Phantom study suggests 2D CT colonoscopy effective for polyp characterization

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Polyp size measurement is critical in both conventional colonoscopy and CT colonoscopy as the risk of established cancer in a polyp measuring 1 cm is less than 1 percent versus 50 percent for those 2 cm or larger. Using a polyp phantom and a variety of conventional and advanced imaging techniques, British scientists have found that measurement error is encountered when the diameter of simulated polyps is estimated by colonoscopy, 2D CT display and 3D endoluminal CT display, according to an article published online in European Radiology.

“Accurate measurement of maximal polyp diameter is therefore central to patient management strategies, especially those using CT colonography, where there is no opportunity for polypectomy at the time a polyp is detected,” the authors wrote.

Researchers from the University College Hospital in London conducted a study to determine the accuracy of polyp diameter measurement for both colonoscopy and CT colonography using a synthetic phantom containing simulated spherical polyps of a known reference diameter.

The phantom consisted of a length of corrugated plastic tubing 3-cm in diameter in which were placed 12 simulated spherical wooden polyps of different diameters (4, 6, 8, 10, 12 and 18 mm).

The phantom was endoscoped using a standard colonoscope by two gastroenterologists blinded to one another’s measurements. The phantom was then scanned on a 64-slice CT system (Somatom 64, Siemens Medical Solutions). Data acquired during the CT scans were imported to a standalone CT colonography workstation (Viatronix 3D) and the images were interpreted by two radiologists.

“Both radiologists were asked to measure the maximal transverse diameter of each polyp identified using two different visualization methods: 3D endoluminal rendering and 2D multi-planar reformatting,” the authors wrote.

The 2D measurements were performed in one sitting, while the 3D measurements were performed the following day; the radiologists did not have access to their 2D measurements, the researchers reported.

The mean difference between observers’ measurements and the reference diameter was smallest for estimates made using 3D CT (_0.09 mm and _0.03 mm) and greatest for endoscopy (_1.10 mm and _1.19 mm), with 2D CT intermediate, according to the scientists. They noted that 95 percent limits of agreement were largest for 3D CT estimates (_4.38 mm to 4.20 mm).

“Estimates by 2D CT consistently overestimated polyp diameter, whereas endoscopy consistently underestimated diameter,” they wrote. “In contrast, measurements by 3D CT were a combination of over- and under-estimates, with a tendency for disagreement to increase with the size of the polyp.”

The researchers stated that the simulated polyps were wooden rather than soft tissue, with a density of approximately _370 Hounsfield units, which may have affected the perceived diameter. Also, the phantom was examined in air rather than submerging it in fluid to simulate soft-tissue attenuation.

In addition, they observed that all CT measurements were made using a single software platform and endoscopy measurements were made using the same endoscopic apparatus, so that the effect of different systems was not investigated.

Although their data found that all methods employed to measure the simulated polyps resulted in error, they believe that 2D CT may provide the smallest margin for error.
“Overall, the narrowest limits of agreement were found for 2D CT estimates of diameter, suggesting to us that this method is the best compromise overall, especially as the measurement error introduced by this approach seems the most predictable,” they wrote.