Virtual
Colonoscopy...At the Forefront of Colorectal Cancer Screening
There is clear evidence that death rates from colorectal cancer can be
reduced by screening asymptomatic populations for the presence of
precancerous adenomatous polyps and their subsequent removal by polypectomy.
Controversy arises because there are multiple different screening tests
available, including fecal occult blood testing, flexible sigmoidoscopy,
double contrast barium enema, and colonoscopy. All of these tests have
advantages and disadvantages, but none are ideal. The proponents disagree
among themselves as to the optimal timing, test intervals, and strategy.
The public has been generally unwilling to accept the concept of colon
screening, as evidenced by the very low participation level when offered
colorectal cancer screening of any type. For this reason, the recent
introduction of virtual colonoscopy, a new CT- scan based imaging technique
for screening the colon, has generated wide interest.
Virtual colonoscopy is done as a 10-minute CT scan, with no injections,
sedatives, or risk of dangerous complications. A small catheter is inserted
into the rectum to pump in air (until colonic gas cramps occur) in order to
improve the visualization of polyps. After the scan, the patient is released
to return home or to work with essentially no after-effects. The only
disadvantage of the study is that a laxative preparation is still required.
Virtual colonoscopy provides complete examination of the colon,
from the rectum to the lower small bowel, and is able to show
important polyps (
> 1 cm) with an accuracy equal to
that of colonoscopy. Very small polyps are not as accurately demonstrated,
but these are clinically irrelevant in the vast majority of patients,
and overlooking them is probably of minor consequence. Virtual
colonoscopy has the additional advantage in that the technique
is able to bypass areas of narrowing or stricture in the colon
in order to demonstrate more proximal abnormalities.
The benefit of virtual colonoscopy is that it provides accurate information
about the status of the colon noninvasively, which allows patients and their
physicians to make an informed choice as to whether any more invasive
procedures, such as conventional colonoscopy, are required. Because the
likelihood that the average adult at age 50 years or greater will have a
large or "significant" polyp is no more than 10%, the chance that any
individual patient will have to return for a colonoscopy to remove such a
lesion is only about 1 in 10. If a follow-up colonoscopy is required, then
it will be very likely that colonoscopy will have a definite therapeutic
benefit rather than being a search and destroy mission, which may do the
patient very little real good. By the same token, a negative result at
virtual colonoscopy would give the patient a high level of reassurance that
there is no lurking colonic tumor, i.e., 95% negative predictive value.
At present, virtual colonoscopy is not reimbursed by Medicare for screening
asymptomatic patients. However, the technique is increasingly available
throughout the US in large urban centers and is disseminating quickly into
community practice. Many private insurance companies are already
reimbursing for virtual colonoscopy when it is performed for specific
reasons, such as a failed conventional colonoscopy. Average charges for
virtual colonoscopy are approximately $1,000, about one-half the current
cost of conventional colonoscopy.
There is currently intense research regarding virtual colonoscopy
on the part of both university investigators and commercial vendors
of imaging equipment and software. New rapid CT scanning techniques
and computer-assisted interpretation methods promise faster scan
times and quicker and more accurate readings. Specialized dietary
kits can avoid the requirement to drink unpleasant laxative regimens
the evening before the study. This so-called "prepless" innovation
has the potential of boosting virtual colonoscopy to the forefront
of colorectal cancer screening techniques.
Sources:
Morrin MM, Farrell RJ, Kruskal JB, LaMont JT. Virtual colonoscopy: a
kinder, gentler colorectal cancer screening test?
Lancet.
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Fenlon HM, Nunes DP, Schroy PC 3rd, Barish MA, Clarke PD, Ferrucci
JT. A comparison of virtual and conventional colonoscopy for the
detection of colorectal polyps.
N Engl J Med. 1999;341:1496-503.
Ferrucci JT. Colon cancer screening with virtual colonoscopy:
promise, polyps, politics.
Am J Roentgenol. 2001;177:975-88.
Dachman AH. Diagnostic performance of virtual colonoscopy.
Abdom
Imaging. 2002;27:260-7.
Gluecker TM, Fletcher JG. CT colonography (virtual colonoscopy) for
the detection of colorectal polyps and neoplasms. Current status and
future developments.
Eur J Cancer. 2002;38:2070-8.
