CT colonography virtual colonoscopy

This new technology (described further later) is not as yet approved for use as a screening test. No randomized controlled studies are available investigating the ability of CTC to reduce CRC morbidity. Until recently, no screening studies were available to evaluate the efficacy of CTC, and the accuracy of the test had to be extrapolated from multiple studies using primarily surveillance-type populations.

The appeal of CTC in the screening setting derives from the fact that the examination is a noninvasive CT scan that uses no sedatives or contrast media, and could provide an attractive alternative for many patients who refuse to or cannot undergo co-lonoscopy. Because it is an imaging test only, and cannot remove polyps, its role is limited compared with colonoscopy.

The first and largest screening study performed by Pickhardt and coworkers [37] at three United States military hospitals revealed that CTC performed as well as or better than optical colonoscopy in a same-day back-to-back correlative comparison of 1233 asymptomatic subjects at average risk for CRC. Five experienced radiologists and 17 experienced colonoscopists were involved. Sensitivity for adenomatous polyps was 94% for CTC versus 92% for optical colonoscopy at the 8-mm diameter threshold and 96% for CTC versus 88% for optical colonoscopy at the 10-mm threshold size. The accuracy of CTC for adenomatous polyps on a perpatient basis was 92% for 8 mm and 96% for 10 mm. CTC depicted 54 (91.5%) of 59 advanced neoplasms, whereas optical colonoscopy depicted 52 (88.1%). The negative predictive value of CTC was 99% for adenomas 8 mm or larger. The authors were the first to use a primary three-dimensional interpretation method after fecal tagging with barium and meglumine diatrizoate, followed by electronic subtraction of labeled stool and fluid using a commercially available, US Food and Drug Administration-approved computer. The technique of segmental unblinding (see later) allowed separate validation of both CTC and colonoscopy. These results closely mimicked or improved on a large number of preceding studies performed in surveillance or mixed-type populations, using combined two-dimensional and three-dimensional interpretations, without fecal tagging and electronic subtraction (Table 2) [2,38,39]. A coincident study, not performed in a screening population and using older technology with less reader experience [40], led to much argument and deliberation in the literature, and served well to point out the necessity of attention to detail (ie, slice thickness, polyp nomenclature, and reader experience) required to achieve robust and reliable results, and to point out the existence of a steep radiologist's learning curve for CTC [41]. Nonetheless, as of this writing, no other large study in screening subjects has been performed to validate the Pickhardt study.

Other screening trials now in progress include a large multicenter trial launched by the American College of Radiology Imaging Network to test two-dimensional versus three-dimensional accuracy in 2600 subjects. The Special Interest Group in Gastrointestinal and Abdominal Radiology (United Kingdom) will compare CTC with barium enema and colonoscopy in 4500 patients. The Italian Multicenter Study on Accuracy of CT Colonog-raphy will enroll 3550 patients.

Because of the novelty of this technique, cost-effectiveness has only been able to be investigated using various statistical models [42-44]. Taking into account compliance, the cost of care from missed polyps, the avoidance of perforations and other factors, and assuming optimal sensitivity and specificity for CTC, some investigators have determined that the cost of CTC has to be significantly lower than colonoscopy (up to 54% less) to be cost effective [42].

Numerous studies have looked at patient comfort and preference for CTC versus colonoscopy [45] and found varying results. In a recent study testing preference immediately after each test and 5 weeks later, CTC with bowel relaxants was preferred over colonoscopy [46].

Although CTC is noninvasive, potentially serious adverse events have been reported in 0.08% of symptomatic patients and perforations have occurred in 0.05% to 0.059% [47,48]. By comparison, the colonoscopy perforation rate reported in the same and similar hospitals was 0.13%, suggesting a much lower risk with CTC [47].

Finally, use of a screening test that uses radiation must address concerns regarding potential radiation-induced cancers. According to one investigator, the best estimate for the absolute lifetime cancer risk using the typical scanner (prone and supine scans, for one CTC only) is about 0.14% in a 50 year old and half of that in a 70 year old. Multiple interval examinations increase these values [49].

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