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Virtual
colonoscopy 'back on the radar'
By Kristin Jenkins
Baltimore,
MD—
Virtual colonoscopy can have the same sensitivity as conventional
colonoscopy in the diagnosis of pre-cancerous colonic polyps, according
to a study published in the New England Journal of Medicine
in December 2003.
“This is an encouraging result in a large, well-designed trial,”
Douglas K. Rex, MD, Professor, Director of Endoscopy at IU Cancer
Centre, Indianapolis, told Gastroenterology Canada. “This
study has put virtual colonoscopy back on the radar.”
Two large studies using 2-dimensional imaging had previously found
that VC was inadequate for polyp detection. However, 3-dimensional
imaging and 16-slice CT scanning in a group of 1,233 asymptomatic
patients in a low prevalence population has produced results “very
favourable for virtual colonoscopy,” said Dr. Rex. “The
software relies on 3-dimensional images so it’s the same kind
of image, moving through the lumen looking for polyps, as you get
on conventional colonoscopy.”
In radiology literature, VC is usually referred to as CT or MR colonography.
However, the term virtual colonoscopy has been kept because of its’
patient appeal, Dr. Rex said at the annual meeting of the American
College of Gastroenterology.
At present, no multidisciplinary guideline group in the U.S. has
endorsed the use of VC for colorectal cancer screening and the technology
has been specifically rejected for this purpose by the American
Cancer Society, the AGA Consortium and the U.S. Preventive Services
Task Force. The American College of Radiology has declined to take
a position on VC.
It has been difficult to assess the effectiveness of the technology
as a population screening tool. While some groups have achieved
acceptable results in high-prevalence populations, others have not,
Dr. Rex said. Results from a Mayo Clinic study of 700 patients with
a low prevalence of disease, for instance, showed that in those
with large polyps, sensitivities using VC were between 32 per cent
and 73 per cent, which is comparable to double contrast barium enema.
Also, low sensitivity would require examinations to be repeated
at five-year intervals rather than at 10-year intervals, making
VC less cost effective than originally thought.
The potential of VC as a means of screening both the colon and the
abdomen and pelvis outside of the colon continues to be of interest,
although large studies have failed to confirm a benefit of extra
colonic screening.
One promising feature of virtual colonoscopy is the potential for
getting new patients in the door for cancer screening. At present,
the preparation for virtual colonoscopy is just as aggressive as
it is for conventional colonoscopy. However, said Dr. Rex, development
of fecal tagging methods that permit the electronic subtraction
of stool could eventually make this kind of cancer screening —
and preparation for it — more acceptable to patients. “This
is an ongoing story that we are watching with interest.”
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