Virtual colonoscopy includes CT Colonography (CTC) and MR (magnetic resonance) colonography. MR colonography is very much in the developmental phase and is not available in Australia and therefore, will not be discussed here.
Technique: CTC involves
a) bowel cleansing,
b) air or CO2 insufflation of the co- lon,
c) CT scanning and
d) image processing and interpretation.
Indications: CTC is indicated :a) when conventional colonoscopy is incomplete as rarely happens with very long and redundant colons, malrotation of the caecum and extreme adhesions , b) in patients with severe premorbid conditions e.g.,elderly with unstable angina or recent stroke precluding sedation, and c) in patients with obstructing tumour preventing progression of the colonoscope when bowel proximal to the tumour requires evaluation prior to surgery.
Contraindications: The American cancer society, the US Multi-society Task Force on colorectal cancer and the US Preventative Services Task Force do not recommend CTC for colon cancer screening given lack of evidence (1-4). American Society of Gastrointestinal Endoscopy does not recommend the use of CTC for the evaluation of the lower bowel symptoms also because of the lack of data supporting its usefulness (5). Conventional colonoscopy remains the gold standard in these situations.
Efficacy of CTC: I have reviewed 15 original publications (references provided on request). Sensitivity (true positives) and specificity (true negatives) of polyp detec- tion by CTC decreases with decreasing size of the polyp. For polyps 10 mm or larg- er, sensitivity varies from 50 to 93% and specificity from 57 to 93%. For polyps 5 to 9mm in size, sensitivity varies from 16 to 82% and specificity from 16 to 92%.Thus CTC does not reach anywhere near the sensitivity (97-87%) and specificity (93-97%) of conventional colonoscopy. I have so far been asked to colonoscope 6 patients who were reported to have polyps on CTC. On colonoscopy, none had polyps found in the anatomical part of the colon specified in the CTC yet all had polyps elsewhere in their bowel. Thus all 6 CTCs were both false positive and false negative. One hopes that with time , the learning curve for the interpretation of CTC will improve. Also use of technologies such as faecal tagging may reduce the incidence of false posi- tives for polyps.
Advantages of CTC: CT colonography does not require intravenous sedation and thus an advantage in seriously frail and sick elderly patients. However, lack of sedation is a handicap for majority of patients who find the process of per anal air insufflation undignified.
Disadvantages: Clearly CTC does not lend itself to removal of polyps , biopsy of tissue or cautery of bleeding points etc as in the conventional colonoscopy.
Risks: Contrary to popular belief, CT colonography is not complication free. Perfo- ration of colon with air insufflation has been reported although likely to be less than conventional colonoscopy. Radiation risks are also there.
Cost effectiveness: Models created in the US indicate that CTC is less
Contrary to popular belief, CT colonography is not complication free. Perfo- ration of colon with air insufflation has been reported although likely to be less than conventional colonoscopy. Radiation risks are also there.
Cost effectiveness: Models created in the US indicate that CTC is less cost-effective than conventional colonoscopy (6). In order for CTC to be cost- effective initial compliance needs to be 15-20% more than that of conventional colonoscopy or the cost of CTC needs to be 54% lower than that of conventional colonoscopy. False positive CTC and incidental discovery of other anomalies lead to additional investigations leading to further costs(7).
Summary: There is no data to date nor recommendations from expert organiza- tions worldwide to support use of CTC for colorectal cancer screening, polyp surveillance or investigation of abdominal symptoms. For these indications , conventional colonoscopy remains the gold standard. The only globally ac- cepted indications for CTC today are incomplete colonoscopy, colon obstructed by tumour preventing passage of the colonoscope and pa- tients with severe premorbid conditions precluding intravenous sedation.