CT Colonography and Non-Polypoid Colorectal Neoplasms

Computed tomographic colonography (CTC) has been reported to be as effective as optical colonoscopy in the detection of significant adenomas. However, there are widely conflicting performance data in relation to detection of flat neoplasia. This article describes the potential and limitations of CTC and computer-aided diagnosis in the detection of flat neoplasms.

Computed tomographic colonography (CTC) is a relatively new development for examining the large intestine. Some comparison with colonoscopy has suggested that CTC is as effective as optical colonoscopy in detection of significant adenomas. However, widely conflicting performance data and critical expert commentaries have questioned whether CTC can detect flat neoplasia. The advent of CTC, particularly when performed using the latest methods for optimizing distension and bowel preparation such as automated colonic insufflators and fecal tagging, provides a potentially accurate technique for detection and characterization of flat polyps.

What is a flat polyp?

Histologically, when the height of the polyp is more than double the thickness of adjacent mucosa, it is frequently described as a flat polyp. However, this definition applies only to the operative specimen and is of no benefit to colonoscopists or radiologists. The most common endoscopic definition of a flat polyp is a polyp whose height is no more than half its diameter and this has also been used for CTC. In 2000, Paris workshop participants attempted to achieve consensus for endoscopy by proposing that closed biopsy forceps (approximately 2.5 mm in height) are placed next to the polyp to estimate its height and categorize it as polypoid (>2.5 mm) or nonpolypoid (<2.5 mm). More recently, a joint working party for CTC defined flat polyps as those with less than 3 mm of vertical elevation above the surrounding colonic mucosa.

In day-to-day clinical practice, for CTC, any lesion that protrudes from the surrounding mucosa or results in focal thickening of the colonic wall, for example, an excavated lesion, is potentially visible (l, lla, IIa+llc) ( Figs. 1–4 ). In contrast, truly flat lesions (IIb) or minimally depressed lesions (IIc) do not protrude and therefore are not generally detected by CTC. Fortunately, llb and IIc lesions are rare, probably accounting for less than 3% of all neoplastic polyps.