A new study found that virtual colonoscopy using non invasive X-ray based computed tomography (CT scanning) was 90 per cent effective at finding large pre-cancerous and cancerous growths, but not very effective at finding much smaller growths.
The study was the work of a large team of scientists from research centres in the US and Canada, including the Mayo Clinic Arizona, where first author Dr C. Daniel Johnson is based. The study is published in the 18th September issue of the New England Journal of Medicine.
The purpose of the study was to establish the accuracy of CT colonography (also known as virtual colonoscopy) in adults, using optical colonoscopy and biopsy as the reference point.
For the study 2,600 participants aged 50 and over with no symptoms who were seen at 15 different centers underwent virtual colonscopy using CT scans.
The researchers reported using standard methods for bowel preparation, “stool and fluid tagging, mechanical insufflation, and multidetector-row CT scanners (with 16 or more rows)”.
Trained radiologists examined the results and pointed out all lesions that were 5 mm or larger in diameter.
97 per cent of the participants also underwent invasive optical colonscopy with biopsy follow up (this is where the surgeon inserts a tube in the patient’s rectum and explores the colon with a small camera, removing tissue samples of any growths or polyps for lab testing afterwards). This was the “reference standard” that the CT colonography results were compared to.
The main outcome measure was how well the CT colonography detected large cancerous and precancerous growths (10 mm or larger in diameter) compared to those confirmed by optical colonoscopy and biopsy. The researchers also analysed detection rates of smaller growths (6 to 9 mm in diameter).
The results showed that the sensitivity of CT colonography compared with optical colonoscopy with biopsy follow up was 90 per cent. This means 1 in 10 large growths was not detected with CT colonography. Results for smaller lesions were less accurate, with sensitivity as low as 65 per cent for 5 mm diameter growths.
The authors concluded that:
“In this study of asymptomatic adults, CT colonographic screening identified 90% of subjects with adenomas or cancers measuring 10 mm or more in diameter. These findings augment published data on the role of CT colonography in screening patients with an average risk of colorectal cancer.”
In an acompanying editorial Dr Robert Fletcher, a retired Harvard Medical School professor noted that 1 in 4 patients that the CT scan method suggested had a growth actually had one confirmed by the optical colonoscopy and biopsy. He also pointed out that because the CT scan looks outside the colon too, it could detect abnormalities in other parts of the bowel, and while these could be real and life threatening, given the high rate of false alarms it could also lead to unnecessary stress and expense in the follow up.
Fletcher also said that the X-ray method was not so good at detecting growths that lay flat against the colon wall, and these could be more cancerous than polyps that stick out. Another potential drawback could be that because virtual colonscopy is recommended every 5 years (compared to optical colonoscopy’s 10 years), this means patients would have cumulative X-ray exposure, which although not as strong as conventional CT scans, the effects are unknown.
Fletcher concluded that these potential drawbacks do not rule out virtual colonoscopy as a potential screening test but they should be taken into account.
The American Gastroenterological Association (AGA) commented that despite these findings they consider optical colonoscopy to be “the definitive test for colorectal cancer screening and prevention. Colonoscopy is the only test that can both detect cancer at an early curable stage and prevent cancer by removing pre-cancerous polyps.”
The AGA’s concerns about virtual colonoscopy are:
- Does CT colonography find all possible cancers? While the detection rate for large growths was 90 per cent, the rate for small and flat growths was much poorer.
- Will the detection rate be the same in all settings? The study investigators used highly trained radiographers, the skill of the radiographer is very important to achieving high accuracy.
- Do patients fully understand the pros and cons of available colorectal screening tests? They may not realize that while the CT based virtual colonoscopy is “non invasive”, the bowel preparation is the same, and if a polyp is found then there is still a need for a full colonoscopy and biopsy and then removal.
- The radiation risks are unknown, especially to DNA damage.
“Patients also need to understand that for most people colorectal cancer screening is not a one-time event and that interval examinations are recommended,” said the AGA.
“Accuracy of CT Colonography for Detection of Large Adenomas and Cancers.”
Johnson, C. Daniel, Chen, Mei-Hsiu, Toledano, Alicia Y., Heiken, Jay P., Dachman, Abraham, Kuo, Mark D., Menias, Christine O., Siewert, Betina, Cheema, Jugesh I., Obregon, Richard G., Fidler, Jeff L., Zimmerman, Peter, Horton, Karen M., Coakley, Kevin, Iyer, Revathy B., Hara, Amy K., Halvorsen, Robert A., Jr., Casola, Giovanna, Yee, Judy, Herman, Benjamin A., Burgart, Lawrence J., Limburg, Paul J.
N Engl J Med September 18, 2008, Vol 359, No 12, pages 1207-1217.
Source: NEJM, AGA, Associated Press.
Written by: Catharine Paddock, PhD