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Advantages of Virtual Colonoscopy for Colon Cancer Screening

March 10, 2009
Written by: , Filed in: Gastrointestinal Imaging
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Colon cancer is a significant health problem. In the United States, it is the third most frequent cancer and the second most frequent cause of cancer death. Unfortunately, by the time patients present and are symptomatic, the disease has already spread.

In order to impact morbidity and mortality, we need to implement screening for colon cancer on a widespread basis. It is a great cancer to screen for because, unlike other cancers, we actually have the opportunity to detect precancerous lesions, as most colon cancers arise from precancerous adenomatous polyps.

Colon cancer screening relies on detecting the precancerous polyps, and one of the problems is polyps are very common. It is estimated that more than half of patients over the age of 50 have some polyps, but not all polyps are adenomatous.

A small number of adenomatous polyps can go on to become cancer, but it takes between 10 and 15 years for an adenoma to become a carcinoma. Many patients have polyps so, in order to screen for colon cancer, you need to screen everybody.

We have a variety of screening choices available. The most definitive one is conventional colonoscopy, but it is fairly involved, requires sedation, and has risk of bleeding and perforation. The risk of death is somewhere between one and three per ten thousand, and sometimes the colonoscopist cannot reach the right colon.

Virtual colonoscopy uses CT to specifically image the colon and to look inside the colon to find polyps. The procedure is quick. Patients are in the room for about 10 minutes and it requires no sedation. They do not have to take the day off work, have somebody drive them home, and there is no anesthesia risk. The scan itself only takes a matter of seconds. These advantages may encourage more people to be screened.

Additionally, you can look at the entire colon with no blind spots, it is less invasive, and is definitely safer with less morbidity. There is a very small chance of perforation but much less than conventional colonoscopy, and it is less expensive as well.

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Bowel cleansing is essential and is done with the same agents used for conventional colonoscopy. Tagging agents assist the radiologist in case there is any fluid or stool left in the colon.

Tagging can be done with Tagitol, a dilute barium agent that the patient drinks the day before while they are taking the prep. It will mix with the residual solid stool and make it look white on the CT scan so there is no way that you will confuse stool with a polyp.

Another option is Gastroview, or an iodinated agent given the night before that will tag any residual fluid so the fluid will look white. So, if there is a polyp hidden in the fluid, you will be able to see it, because the fluid will be white and the polyp will be soft-tissue density.

There might be some sensitivity to the contrast agents, but in general the risk to reward ratio of performing virtual colonoscopy makes it an excellent choice for screening patients to rule out colon cancer fairly quickly and easily.

If you want to perform virtual colonoscopy in your practice, you need to be trained. You need to be trained in how to set it up, the bowel prep needed, tagging agents, and the details in performing the study. You especially need to be trained in how to read the study.

The best way is to look at real cases with colonoscopy or surgical proof. You need to have these data sets and quiz cases, and you should look at a minimum of 50 well-chosen cases that include examples of various pathology of the colon in order to discern colon cancer from other colon health issues.

In the beginning, it will take you about 45 minutes or an hour per virtual colonoscopy. By the time you finish the 50 cases, you will be down to about a half hour, and that will get better over time as you practice reading more scans.

If the virtual colonoscopy detects polyps suggestive of colon cancer, then a full colonoscopy will need to be scheduled.

Virtual colonoscopy is not a substitute for colonoscopy in cases of family history, or follow up colon cancer screening if there have been previous findings suggestive of colon cancer. But is it another valuable tool in our gastrointestinal imaging arsenal which can help detect and prevent colon cancer when still in its early stages.

Author: Karen Horton, M.D.


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