Differential diagnosis of polypoid lesions seen at CT colonography (virtual colonoscopy).

Computed tomographic (CT) colonography, also referred to as virtual colonoscopy, holds significant promise for effective large-scale colorectal cancer screening. Two-dimensional (2D) and three-dimensional (3D) displays of the CT data are employed, both of which are critical for proper evaluation. Although many radiologists continue to use the 2D images for polyp detection, more emphasis on the 3D images for primary detection of polyps has yielded the best results for screening detection. The primary target lesion for colorectal screening is the adenomatous polyp, since detection and removal of all larger or advanced lesions could potentially prevent approximately 95% or more of all colon cancers. Frankly invasive adenocarcinoma is rarely encountered in an average-risk asymptomatic screening population, but it is of course another important target. In addition to these clinically significant epithelial neoplasms, however, a host of additional lesions and pseudolesions may be encountered that appear polypoid at CT colonography. A subset of "don't touch" lesions, which should not be confused with potential neoplasms, can also be recognized at CT colonography. A variety of useful techniques and observations can be used to increase the specificity of CT colonography for distinguishing false polyps from true polyps. (c) RSNA, 2004.