Dr. Stephen Amann's excellent article on colon cancer (For Your Health, March 4, 2004) described the challenges and benefits of early detection of colon cancer. As America shifts its focus from disease treatment to disease prevention, methods to screen the population for all types of cancers becomes critical. Mammography, an X-ray procedure to image the breast, has proved to be the best means of screening for cancer of the breast when coupled with self breast examination and physical examination by a health professional. For prostate cancer, a blood test for Prostatic Specific Antigen (PSA), a chemical produced by the prostate, has emerged as an excellent screening method and increased the survival for prostate cancer. To be effective as a screening tool, a diagnostic procedure must be accurate, economical and agreeable to the patient.
As Dr. Amann pointed out, testing fecal material for occult blood is one method of screening for colon cancer. It is economical and easy to perform but suffers from being non-specific and less sensitive. Other forms of disease such as diverticulosis and hemorrhoids will test positive for occult blood and when the blood is secondary to cancer, the disease is often advanced, reducing the likelihood of a cure with treatment.
Over the past couple of years, virtual colonoscopy has emerged as a viable method to screen for colon polyps and colon cancer. Recent articles in various medical journals have indicated that virtual colonoscopy is as sensitive for more sensitive in detecting polyps, especially smaller ones, than fiber optic colonoscopy. But fiber optic colonoscopy is the ultimate goal standard, necessary for verification of suspicious lesions and biopsy or removal of any polyps discovered during virtual colonoscopy.
Virtual Colonoscopy has become possible with the development of fast, high-speed computed tomography (CT) scanners coupled with powerful, less expensive computer systems that allow three dimensional visualization of the body. Newer CT scanners are capable of obtaining 800 to 1000 thin, 1 millimeter thick images of the body in 10 to 15 seconds. These thin images can be utilized by computers to reconstruct detailed images of the body in almost any imaginable form.
As with fiber optic colonoscopy, it is necessary to clean the colon of stool and debris. A special diet is required for two days prior to the exam with laxatives the day before. To perform a virtual colonoscopy, the colon is dilated with carbon dioxide producing a sensation of needing to go to the bathroom. Once the colon is adequately filled with gas, two rapid CT scans are performed, one with the patient lying on their back and one with them on their stomach. The two “views” are obtained in case there is any fluid in the colon. The fluid will layer dependently, allowing full visualization of both the front and back of the colon. The images from the CT scan are then sent to a powerful computer system, providing a three dimensional image of the inside of the colon. The colon is carefully evaluated by a qualified physician in a variety of ways to document the presence or absence of polyps or masses. If any abnormalities are discovered, the computer provides a detailed view of the location of the abnormality so that the gastroenterologist can locate, biopsy or remove the abnormality.
C. Michael Currie, M.D., is a board certified neuroradiologist and owner of The Imaging Center at Gloster Creek Village, an independent medical imaging facility.