Computed Tomographic Colonography (Virtual Colonoscopy)

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Abstract
Colorectal cancer is a major public health issue that has received much attention in recent years. Conventional colonoscopy is widely accepted as the best available method for detection and exclusion of precursor lesions, with the advantage that most can be removed at the same examination.1,2 However, there is consumer resistance to colonoscopy, which may be perceived as invasive and not without risk. There is a need for simpler screening methods that would allow colonoscopy to be used more selectively and efficiently.3,4 Computed tomographic colonography (CTC), sometimes called virtual colonoscopy, is a promising candidate.5-9 Computed tomographic colonography involves helical computed tomographic scanning of the colon after cathartic preparation and colonic distension. Several single-center studies have reported sensitivities of more than 90% for detection of lesions sized 10 mm or more,10-13 but other studies reported lower data ranging from 61% to 78%.14-17 The largest recent single-center study reported poor results and considerable variation between readers, with sensitivities of 32%, 34%, and 72%.18 Although detection (and exclusion) of all lesions is the ultimate goal, the key screening parameter is the ability to detect participants with clinically significant lesions because the detection of any lesion would lead logically to colonoscopy, which should detect nearly all lesions. The definition of a clinically significant lesion is important. Most physicians agree that it is crucial not to miss participants with lesions sized more than 10 mm in diameter and it is desirable to detect all lesions sized more than 6 mm. Studies have reported CTC sensitivities of 85%, 90%, 96%, and 100% for the detection of participants with lesions sized at least 10 mm and sensitivities of 84%, 88%, 93%, and 94% with the threshold at 6 mm.11,12,14,15 Most of these studies were initiated by committed radiologists, many of them pioneers in the technique, and were restricted to single centers. To be valuable as a screening tool, CTC must perform well in routine practice. Our goal was to assess the accuracy of CTC in a large number of participants across multiple centers. Since this study was completed, good results have been reported from a study conducted in 3 US Armed Services Hospitals.19

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