Polyp size measurement at CT colonography: what do we know and what do we need to know?

Abstract

Polyp size is a critical biomarker for clinical management. Larger polyps have a greater likelihood of being or of becoming an adenocarcinoma. To balance the referral rate for polypectomy against the risk of leaving potential cancers in situ, sizes of 6 and 10 mm are increasingly being discussed as critical thresholds for clinical decision making (immediate polypectomy versus polyp surveillance) and have been incorporated into the consensus CT Colonography Reporting and Data System (C-RADS). Polyp size measurement at optical colonoscopy, pathologic examination, and computed tomographic (CT) colonography has been studied extensively but the reported precision, accuracy, and relative sizes have been highly variable. Sizes measured at CT colonography tend to lie between those measured at optical colonoscopy and pathologic evaluation. The size measurements are subject to a variety of sources of error associated with image acquisition, display, and interpretation, such as partial volume averaging, two- versus three-dimensional displays, and observer variability. This review summarizes current best practices for polyp size measurement, describes the role of automated size measurement software, discusses how to manage the measurement uncertainties, and identifies areas requiring further research.

Figure 1:

Hypothetical graph shows polyp sizes measured at optical colonoscopy (OC), CT colonography (CTC), and pathologic evaluation compared with “true” (in vivo) size, estimated from analysis of literature. Although the literature is conflicting, CT colonographic measurements tend to lie between those of optical colonoscopy and pathologic evaluation and may be closest to “true” size as reflected in the graph. The differences, about ±2 mm, are comparable to and potentially sum with other measurement errors, such as observer variability and method of measurement (eg, 2D vs 3D CT colonography). Horizontal thick lines show lower boundaries of C-RADS C2 (6–9 mm) and C3 (10 mm and larger) categories according to the CT colonographic polyp size measurement. If C-RADS size boundaries were applied to optical colonoscopic or pathologic measurements, the boundaries would need to shift up or down, respectively, to keep the “true” polyp size constant. The C-RADS guidelines were determined to some extent by combining knowledge of size-based polyp risk gained from the optical colonoscopy and pathology literature with the ability of CT colonography to help identify polyps of clinically relevant size.

Figure 2:

Graph of risks and referral rates as a function of polyp size. Risks of high grade dysplasia (HGD) in all polyps and cancer in adenomatous or all polyps are from references (2,7,11). For comparison, test-positive rates for CT colonography are shown as reported in references (10,61). Patients with positive results at CT colonography would be referred for colonoscopy (OC). Risks are estimated from graphs or tables in the cited articles and modified to fit the format of this graph; they should be viewed as approximate. Lower risks of cancer and high-grade dysplasia by Lieberman et al are attributable to differences in patient population and denominators used for calculating risk. Note that small shifts (for example, ±2 mm) in size threshold (along x-axis) greatly affect referral rates (because the referral rate curves have steep slopes) but affect risk relatively less (because the risk curves have shallower slopes).