CT colonography predictably overestimates colonic length and distance to polyps compared with optical colonoscopy.

OBJECTIVE: The purpose of this study was to determine the discrepancy between CT colonography (CTC) and optical colonoscopy (OC) measurements for both anus-to-cecum length and anus-to-polyps distance and then determine whether a conversion factor could be generated to equate these CTC and OC distances. MATERIALS AND METHODS: We retrospectively reviewed CTC and OC reports from patients who had undergone both procedures as part of an established protocol. The anus-to-cecum measurement recorded on a single proprietary CTC workstation was compared with the OC cecal length for each patient. Likewise, anus-to-polyp distances were compared as measured by the radiologist and endoscopist. RESULTS: Three hundred thirty-eight patients and 437 polyps were identified with complete data from both CTC and same-day OC. The average anus-to-cecum distance measured at CTC was 189 cm (range, 75-257 cm) and at OC, 108 cm (range, 65-150 cm). For polyps proximal to the splenic flexure (n = 145), the CTC anus-to-polyp measurement was on average 1.7 times that measured at OC. For left-sided polyps (n = 292), the CTC measurement was, on average, within 12 cm or 1.3 times that of the OC anus-to-polyp measurement. All the differences between CTC and OC measurements of cecal length and polyp distances were found to be statistically significant using a paired Student's t test of means (p < 0.001). CONCLUSION: Anus-to-cecum and anus-to-polyp distances are disparate but comparable using a conversion factor of 0.57 for the CTC anus-to-cecum measurement and 0.59 for right-sided CTC anus-to-polyp or 0.78 for left-sided CTC anus-to-polyp measurements. These anus-to-polyp conversion factors could potentially augment current CTC guidelines for accurate and precise polyp localization and removal at endoscopy.