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| Colon Cancer |
| Virtual Colonoscopy |
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By Larry Prensky, MS, CGC, CCGC
Reviewed
by Miriam Komaromy, MD
Colonoscopy is the method of choice for detecting colon cancer at an early stage. But despite the procedure's accuracy, many people avoid having a colonoscopy because of it's drawbacks – it's uncomfortable, expensive, and somewhat embarrassing. Fortunately, a new screening method called virtual colonoscopy may prove to have some advantages over the traditional methods.
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Current Screening Methods
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When screening for colon cancer, a doctor looks for either cancerous masses or mushroom-shaped growths called polyps on the inside of the colon. Although polyps are not cancers, they can turn into cancer if they aren't removed. There are currently four main ways that a physician can screen for polyps or cancer. These are:
- Fecal occult blood test (FOBT), in which the doctor examines a person's stool for blood. Blood can indicate the presence polyps or cancer.
- Barium enema, in which a doctor pumps barium into the colon so any polyps or cancers will show up in an X-ray.
- Sigmoidoscopy, which uses a lighted tube to examine the lower portion of the colon.
- Colonoscopy, which uses a longer lighted tube to examine the entire colon.
Each of these methods has its advantages and disadvantages. Overall, though, colonoscopy is more accurate than FOBT, barium enema, and sigmoidoscopy in screening for cancer and catching many other abnormalities. It is considered the gold standard for colon screening, but has the disadvantage of being invasive, embarrassing, uncomfortable, and carries a risk (albeit a small one) of puncturing the colon, which can lead to internal bleeding.
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| Virtual colonoscopy is more accurate than FOBT, barium enema, and sigmoidoscopy, and is more comfortable and less embarrassing than a colonoscopy. |
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Virtual colonoscopy may combine the benefits of the four screening methods: it is more accurate than FOBT, barium enema, and sigmoidoscopy, while being more comfortable and less embarrassing than a colonoscopy. If the technique continues to do well in clinical trials, it may make regular screening for colon cancer more appealing for those who want to avoid the discomforts of "real" colonoscopy.
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How Virtual Colonoscopy Works
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| In a virtual colonoscopy, a computer assembles hundreds of pictures of the colon into a 3-dimensional image. |
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Virtual colonoscopy starts out similar to a colonoscopy, sigmoidoscopy, or barium enema. The day before the procedure, you completely clear your colon. Then a physician inserts a small tube into the anus in order to inflate the colon with air. This step makes sure that the walls of the colon are not touching each other. A computed tomographic (CT) scanner is then used to take hundreds of pictures of the inside of the colon. Each of these pictures shows only a narrow slice, but taken together the pictures combine to give a complete view of the colon. Using a computer to put the images together, a radiologist can view the colon from many different angles.
It only takes a couple of minutes to take the necessary pictures, and the results are ready for the radiologist to interpret in minutes. However, the results cannot be interpreted on the spot, unlike the results of sigmoidoscopy or colonoscopy. According to Dr. Judy Yee, Associate Professor of radiology at the University of California, San Francisco, "Virtual colonoscopy can take from 20 minutes to an hour to interpret depending upon the experience of the radiologist." However, Yee adds that this time could shorten with the help of computer-aided diagnosis.
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Advantages and Disadvantages
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Like all medical procedures, virtual colonoscopy has its advantages and disadvantages. According to Yee, the major advantages of the procedure are that it:
- Is less embarrassing and uncomfortable than other procedures.
- Doesn't require that patients be sedated.
- Presents less risk of puncturing the colon.
- Takes less time than colonoscopy.
- Allows physicians to see abnormalities outside of the colon that could affect the patient's health.
Despite these advantages, virtual colonoscopy also has some drawbacks. Most important, it only detects 60 to 90 percent of polyps compared to almost 100 percent for colonoscopy. A person would still need to have occasional colonoscopies, or have a colonoscopy to verify any abnormal finding. Virtual colonoscopy is also more expensive, and at this time few radiologists are trained to provide the technique.
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Who Will Benefit
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| Virtual colonoscopy will probably be of most benefit to people at average risk of colon cancer. |
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If virtual colonoscopy pans out in trials, Yee expects that it will have the biggest impact on those individuals who are at average risk for colon cancer and just need routine surveillance. Currently, fewer than 40 percent of people who should be having colon screening actually receive it, usually because they fear conventional colon screening. Yee doesn't expect virtual colonoscopy to replace conventional screening, but hopes that it will be used in patients who are not now being screened but should be. This would help find cancers at an earlier stage, thereby preventing cancer-related deaths in people who might not ordinarily have received screening.
People who are high risk for colon cancer are less likely to benefit from virtual colonoscopy because of its lower accuracy, Yee says. These people are often at such high risk that they need the sensitivity of colonoscopy. High-risk individuals will likely need to continue getting regular colonoscopies, at least until the accuracy of virtual colonoscopy improves.
Virtual colonoscopy is still three to four years way from being used routinely to check for colon cancer, Yee says. However, she expects it to replace colonoscopy, at least for people at average risk. Currently, the procedure is being used in only a few hospitals, such as the Mayo Clinic and Bowman Gray at Wake Forest, but it is being used in about 20 research protocols around the world. The number of centers that will be using virtual colonoscopy in a research protocol is expected to double in the near future due to collaborations between the centers.
However, Jonathan Terdiman, assistant clinical professor of medicine at the University of California, San Francisco and the clinical director of the UCSF Colorectal Cancer Prevention Program, says that the role for virtual colonoscopy is not absolutely clear. He calls the technique "promising," but says, "It is nowhere near ready."
Terdiman says that the biggest barrier to getting a colonoscopy is the preparation the day before – a step that isn't eliminated by virtual colonoscopy. Given this, he's not sure that people will prefer virtual colonoscopy to the real thing. Add to that the high cost and decreased sensitivity of virtual colonoscopy, Terdiman says that it's too soon to be touting the merits of virtual colonoscopy. "It may end up playing an important role, but how it will be implemented is totally unresolved."
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Other Up-and-Coming Screening Methods
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| There
is no single, effective screening test for ovarian
cancer. |
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Terdiman points out that virtual colonoscopy isn't the only new tool being developed for colon cancer screening. A procedure that looks for suspicious DNA in stool samples is also being tested at several clinics. This screen can detect mutated DNA that tends to occur in colon polyps. Although this screening method is not yet generally available, Terdiman calls it a "promising technology." He says that the stool test and virtual colonoscopy will both probably have a role in screening recommendations of the future. (For recent news about the stool assay, see Related News below.)
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References
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VJohnson, C.D and Ahlquist, D.A. (1999) Computed tomography colonography (virtual colonoscopy): a new method for colorectal screening. Gut. 44(3): 301-305.
Royster, A.P., et al. (1997) CT colonoscopy of colorectal neoplasms: Two-dimensional and three-dimensional virtual reality techniques with colonoscopic correlation. AJR. 169; 1237-1242.
Morrin, M.M, et al. (2000) Role of virtual computed tomographic colonography in patients with colorectal cancers and obstructing colorectal lesions. Dis Colon Rectum. 43(3); 303-311.
Johnson, C. D., et al. (1997) Computed tomographic colonography (virtual colonoscopy): a new method for detecting colorectal neoplasms. Endoscopy. 29;454-461.
McFarland, E.G. and Brink, J.A. (1999) Helical CT colonography (virtual colonoscopy): The challenge that exists between advancing technology and generalizabililty. AJR. 173; 549-559.
Personal Communication (2000): Dr. Judy Yee, Associate Professor of radiology at the University of California, San Francisco, and Chief of Computed Tamography and GI at the VA Medical Center in San Francisco.
Personal Communication (2000): Dr. Jonathan Terdiman, assistant clinical professor of medicine at the University of California, San Francisco and the clinical director of the UCSF Colorectal Cancer Prevention Program
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D. et al. (1997). Is incidental prophylactic oophorectomy
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Stratton,
J. F. et al. (1997). Contribution of BRCA1 mutations
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