In this section on accuracy, a distinction between high and low prevalence has been made. The difference that is made in this spectrum is of course arbitrary and is not absolute. However by drawing this line, a difference in performance of both methods is apparent.

In studies with a high prevalence for colorectal polyps or cancer, primary 2D reviewing performed well (Pineau et al. 2003; Johnson et al. 2003b; Iannaccone et al. 2004a) and in comparative studies (primary 2D vs primary 3D) (McFarland et al. 2001; Macari et al. 2004; Iannaccone et al. 2004a) no difference in sensitivity was detected between primary 2D and primary 3D review methods. It is noteworthy that in these comparative series most likely conventional 3D methods with suboptimal surface visualization were used. This may have underestimated the diagnostic value of 3D.

In the low prevalence group a discrepancy can be seen between studies with good results (Pickhardt et al. 2003;van Gelder et al. 2004b) and moderate outcomes (Johnson et al. 2003a; CoTTon et al. 2004). These controversial results have resulted in speculations about its cause: the review method (primary 2D or primary 3D), the bowel preparation (with or without oral contrast), scanning parameters (5 mm vs thinner), the role of reviewer experience, etc.

Although we cannot determine the definite cause for the differences in the mentioned papers, it is striking that the two studies that reported the highest sensitivity used primary 3D review methods with improved surface visualization to detect polyps, whereas the two studies with the lower sensitivity used a primary 2D review method.

In all direct comparative studies there was no statistical difference between both methods, although the differences between both methods described by van Gelder were very close to significance (p=0,06).

It is widely known that polyp size is a major determinant of detection. Comparing visualization methods in populations with large polyps that are detected easily, may obscure difference that emerge in average risk patients with generally smaller polyps. The differences between primary 2D and primary 3D must be assessed in the situation to which it most likely applies: patients with a low prevalence of polyps.

The potentially superior performance of 3D is most likely based on the more intuitive presentation (Fig. 9.13) and the longer exposure time to polyps (Lee and Pickhardt, 2004). Lee and Pickhardt compared exposure time of 20 polyps in an axial 2D method with a conventional endoluminal 3D method. Lee concluded that the opportunity of polyp detection, including both exposure time and distance of polyp visualization, is significantly greater for the 3D endoluminal display. A consequence may be that less perceptive errors are made by the reviewer.

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