CT scan, combined with computer software that enables the reader to evaluate the colon for polyps in a minimally invasive way (Fig. 1). The technique was first described in 1994 and has undergone tremendous advances over the past 15 years.(9) Despite the minimal invasiveness, current techniques rely on thorough bowel cleansing and the colon does need to be distended with gas, typically CO2 which is rapidly absorbed across the colonic mucosa. Sedation is not required and the entire procedure time in the CT suite is less than 10 minutes.(9) This noninvasive examination has been shown to increase compliance with colon cancer screening and may increase the number of individuals that ultimately undergo colon cancer screening.
VC has been used to evaluate the colon after an incomplete colonoscopy, in elderly individuals and those with other contraindications to OC, and to evaluate the colon proximal to an obstructing mass.(9) Recent large studies, using current multidetector CT techniques, 2D and 3D visualization platforms, fluid and fecal tagging, and with readers who have been trained in VC interpretation, have shown that VC also performs very well for colon cancer screening.(5,8-11) As a result of continued advances in the field of CTC, the American Cancer Society has now endorsed CTC as an approved method for colorectal cancer screening.(12)
A comparative screening study that evaluated the prevalence of advanced neoplasia (defined as an adenoma³10 mm or one of any size with villous histology or high grade dysplasia) in a cohort of >3,000 individuals, each undergoing either OC or VC, was identical.(7) In the OC screening group, many more polypectomies needed to be performed to detect the same number of advanced lesions. In this study, the prevalence of advanced neoplasia was approximately 8%, implying that the vast majority of individuals undergoing VC screening would not need OC for polypectomy. In the OC group, eight colonic perforations occurred, with zero in the VC group. The recently completed multicenter ACRIN trial screened >2,500 individuals from both the private and academic sectors with VC and OC. In that study, the detection rate of persons with at least one adenoma³10 mm was 90% for VC. This is clearly superior to FOBT and sigmoidoscopy and approaches that of colonoscopy.
In order to impact on the morbidity and mortality related to colon cancer we need to improve upon current screening techniques. By including VC in the colon screening armamentarium the incidence of colon cancer should decrease. Of course there are still hurdles. Training, implementation, quality assurance, and reimbursement are all important issues that are being investigated and will continue to need to be addressed. With input from the radiology and gastroenterology communities, VC can be an important contributor to colon cancer prevention.
Figure 1. 58 year-old man with 10 mm wide and 1.5 mm thick flat lesion in the sigmoid colon.
A. Axial CT image obtained after bowel cleansing in same patient in same area performed 9 days later shows flat morphology (arrow) of the lesion.
B. 3D CT endoluminal image confirms flat morphology of the lesion (arrow).
C. Endoluminal image from optical colonoscopy shows a 10 mm flat hyperplastic polyp (arrow).
References
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- Pickhardt PJ, Choi JR, Hwang I, et al. 2003. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med; 349:2191-2200.
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- Baxter NN, Goldwasser MA, Paszat LF, et al. 2008. Association of colonoscopy and death from colorectal cancer. Ann Intern Med; 9; 150:1-8.
- Kim DH, Pickhardt PJ, Taylor AJ, et al. 2007. CT colonography versus colonoscopy for the detection of advanced neoplasia. N Engl J Med; 357:1403-1412.
- Macari M, Bini EJ. 2005. Special Review: CT colonography: where have we been and where are we going? Radiology; 237:819-833.
- Johnson CD, Chen MH, Toledano AY, et al. 2008. Accuracy of CT colonography for detection of large adenoma and cancers. N Engl J Med; 359:1207-1217.
- An S, Lee KH, Kim YH, et al. 2008. Screening CT colonography in an asymptomatic average-risk Asian population: a 2-year experience in a single institution. AJR; 191:W100-W106.
- Graser A, Stieber P, Nagel D, et al. 2009. Comparison of CT colongraphy, colonoscopy, sigmoidoscopy, and faecal occult blood tests for the detection of advanced adenoma in average risk population. Gut; 58:241-248.
- Levin B, Lieberman DA, McFarland B, et al. 2008. Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: a joint guideline from the American Cancer Society, the multi-Society taks force on colorectal cancer, and the american College of Radiology. CA Cancer J Clin; 58:1-31.
