CT Colonography

CT colonography (CTC) is popularly termed "virtual colonoscopy."  This technology combines multiple helical CT scans to create two- or three- dimensional images showing the interior of the colon.  In the past, this modality was not included among recommended screening methods.  However, due to improvements in technology, the 2008 ACS guideline includes CTC as an acceptable option.

There have been no prospective, randomized controlled trials of CTC.  Available studies suggest better sensitivity for polyps ≥ 1 cm than for smaller polyps.  Potential limitations in the use of CTC include variations in user skill, a relatively high number of false positives and the inability to remove detected polyps, leading to the need for follow-up colonoscopy.  CT colonography may be less able than colonoscopy to detect flat adenomas, which appear to be less common but more aggressive than adenomatous polyps. (17)

Performing the Test

Virtual colonscopy requires bowel preparation similar to what is needed for a colonoscopy.  Before the CT image is taken, air is introduced into the colon through a rectal catheter.  Each scan is done during a single breath hold; scans must be done in both the supine and prone positions.  The test itself takes about 10 minutes.  CTC does not require sedation and recovery time is minimal.

Risks of the Test

Risk of colonic perforation with CTC appears to be low in the general population, with one study recording only one symptomatic perforation in almost 22,000 procedures.  In symptomatic patients the risk may be higher, with reported rates ranging from 0.03% to 0.06%. (2)

The level of radiation involved in CTC has been examined for its potential to contribute to colon cancer risk.  In a 50-year-old subject, the radiation dose may add 0.044% to the lifetime risk.  Organ radiosensitivity decreases with age, so this estimate is lower in older patients.  Risk can be reduced with low-dose protocols. (2)

Another concern is the risk of incidental findings outside the colon, which can require additional testing.

Interpreting the Test

Currently, ACS suggests colonoscopy for patients who are found to have one or more polyps ≥ 6mm.  For patients who can not have or do not want colonoscopy, surveillance with CTC appears to be an acceptable option.  However, research on appropriate follow-up is limited.  There is no strong recommendation for screening interval in patients with negative findings, but a 5-year interval is offered as reasonable in the ACS guideline.

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Module III - Table of Contents Page 5