Computed tomography colonography (CTC) is a minimally invasive method of examining the colon and rectum. It requires bowel preparation and the oral administration of faecal tagging agents prior to the insertion of a rectal tube, which is used to inflate the colon with carbon dioxide. A low-dose CT scan is then performed in two positions, comprising a supine scan and then either a prone or lateral decubitus study. Advanced post-processing techniques and dedicated imaging software enable the colon to be examined in both a multi-planar two-dimensional and a three-dimensional ‘virtual colonoscopy’ mode which simulates traditional endoscopic views. The procedure is well tolerated, does not require sedation and is extremely safe, with a perforation rate of 0.04%, the vast majority of which are asymptomatic and managed conservatively. CT colonographyAlso known as virtual colonoscopy. can be performed immediately following a simple polypectomy but should be delayed in patients who have undergone complex endoscopic intervention as this increases the risk of perforation. Likewise, CTC should be avoided in patients with active colitis or obstructing strictures.
Overview of evidence (non-systematic literature review)
No systematic reviews were undertaken for this topic. Practice points were based on selected evidence and guidelines (see Guideline development process).
Polyp detection rates
In a study with over 1200 patients comparing same-day CTC with segmentally unblinded optical colonoscopy (OC), CTC had a sensitivity of 94% for the detection of polyps over 10mm, performing as well as OC. The high sensitivity of CTC for the detection of colorectal cancer (CRC) has been confirmed in a subsequent meta-analysis involving 49 studies and 11,151 patients.
The sensitivity of CTC for the detection of polyps 6-9 mm is variable, with one meta-analysis reporting a sensitivity of 59% for these diminutive lesions. A limitation of this analysis is that many of the included studies were published in 2005 or before, with some dating back to 1997, and therefore the data do not take account of technological advances in hardware and software, improved reader training, and faecal tagging which are routinely used today.
The natural history of polyps measuring 6–9mm is yet to be fully defined. Radiologists do not report polyps that are less than 6mm, as the overwhelming majority of these do not harbour advanced histology.Back to top
Interval cancerA cancer whose presence is diagnosed in the time between scheduled screening tests or surveillance procedures. rates
The interval cancerA cancer whose presence is diagnosed in the time between scheduled screening tests or surveillance procedures. rates following a negative CTC are low and in one study involving 1050 patients with a negative CTC and follow-up average of 4.7 years found one interval cancerA cancer whose presence is diagnosed in the time between scheduled screening tests or surveillance procedures. while another study with 1429 patients with negative CTC and mean follow-up of 5.7 years found two interval cancers, one occurring 5 years post CTC and the other 10 years post initial CTC. Reader training and experience is vital to maintain the high accuracy of CTC and the low interval cancerA cancer whose presence is diagnosed in the time between scheduled screening tests or surveillance procedures. rate, so CTC should only be reported by radiologists who are accredited for CTC interpretation by the Royal Australian and New Zealand College of Radiologists (RANZCR).
Radiation dose and cancer risk
CTC requires the use of ionising radiation which carries a risk of producing radiation induced malignancy. The inherently high contrast between the gas containing gut lumen and soft tissue colonic wall allows for a low dose CT to be performed without reducing the sensitivity of the examination. Typical radiation doses for CTC are 5mSv or less, while the use of modern iterative reconstructive methods is allowing the dose to fall as low as 1 mSv which is less than half of the annual natural background radiation dose. Modelling of CTC every 5 years between the ages of 50 and 80 years, and using a relatively high dose of 7–8 mSv would prevent between 24 and 35 CRCs for every radiation-induced malignancy. The radiation dose of CTC is significantly lower than the dose acquired during inferior tests such as barium enema.
CTC examines not only the colonic mucosa but also the contents of the abdominal and pelvic cavities, the spine and lung bases. Hence extracolonic findings are frequently encountered, the vast majority of which can be accurately characterised as benign and of no clinical significance. The rates of potentially important findings, such as extracolonic malignancy and vascular aneurysms, varies and is up to 16% depending upon the definition used, the CTC technique and the population being studied. The diagnosis of these conditions has potential benefit to patients, but may require further investigations.
Due to its excellent safety profile and high accuracy for detecting colonic carcinoma, CT colonographyAlso known as virtual colonoscopy. is an alternative for patients unable to have colonoscopy. Bowel preparation is still required prior to the examination.
In patients at risk of colorectal carcinoma who have had an incomplete colonoscopy, CT colonographyAlso known as virtual colonoscopy. should be performed to allow assessment of the entire colonic mucosa.
It is safe to perform same-day CT colonographyAlso known as virtual colonoscopy. following incomplete colonoscopy, including in patients who have had a biopsy or simple polypectomy. However, CT colonographyAlso known as virtual colonoscopy. should be delayed in patients with complex endoscopic intervention and in patients at high risk of perforation such as active colitis or high-grade stricture.
CT colonographyAlso known as virtual colonoscopy. should only be interpreted by radiologists who have undergone specialist training and are accredited by RANZCR.
Patients with a CT colonographyAlso known as virtual colonoscopy. detected polyp over 10mm should be referred for polypectomy. Patients with polyps 6–9mm can be offered either polypectomy or repeat colonic examination at 3 years.
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