Virtual
Colonoscopy...Is Not Yet an Acceptable Option for Colorectal Cancer
Screening
Evidence-based guidelines recommend that all asymptomatic, average-risk men and
women be offered screening for colorectal cancer beginning at age 50 using one
of five options: annual fecal occult blood testing; flexible sigmoidoscopy
every five years; the combination of fecal occult blood testing and flexible
sigmoidoscopy; double-contrast barium enema every five years; or colonoscopy
every 10 years.
Most gastroenterologists prefer the option of direct screening with colonoscopy
because it is the most accurate way of detecting early cancers and premalignant
polyps, and infrequent screening is effective. However, the resources and
capacity to conduct colonoscopy screening of the entire average-risk population
are currently lacking. Moreover, it is not certain that colonoscopy will be
acceptable to all candidates for screening.
Virtual colonoscopy (CT colonography) is a promising new technique that might
bolster our screening capacity and increase overall screening compliance.
According to the guidelines, in order for a new test to replace any of the
established screening options, it first should be shown to be as safe,
acceptable, available, effective, and cost-effective as the method it is
replacing.
Virtual colonoscopy already has been shown to be more accurate than
double-contrast barium enema for detecting colorectal polyps. In addition,
some, but not all, studies indicate that this method is nearly as accurate as
colonoscopy for detecting advanced (
> 1 cm) polypoid adenomas, although accuracy
rapidly drops off for smaller polyps. Virtual colonoscopy has several obvious
advantages over conventional colonoscopy. Examination time is shorter and there
is no need for IV conscious sedation with its attendant cost and complications.
The procedure has little risk, allows scrutiny of both sides of the bowel wall
and of bowel folds, and precisely localizes lesions. It can examine the
proximal colon before surgery when a distal obstructing cancer prevents passage
of a colonoscope.
Disadvantages of virtual colonoscopy include the need for a very thorough bowel
cleansing preparation and for a somewhat disagreeable gas distention of the
colon. Colonic spasm or retained stool or fluid substantially interferes with
the interpretation of findings. Many centers report a long learning curve to
set up and read these complex scans, and reading requires appreciable expensive
radiologist time. Several studies report a low accuracy for detecting flat
sessile lesions, and there are many false-positive scans. A final obvious major
limitation of virtual colonoscopy is that it is diagnostic only. Whenever a
clinically significant neoplasm is found, the patient must undergo a colonoscopy
to biopsy or resect the lesion. This follow-up endoscopy usually must be
scheduled on a different day and the patient therefore must undergo a second
bowel preparation.
The published sensitivity of virtual colonoscopy for detecting advanced
adenomatous polyps (> 1 cm) in three experienced US centers was 75.2% to 91%.
However, not all centers that are currently performing virtual colonoscopy can
achieve this level of accuracy. For example, a recent multicenter US study
reported that the sensitivity for detecting 1 cm polyps in over 500 patients in
nine centers ranged from about 8% to 83%. A Danish study of 96 patients who had
206 polyps and five cancers reported sensitivity for detecting polyps >1 cm of
only 77%, and one flat sessile cancer was missed. In the best US studies, the
sensitivity of virtual colonoscopy for detecting medium-size polyps (5 to 10 mm)
was only 47.2% to 82%.
Radiologists correctly emphasize that an imaging method that detects larger,
advanced polyps, but misses some smaller ones, might, nevertheless, be an
effective screening option. Most small adenomatous polyps are clinically
unimportant tubular adenomas that likely will never develop the additional
acquired genetic alterations that cause them to grow and become malignant. Most
gastroenterologists now agree that missing diminutive polyps - 5 mm in size - has
very little clinical importance.
Currently, however, the controversial area has to do with adenomas of
intermediate size (5 to 9 mm). Such polyps pose a low cancer risk in the short
run; however, most clinicians and many patients may not be willing to have such
lesions missed unless they know that repeat screening will be carried out within
3 to 5 years. Increasing the frequency of screening, however, greatly increases
the cost of a screening option, and may not allow it to compete with the option
of performing direct colonoscopy screening every 10 years.
Current charges for screening virtual colonoscopy are similar to those of an
abdominal/pelvic CT scan. However, if the indication for an examination is to
screen for colorectal neoplasia, additional colonoscopies will be needed in 10%
to 20% of patients to assess findings or to resect polyps. In these cases, a
more cost-effective approach may be to do an initial colonoscopy, that is both
diagnostic and therapeutic, in a single sitting with a single bowel preparation.
To compete with direct colonoscopy screening, the price of a screening virtual
colonoscopy would have to drop substantially below that of conventional
colonoscopy in order not to increase dramatically the overall cost of a
screening program. Expected advances in automated reading of virtual
colonoscopy scans performed with super rapid CT scanners may bring the cost down
to a level that would allow it to compete with other established screening
options.
A recent cost-effectiveness analysis underscores this point. Sonnenberg et al
used a Markov mathematical model to compare the cost-effectiveness of screening
virtual colonoscopy with that of conventional colonoscopy in the US. Even when
the authors assumed 100% sensitivity and specificity for virtual colonoscopy,
conventional colonoscopy was more cost-effective. Only when the cost of virtual
colonoscopy was assumed to be < 55% of that of colonoscopy, or the compliance
rate was assumed to be 15% to 20% higher, did virtual colonoscopy become the
more cost-effective option.
It is not yet known if the at-risk public will prefer virtual colonoscopy over
other screening options. Many are attracted to the concept of a "virtual"
examination that does not require intubation of the colon. However, when they
learn that they first must undergo a vigorous cathartic prep and then be
subjected to rectal instillation of gas, their acceptance of virtual colonoscopy
decreases. Akerkar et al conducted a satisfaction survey of 295 patients who
had both virtual and conventional colonoscopy. Both immediately after the
procedure, and 24 hours later, these patients rated colonoscopy superior with
regard to pain, discomfort, and personal embarrassment. When asked if they
would be willing to repeat either test, they indicated a preference for
conventional colonoscopy. In contrast, a more recently reported study of 45
patients indicated that patients preferred virtual colonoscopy over conventional
colonoscopy because the latter caused more discomfort. However, when informed
that they would need a colonoscopy anyway if a virtual colonoscopy was positive,
77% stated they would choose direct colonoscopy for future screening.
These observations show that for virtual colonoscopy to play a major role in
population-based screening, better-tolerated methods of bowel cleansing and gas
distention need to be developed. If current efforts to find a way to do virtual
colonoscopy without the need for a cathartic preparation-by tagging stool with
oral contrast so that the computer can separate retained luminal contents from
tissue (a "virtual preparation")-- virtual colonoscopy likely would become the
preferred screening option for many people.
In summary, although virtual colonoscopy appears to have a promising future, it
is not yet sufficiently accurate, available, acceptable, or cost-effective to be
promoted as a population-based screening option for average-risk patients.
Further refinement is needed in order to improve its accuracy for detecting
medium-size and advanced adenomas and flat sessile lesions. Automated
computerized reading should reduce the need for expensive radiologist time and
allow virtual colonoscopy to compete cost-wise with other established options.
Better-tolerated methods of preparing the colon should make this a more
attractive choice for many patients. After these issues are satisfactorily
addressed and the procedure becomes available in more community medical centers,
the addition of virtual colonoscopy to the menu of screening options should help
improve screening compliance and favorably impact outcome from colorectal
cancer, the second most common cancer killer of Americans.
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