Click a topic to jump down the page.
 Introduction
to Virtual Colonoscopy (CT Colonography)
What is the test?
What is the rationale for the test?
How
do I schedule an exam for a patient?
What
are the current
practice guidelines?
What
are the inclusion and exclusion criteria?
What
are some of the current research questions under investigation?
Conclusion
Introduction to Virtual Colonoscopy (CT
Colonography)
CT colonography (virtual colonoscopy) is a promising new method
for detecting colorectal polyps and cancers. Air or carbon dioxide
is insufflated into a cleansed colon in order to acquire high resolution,
thinly collimated helical CT slices. The slices are then examined,
in conjunction with three-dimensional reconstructions of the colon,
for the presence of polyps or tumors. Research at BWH, MGH, UCSF,
the Mayo Clinic and elsewhere has shown that CT colonography is
nearly or as accurate in detecting polyps as conventional endoscopic
colonoscopy, the gold standard for colorectal screening, and more
accurate than barium enema, formerly the leading alternative to
endoscopic colonoscopy. In addition, many patients report that
the CT colonography technique is more comfortable than either endoscopic
colonoscopy or barium enema. CT colonography is therefore best
suited and highly recommended for those patients who are unable
or unwilling to undergo conventional colonoscopy.
What is the test?
CT colonography is an imaging test that makes use of the principles
of computed tomography to produce high quality cross-sectional
images of the colon and rectum. The test is performed in both
the supine and prone positions while air or carbon dioxide is
insufflated into the colon. The entire exam takes approximately
ten minutes, and the radiation exposure is approximately forty
per cent of the exposure for a typical CT exam.
Prior to the exam, patients must follow a bowel cleansing regimen
to remove stool from the colon. Stool has a similar appearance
to polyps on CT and may make diagnosis difficult or impossible.
In the day immediately preceding the exam, patients should consume
only clear liquids, tea, coffee, fruit juice, jello, or popsicles.
Patients will also be asked to ingest an over-the-counter laxative
that helps cleanse the bowel. Specific preparatory instructions
will be provided. Strict adherence to the purgative regimen is
important to ensure a high-quality exam.
The cost of colonography is approximately $1300. Currently, it
is not covered by most health insurance plans. The bill will be
submitted to the insurer if the patient so wishes, but the patient
will also be asked to sign a waiver assuming responsibility for
payment.
What is the rationale for the test?
Colorectal cancer (CRC) is the second most common cause of cancer-related
death in the United States. Each year, 65,000 people in the United
States die from the disease, while 155,000 new cases are diagnosed.
Multiple studies demonstrate that CRC usually develops from a
precursor lesion, most commonly a form of growth known as a adematous
polyp. Early detection of CRC improves survival dramatically.
When detected in its earliest stage, CRC has a five-year survival
rate of 85-100%, while in later stages the survival rate drops
to 50% or lower. Furthermore, detection of precancerous polyps
may prevent cancer from developing completely.
For those reasons, colorectal screening is strongly recommended
for all individuals over the age of fifty, particularly those with
a family history of colorectal cancer. However, many patients are
unable or unwilling to undergo endoscopic colonoscopy, and for
those patients, CT colonography is often appropriate. Several studies
have evaluated the accuracy of colonography screening in symptomatic
patient groups or patients with known risk factors for colorectal
cancer. Those studies have found that colonography has a sensitivity
of 75-91% and specificity of 90+% in comparison to conventional
colonoscopy for detection of polyps that pose a greater than 1%
chance of becoming cancerous (i.e. those > 1 cm in size).
How do I schedule an exam for a patient?
The Brigham does not allow patients to schedule exams directly. If you have
a patient who wishes to undergo virtual colonoscopy, please have your office
call to schedule the exam. The scheduling section
has additional information, as well as order forms and
other useful documents.
As the scheduling section explains, there are two types of CT
colonography. Screening colonography is performed on asymptomatic
patients. It is not covered by most health insurance plans. The
patient must pay for the procedure. Diagnostic colonography is
performed on symptomatic patients. It is a combination of a CT
of the abdomen and pelvis and a review of the colon for polyps.
It is reimbursable if the patient has symptoms that Medicare deems
acceptable justification for a CT of the abdomen and pelvis on
the same day.
What are the current practice guidelines?
It is important to understand the role of CT colonography in screening
for colorectal cancer. CT colonography is an alternative to,
not substitute for, conventional colonoscopy. It is appropriate
for individuals unwilling to undergo colonoscopy and patients
who, for a number of reasons, cannot or should not undergo colonoscopy,
as well as for those people in whom colonoscopy is attempted
but must be halted before it is finished (due to an unusually
tortuous colon, the presence of obstructing growths, strictures
in the colon, etc). As with any medical examination, the decision
to undergo CT colonography should be made in consultation with
a physician. Patients should always involve their primary care
physicians in the screening process.
What are the inclusion and exclusion
criteria?
Inclusion criteria
- Patients who are unable or unwilling to undergo conventional
colonoscopy.
- Patients with absolute contraindication to conventional colonoscopy
- Patients
with relatively contraindication to conventional colonoscopy
(cardiovascular diseases that increase risk of sedation, stricture,
stenosis,
or obstructive
cancer).
Exclusion criteria
- Active inflammation
- History of Crohn’s disease
- Lack of intact ileocecal valve
(history of right colectomy)
- Recent colectomy
- Absence of anorectum
- History of pelvic irradiation (relative
contraindication)
What are some of the current
research questions under investigation?
Although several large studies have documented the accuracy of
CTC as a method for detecting polyps in patients with symptoms
or known risk factors, its accuracy for polyp detection in an
asymptomatic screening population is still under investigation.
Several large multi-centered trials are underway to address this
question, including one sponsored by the American College of
Radiology and Radiological Society of North American investigating
the accuracy of CTC in a screening population of men and women
aged 65 or older.
Research efforts are also focused on improving test accuracy and
patient acceptance. Among the possibilities under investigation
are modifications to the CT scan protocol itself (e.g. the use
of intravenous contrast), improvements to the bowel cleansing agents,
and the use of specialized computer software to aid polyp detection.
Not surprisingly, one of the foremost barriers to patient acceptance
is the need to follow a bowel cleansing regimen prior to the test.
The possibility of modifying or eliminating the preparatory regimen
is under investigation, such as by developing computer-aided methods
to identify and mark stool prior to reading. One such method would
rely on the use of a non-toxic stool tagging agent ingested by
the patient prior to the exam.
Conclusion
The American Medical Association recommends that all individuals
aged 50 or older undergo screening for colorectal cancer every
seven years. Currently, only 20% of people in the United States
comply with that recommendation. CT colonography has the potential
to increase compliance significantly by offering an alternative
screening method for appropriate patients. Current research promises
to improve CTC further, increasing its accuracy as a screening
test and minimizing the discomfort to patients.
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