Colon Cancer Screening (cont.)
Dennis Lee, MD
Dr. Lee was born in Shanghai, China, and received his college and medical training in the United States. He is fluent in English and three Chinese dialects. He graduated with chemistry departmental honors from Harvey Mudd College. He was appointed president of AOA society at UCLA School of Medicine. He underwent internal medicine residency and gastroenterology fellowship training at Cedars Sinai Medical Center.
In this Article
- Introduction to colon cancer screening and surveillance
- Screening recommendations for individuals with average risk of colon cancer
- Fecal occult blood tests
- Flexible sigmoidoscopy
- Screening colonoscopy
- Virtual colonoscopy
- Air contrast (double contrast) barium enema
- Surveillance recommendations for individuals with higher-than-average risk of colon cancer
- Patients with history of colon polyps
- Patients with history of colorectal cancer
- Patients with ulcerative colitis
- Family history of colorectal cancer
- What are hereditary colon cancer syndromes?
- Who should consider genetic counseling and testing and how is it conducted?
- Summary of colon cancer screening
Virtual colonoscopy is a new technique that uses CT to construct virtual images of the colon that are similar to the views of the colon obtained at colonoscopy. The virtual colonoscopic images are produced by computerized manipulation of two-dimensional images obtained by a CT scanner rather than direct observation through the colonoscope. The colon is cleaned-out using laxatives the day prior to the virtual colonoscopy examination. A tube then is inserted into the anus and is used to inject air into the colon. The CT scans then are performed, and the scans are analyzed and manipulated to form a virtual image of the colon.
Properly performed virtual colonoscopy can be very good. It can even find polyps "hiding" behind folds that occasionally are missed by colonoscopy. Nevertheless, virtual colonoscopy has several limitations. They are:
- Virtual colonoscopy cannot find small polyps (less than 5 mm in size) that are easily seen at colonoscopy.
- Virtual colonoscopy is not as accurate as colonoscopy at finding flat cancers or premalignant lesions that are not protruding, that is, are not polyp-like.
- Small pieces of stool can look like polyps on virtual colonoscopy and lead to a diagnosis of polyps when there are none.
- Virtual colonoscopy cannot remove polyps. Thirty to forty percent of people have colon polyps. If polyps are found by virtual colonoscopy, then colonoscopy must be done to remove the polyps, and, therefore, many individuals having virtual colonoscopy will have to undergo a second procedure, colonoscopy.
- There have not been studies to compare the discomfort levels of colonoscopy versus virtual colonoscopy, and comparisons will be difficult to do. The discomfort of colonoscopy is from the insertion of the colonoscope and air being pushed into the colon to inflate or open for viewing areas that might otherwise be collapsed. This technique is called air insufflation. The discomfort of virtual colonoscopy is from air insufflations. Patients' perceptions of discomfort from both procedures are highly variable. What makes the discomfort difficult to compare is that patients undergoing colonoscopy usually are sedated intravenously, while patients undergoing virtual colonoscopy are not sedated. As a result, patients may actually find colonoscopy more comfortable than virtual colonoscopy. On the other hand, sedation increases the risk of complications from colonoscopy.
Because of these limitations, virtual colonoscopy has not replaced colonoscopy as the primary screening tool for individuals at either normal or high risk for polyps or colon cancer. It is currently a good option for individuals who cannot or will not undergo colonoscopy
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