Report in English with a Dutch summary (KCE reports 45A)

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88 Screening for Colorectal Cancer KCE reports vol.45 the most relevant analysis for the purpose of assessing screening performance of CT with referral to optical colonoscopy is a per-patient analysis. A perpatient analysis uses the patient as the unit of analysis, and assesses the capability of CT colonography to detect or rule out a patient with at least 1 lesion of a particular minimum size. The per-patient analysis must specify the size threshold for referral, because the rational case for CT colonography relies on only referring patients with a specific threshold size for colonoscopy. Many studies only calculated per-patient sensitivity and specificity for detection of any polyp regardless of size, a strategy which refers a very high proportion of patients to colonoscopy. A few studies used clearly flawed methods in that different size thresholds were apparently used in the calculations of sensitivity and specificity. For example, in a study by Rex et al. 388, a threshold of any polyp seen on CT, regardless of size, was used to calculate sensitivity to detect a patient with a polyp of 10 mm or larger. However, specificity was calculated based on whether CT showed a false-positive polyp of greater than 10 mm. Other studies were excluded because it was unclear whether they used similar diagnostic thresholds for polyp size in the calculations of sensitivity and specificity. It is also important to consider the reference standard in assessing the performance of CT colonography. Most studies use colonoscopy as the reference standard; although colonoscopy is imperfectly sensitive, it is highly likely to be close to 100% specific. To the extent that CT colonography detects some polyps that are missed by colonoscopy, these potentially true positives are instead classified as false positives. Thus, both the sensitivity and specificity of CT are downwardly biased from their true values when colonoscopy alone is used as a reference standard. A few studies used unblinded colonoscopy as the reference standard, where the CT colonography findings are sequentially revealed to the colonoscopist, who can then investigate all polyps thought to be seen with CT 320, 389, 329. By rechecking areas of the colon that CT identified as having polyps, lesions that might be classified as false positive on CT can be correctly reclassified as true positive if a polyp is seen on reexamination with colonoscopy. Although polyps that miss detection by either method are still uncounted, this provides a lessbiased estimate of diagnostic performance, and it also allows measurement of the performance of colonoscopy (where colonoscopy performance is based on blinded colonoscopy findings, using unblinded colonoscopy as the reference standard). Overall, sensitivities are quite variable between studies, from as low as 35% to as high as 100% for detecting patients with 10 mm or larger lesions. The larger studies with more stable estimates of sensitivity ranged from 55% to 94%. Specificities were less variable, and most studies reported specificities greater than 90%. At a smaller size threshold of detection CT colonography was both less sensitive and less specific. Variable performance of CT colonography may be associated with interpreter experience or other technical factors. Such evidence, however, does not allow conclusions on the effect of CT colonography in improving health outcomes. Positive findings on CT colonography require referral for colonoscopy to confirm findings and remove polyps. The appropriate minimum size of polyp that should be referred and the appropriate screening interval are unknown. It would defeat the purpose of initial noninvasive screening to refer patients with any polyp for colonoscopy, because the prevalence of polyps is so high that a large proportion of patients would need to undergo both procedures. Because known polyps are left
KCE reports vol.45 Screening for Colorectal Cancer 89 behind, and sensitivity for small polyps is known to be less than colonoscopy, CT colonography is meant to be used more frequently than colonoscopy in a screening program. The performance of CT colonography as a (diagnostic) test has varied widely across studies 194, 246, 270, 219 and the reasons for these discrepancies are poorly defined. In order to clarify this Mulhall et al. 222 conducted a systematic review on the test performance of CT colonography compared to colonoscopy or surgery with assessment of variables that may affect its 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. To be included, studies needed to use state-ofthe-art technology d . The evaluators of the colonographies had to be unaware of the results of the gold standard test. Data on sensitivity and specificity overall and for the 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. Thirty-three studies provided summary statistics on 6.393 patients. The sensitivity of CT colonography was heterogeneous but improved as polyp size increased. 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 (Table 24). The studies differed widely, and the extractable variables explained only a small amount of the heterogeneity. Obviously, only a few studies examined the newest CT colonographic technology. Table 24: Meta-analysis of per patient sensitivity and specificity of virtual colonoscopy 222 Sensitivity Specificity Polyp size Estimate 95% CI Estimate 95% CI polyps < 6 mm 48% 25% - 70% 92% 89% - 96% polyps 6 to 9 mm 70% 55% - 84% 93% 91% - 95% polyps > 9 mm 85% 79% - 91% 97% 96% - 97% The heterogeneity of virtual colonoscopy raises concerns about consistency of performance and about technical variability in daily imaging practice. These issues must be resolved before CT colonography can be advocated for generalized application for diagnostic, let alone screening purposes. d 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
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KCE reports 1. Effectiviteit en kos
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