Brian P. Mulhall, MD, MPH; Ganesh R. Veerappan, MD; Jeffrey L. Jackson, MD, MPH
Computed tomographic (CT) colonography, also called virtual colonoscopy, is an evolving technology under evaluation as a new method of screening for colorectal cancer. However, its performance as a test has varied widely across studies, and the reasons for these discrepancies are poorly defined.
To systematically review the test performance of CT colonography compared to colonoscopy or surgery and to assess variables that may affect test performance.
The PubMed, MEDLINE, and EMBASE databases and the Cochrane Controlled Trials Register were searched for English-language articles published between January 1975 and February 2005.
Prospective studies of adults undergoing CT colonography after full bowel preparation, with colonoscopy or surgery as the gold standard, were selected. Studies had to have used state-of-the-art technology, including at least a single-detector CT scanner with supine and prone positioning, insufflation of the colon with air or carbon dioxide, collimation smaller than 5 mm, and both 2-dimensional and 3-dimensional views during scan interpretation. The evaluators of the colonogram had to be unaware of the findings from use of the gold standard test.
Data on sensitivity and specificity overall and for detection of polyps less than 6 mm, 6 to 9 mm, and greater than 9 mm in size were abstracted. Sensitivities and specificities weighted by sample size were calculated, and heterogeneity was explored by using stratified analyses and meta-regression.
33 studies provided data on 6393 patients. The sensitivity of CT colonography was heterogeneous but improved as polyp size increased (48% [95% CI, 25% to 70%] for detection of polyps <6 mm, 70% [CI, 55% to 84%] for polyps 6 to 9 mm, and 85% [CI, 79% to 91%] for polyps >9 mm). Characteristics of the CT colonography scanner, including width of collimation, type of detector, and mode of imaging, explained some of this heterogeneity. In contrast, specificity was homogenous (92% [CI, 89% to 96%] for detection of polyps <6 mm, 93% [CI, 91% to 95%] for polyps 6 to 9 mm, and 97% [CI, 96% to 97%] for polyps >9 mm).
The studies differed widely, and the extractable variables explained only a small amount of the heterogeneity. In addition, only a few studies examined the newest CT colonography technology.
Computed tomographic colonography is highly specific, but the range of reported sensitivities is wide. Patient or scanner characteristics do not fully account for this variability, but collimation, type of scanner, and mode of imaging explain some of the discrepancy. This heterogeneity raises concerns about consistency of performance and about technical variability. These issues must be resolved before CT colonography can be advocated for generalized screening for colorectal cancer.
Summary statistics: For polyps <6 mm, 0.48 (95% CI, 0.25 to 0.70); for polyps 6–9 mm, 0.70 (CI, 0.55 to 0.84); for polyps >9 mm, 0.85 (CI, 0.79 to 0.91).
Appendix Table 1.
Appendix Table 2.
The QUORUM (Quality of Reporting of Meta-Analyses) guidelines for reporting of meta-analyses were used. CRC = colorectal cancer; CT = computed tomographic.
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Eric R Frizzell
Walter Reed Army Medical Center
April 19, 2005
Computed Tomographic Colonography- Radiation Risk and Informed Consent
TO THE EDITOR:
We read with interest the recent meta-analysis of computed tomographic colonography (CTC) by Mulhall and colleagues. Although we agree that this novel modality holds promise, the radiation risk from potential repeated exams has not yet been well addressed.
Given that small (<5-8 mm) adenomas are common, and their risk of malignancy is low, guidelines have been proposed suggesting that 5-8 mm polyps on CTC be left in place with CTC surveillance in 1-3 year intervals to assess for stability of polyp size. (1) This poses a potential health risk, as a radiation dosage of 8 to 12 mSv, which is the estimated radiation exposure with a single CTC, has been reported to increase the risk of malignancies.(2-4) To date, there has been one publication estimating the life-time risk of malignancy from radiation exposure during CTC, as a single screening test or for surveillance with a risk of radiation related death of between 0.3% and 0.24% with a 3 or 5 year surveillance interval, respectively.(5)
Given this potential risk, it may be important for physicians to accurately inform patients of the risk of ionizing radiation with CTC. While the risk of malignancy from a single CTC is likely low, the risk from repeated surveillance with CTC may not be acceptable for many patients. These risks may be further clarified with studies of the actual growth rate and neoplastic potential of small adenomas versus the actuarial cancer risk from ultra-low dose CTC .(3, 4)
Alternatively, CTC may be envisioned as a preliminary screening tool to identify individuals with large polyps who should be referred for polyp removal and future surveillance colonoscopy. Before CTC is embraced as an accepted standard of care for colorectal cancer screening or surveillance we need to develop better guidelines on its usage, better define the risks from ionizing radiation, and adequately inform patients of the risks of ionizing radiation exposure, ideally through well designed clinical trials. Informed consent for patients undergoing elective CTC for screening, to include the potential risk of radiation related secondary malignancies should be considered.
Eric Frizzell, M.D.
Inku Hwang, M.D.
Walter Reed Army Medical Center
Washington, DC 20307-5001
Disclaimer: The opinions and assertions contained herein are the private views of the authors and are not be to be construed as reflecting the views of the Department of the Army or the Department of Defense
1. Pickhardt PJ, Choi JR, Hwang I, et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med. 2003;349(23):2191-200. 2. Brenner D, Elliston C, Hall E, Berdon W. Estimated risks of radiation- induced fatal cancer from pediatric CT. AJR Am J Roentgenol. 2001;176(2):289-96. 3. Macari M. Virtual colonoscopy: clinical results. Semin Ultrasound CT MR. 2001;22(5):432-42. 4. van Gelder RE, Venema HW, Florie J, et al. CT colonography: feasibility of substantial dose reduction--comparison of medium to very low doses in identical patients. Radiology. 2004;232(2):611-20. 5. Wise KN. Solid cancer risks from radiation exposure for the Australian population. Australas Phys Eng Sci Med. 2003;26(2):53-62.
Mulhall BP, Veerappan GR, Jackson JL. Meta-Analysis: Computed Tomographic Colonography. Ann Intern Med. 2005;142:635-650. doi: 10.7326/0003-4819-142-8-200504190-00013
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Published: Ann Intern Med. 2005;142(8):635-650.
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