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Normalized Distance Along the Colon Centerline: A Method for Correlating Polyp Location on CT Colonography and Optical Colonoscopy

¹ Radiology and Imaging Sciences, National Institutes of Health Clinical Center, Bldg. 10, Rm. 1C368X, MSC 1182, Bethesda, MD 20892-1182.
² Walter Reed Army Medical Center, Washington, DC.
³ Department of Radiology, University of Wisconsin School of Medicine, Madison, WI.

OBJECTIVE. The ability to accurately locate a polyp found on CT colonography (CTC) at subsequent optical colonoscopy (OC) is an important part of the successful implementation of CTC for colorectal cancer screening. The purpose of this study was to determine whether a polyp's normalized distance along the colon centerline derived from CTC data can accurately predict its location on OC.

MATERIALS AND METHODS. The polyp population consisted of 152 polyps in 121 patients. CTC polyp findings were verified by same-day segmentally-unblinded OC. Each polyp's normalized distance along the colon centerline was computed by dividing its distance from the anorectal junction measured along the colon centerline by the length of the colon at CTC. The predicted polyp location at OC was computed by multiplying the normalized distance along the colon centerline by the colon length at OC (i.e., the distance to the cecum as determined at full colonoscope insertion). The differences between the true and predicted polyp locations at OC were compared using paired Student's t tests, linear regression, prediction interval assessment, and Bland-Altman analyses.

RESULTS. The differences between the true and predicted polyp locations at OC using the supine and prone CTC-normalized distances along the colon centerline were 2.2 ± 10.5 cm (mean ± SD; n = 136) and 1.5 ± 10.5 cm (n = 135), respectively. The predicted location was within 10 cm of its true location for 71.3% (97/136) to 74.8% (101/135) of polyps and within 20 cm of its true location for 93.3% (126/135) to 93.4% (127/136) of polyps.

CONCLUSION. By computing the normalized distance along the colon centerline of a polyp found at CTC, the location of a polyp at OC can be predicted to within 10 cm (i.e., 1 colonoscope mark) for the majority of polyps.

Keywords: colorectal cancer screening • colon polyp • CT colonography • optical colonoscopy

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