Colon Cancer Screening: Virtual Colonoscopy / CT Colonography

Terminology:

Computed tomographic (CT) scanning of the colon for masses is often referred to as Virtual Colonoscopy. It is a reasonable, but incomplete and potentially confusing term. The three dimensional rendering and viewing of the inside of the colon by CT (the part that looks like a fiberoptic colonoscopy, thus “virtual colonoscopy”) is only one of several ways the colon is evaluated as part of a CT colon study. The preferred term currently is CT Colonography. This also helps avoid confusion with fiberoptic colonoscopy when discussing or ordering these studies

What is CT colonography?:

CT colonography is a high-speed CT study that captures images of the air-distended colon. The images are acquired as a volume that can be viewed as two- dimensional slices and three-dimensional (colonoscopy-like) images. These images are processed and viewed at a specialized computer workstation that allows real-time interactive evaluation for colon masses.

How is it performed?:

  • Patients undergo a two-day colon preparation similar to that for a barium enema
  • The may ingest a small amount of water-soluble oral contrast to tag fluid
  • The patient is interviewed to ensure that the prep has been followed and that the fecal stream is clear
  • The patent lies on the CT table and small tube is placed in the rectum
  • Air is placed through the tube to distend the colon.
  • A “scout” image is taken to ensure adequate distension
  • The patient is scanned once lying on their back and once lying on their stomach
  • No needle stick (IV) needed

How is the study interpreted?:

The studies are analyzed on a state-of- the-art Vital Images workstation by a radiologist specialized in body imaging. These studies generate nearly a thousand images to be reviewed in several different formats and several radiologists may review the images, thus interpretation often takes longer than a typical radiology study. In addition, as we build our local experience, we are currently having some studies overread by an outside consultant. Thus final results may not be available for up to one to two weeks. Please plan patient follow up appointments accordingly. If more immediate results are needed a preliminary report can generally be issued.

What are the advantages and disadvantages of CT colonography compared to fiberoptic colonoscopy?

Advantages:

  • No sedation
  • Minimal Risk
  • Short procedure time
  • Allows examination above a tight stricture
  • Lower volume bowel prep
  • Assessment of other organ systems

Disadvantages:

  • Still need some colon prep currently
  • Some (although brief) patient discomfort
  • Suboptimal prep or bowel distention can severely limit examination
  • Cannot biopsy/remove lesion (therefore follow up colonoscopy needed for significant lesions)
  • Follow up standards for small lesions not yet established
  • Currently not reimbursed for screening purposes

For whom is CT Colonography appropriate? (Indications and contraindications):

There is no “official” answer to this question. General screening guidelines, published studies on CT colonography and consultation with his or her personal physician, the radiologist, and as needed a gastroenterologist, can help determine if CT colonoscopy is a reasonable alternative for any given patient.

The American Cancer Society recommends that average risk people 50 years of age and older be screened for colon cancer by one of the following:

  • Yearly fecal occult blood test
  • Flexible sigmoidoscopy every 5 yrs (+/- yearly occult blood test)
  • Double contrast barium enema every 5 yrs
  • Colonoscopy every 10 years

Based on published studies to date, presentations from centers with experience in CT colonography and our own experience with these studies, we have a general sense of where CT colonography may fit into a screening program. Although not quantitatively proven, CT colonography can probably be considered at least a barium enema equivalent, and many with experience in the field consider CT colonography superior to barium enema for lesion sensitivity and patient comfort.

Potential Indications for CT colonography:

  • Incomplete colonoscopy
  • Low to average risk patients who prefer not to have colonoscopy
  • Frail patients, unable to tolerate colonoscopy
  • Relative contraindications*:
  • Patients whose symptoms suggest a high risk for colon cancer
  • Moderate to high risk patients
  • History of adenomatous polyps
  • Personal or significant family history of colon cancer
  • Inflammatory bowel disease

Given a relatively high prior probability of colon cancer in these patients consider starting with fiberoptic colonoscopy unless there is a compelling reason use an alternative examination

Referrals:

We do not accept patient self-referral at this time. Referrals must be made through the physician’s office. We feel that this approach is necessary to facilitate any necessary follow up care or patient counseling by the referring physician.

Payment:

Screening (asymptomatic) patients: CT colonography for asymptomatic patients is not currently covered by insurance. Patients are expected to pay for screening CT colonography at the time of service.

Diagnostic (symptomatic) patients: See note on indications above. Insurance carriers may cover some or all of the costs of the base, noncontrast CT and the multiplanar reformation charges.

References:

1: Chen SC, CT colonography: value of scanning in both the supine and prone positions.

AJR 1999;172:595-9.

2: Fenlon HM, A comparison of virtual and conventional colonoscopy for the detection of colorectal polyps. NEJM. 1999;341:1496-503.

3: Kuwayama H, Virtual endoscopy: current perspectives.J Gastroenterol. 2002;37 Suppl 13:100-5.

4: Levin B, Emerging technologies in screening for colorectal cancer: CT colonography, immunochemical fecal occult blood tests, and stool screening using molecular markers.CA Cancer J Clin. 2003;53:44-55.

5: McFarland EG, CT colonography: multiobserver diagnostic performance. Radiology. 2002 ;225:380-90.

6: Royster AP, CT colonoscopy of colorectal neoplasms: two-dimensional and three-dimensional virtual-reality techniques with colonoscopic correlation.AJR.1997;169:1237-42.

7: Svensson MH, Patient acceptance of CT colonography and conventional colonoscopy: prospective comparative study in patients with or suspected of having colorectal disease. Radiology 2002;222:337-45.