FAQ When Considering Colorectal Cancer Screening Test
Results from two studies comparing colorectal cancer screening by conventional or optical colonoscopy versus CT colonography (sometimes called virtual colonoscopy) have sparked interest in colorectal cancer screening and may raise questions in the minds of consumers. The studies, one a large multi-center trial sponsored by the NIH known as ACRIN, and the other from the University of Wisconsin published in the New England Journal of Medicine yesterday, assessed the accuracy of a CT scan of the abdomen to detect polyps or cancer in the colon compared to optical colonoscopy, the current gold standard for colorectal cancer screening.
The American College of Gastroenterology, an organization of more than 10,000 digestive disease specialists, wants consumers to understand the issues surrounding both of these tests for colorectal cancer and has developed this FAQ. Reporters interested in interviewing physician experts on colorectal screening may contact Anne-Louise Oliphant at 301-263-9000.
What are the Advantages of Optical or Conventional Colonoscopy?
Recommended by medical and cancer groups for colorectal cancer screening, conventional colonoscopy utilizes direct visualization of the entire colon to detect pre-cancerous growths, including smaller polyps, even below 1 cm. This test allows removal of suspicious polyps without surgery at the time of the exam. Three studies have shown that colonoscopy prevents about 80 percent of colorectal cancers from developing by removing pre-cancerous polyps. Traditional colonoscopy is one of the most powerful prevention tools in clinical medicine.
What Happens During a CT Colonography?
CT colonography or "virtual colonoscopy" is an X-ray test designed to look for colon polyps and cancers. First, a radiology technician inserts a tube into your rectum and air is pumped into the colon until it is fully distended. Then, you are asked to hold your breath while lying on your back and a CT scan is performed. You then turn over onto your stomach and you again hold your breath while a second CT scan of the abdomen and pelvis is performed.
Does the Test Require Bowel Cleansing (Laxatives)?
Yes. The bowel-cleansing regimen is the same for both a regular colonoscopy and CT colonography. On the day before the procedure, you stay on clear liquids all day and on the evening before and the morning of the procedure, laxatives are taken to flush waste from the colon. In addition, contrast imaging tablets need to be taken three times a day for one to two days prior to the test.
Which Test is Closest to Being Pain-Free?
With CT colonography, because no sedation is used, the distention of the colon with gas can be painful. In several studies, patients experience more pain and discomfort with CT colonography than with a regular colonoscopy, because the latter involves the administration of sedatives.
Is CT Colonography an Accurate Test?
New findings from a multi-center study of the accuracy of CT colonography suggest that it can detect about 75 to 85 percent of intermediate and large polyps. It is not as accurate in identifying smaller polyps, which also have the potential to develop in to cancer. Previous studies of CT colonography had shown a very wide range of results. On average, previous results showed that CT colonography was clearly inferior to regular colonoscopy for detection of colon polyps, with many false negatives and false positives.
How Often is a Follow-Up Colonoscopy Needed After CT Colonography to Remove Polyps?
The ACRIN trial found the incidence of ademonas 6 mm or larger was 8.36 percent, suggesting that those patients would need to have an optical colonoscopy to remove suspicious polyps. Earlier studies ranged from at least 30%, and in some studies 50%, of patients having a CT colonography will need a regular colonoscopy to remove detected polyps. The ACRIN trial, sponsored by the NIH, was designed in part to address some of the methodological inadequacies of these earlier studies.