Background The natural history of small colorectal polyps is an important area for which major evidence gaps persist. We report the results of a prospective trial assessing the behavior of small (6-9 mm) colorectal polyps through in vivo growth rates at longitudinal CT colonography (CTC) evaluation. Methods In vivo CTC surveillance was performed on 306 small (6-9 mm) polyps initially detected at screening CTC in 243 consenting asymptomatic adults (mean interval, 2.3 years; range, 1-7 years). Volumetric and linear polyp measurements at initial and surveillance CTC were correlated with histologic subgroups. Histology was established in 132 lesions at post-CTC colonoscopy. The trial is registered (ClinicalTrials.gov Identifier: NCT00204867) Findings Applying a polyp volume threshold of ±20% change per year to categorize growth, 22% (68/306) of all polyps progressed, 50% (153/306) were stable, and 28% (85/306) regressed, including apparent resolution in 10% (32/306). 91% (21/23) of proven advanced adenomas progressed, compared with 37% (31/84) of proven non-advanced adenomas, and 8% (15/198) of all other lesions (p<0.0001). Odds ratio for a growing polyp at CTC surveillance to represent an advanced adenoma was 15.6 (95%CI, 7.6-31.7) compared with 6-9 mm polyps detected and removed at initial CTC screening (without surveillance). Mean polyp volume change was +77%/year for proven advanced adenomas (n=23), +16%/year for proven non-advanced adenomas (n=84), and -13%/year for all proven non-neoplastic or unresected polyps (p<0.0001). An absolute polyp volume >180 mm3 at surveillance CTC identified proven advanced neoplasia with a sensitivity of 92% (22/24), specificity of 94% (266/282), PPV of 58% (22/38), and NPV of 99% (266/268). In general, volume changes amplified small or absent linear size changes, as only sixteen 6-9 mm polyps (6%) exceeded 10 mm at follow-up. Interpretation Volumetric growth assessment of small colorectal polyps represents a powerful biomarker for determining clinical importance. Advanced adenomas demonstrate more rapid growth than non-advanced adenomas, whereas most other small polyps remain stable or regress over time. These findings may allow for less invasive surveillance strategies, reserving polypectomy for lesions that demonstrate significant growth. Ongoing research will eventually provide more information regarding the ultimate fate of unresected small polyps without significant growth.

The overall detection rate of unsuspected cancer is approximately one per 200 asymptomatic adults undergoing routine screening CT colonography, including about one invasive CRC per 500 cases and one extracolonic cancer per 300 cases. Detection and treatment at an early presymptomatic stage may have contributed to the favorable outcome.

These findings show the importance of providing CTC as an alternative screening option for CRC at our institution, which may increase CRC adherence screening rates.

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.

CT colonography has become a potential alternative technique to optical colonoscopy for the detection of colorectal polyps and cancer. While considered safer than optical colonoscopy, CT colonography is not without risk. We report a case of colonic perforation during CT colonography using automated CO(2) insufflation and present procedural changes to help minimize the adverse effects of perforation when it occurs.