Study Adds Weight to Virtual Colonoscopy for Average-Risk Patients

Article date: September 23, 2008

Virtual colonoscopy, or computed tomographic (CT) colonography, is about as effective as traditional colonoscopy in finding colorectal cancer and large polyps, according to results from a large clinical trial comparing the two tests published in The New England Journal of Medicine. The study confirms results from smaller trials.

But while CT colonography may give some people another option for screening, it isn't recommended for people at high-risk for colorectal cancer. For them, colonoscopy is still the gold standard for finding colorectal cancer early.

"This study showed that CT colonography is a good screening test for cancer for those at average risk," said Durado Brooks, MD, American Cancer Society’s director of prostate and colorectal cancer. "But patients should be aware that the test is less likely to pick up smaller, possibly pre-cancerous lesions, and if abnormalities are found, patients will have to have a colonoscopy anyway."

Another Option for Screening

CT colonography, an advanced type of CT (or CAT) scan of the colon and rectum, uses x-rays to create two- and three-dimensional images, which the doctor can use to view the colon and rectum for polyps and other possible pre-cancerous changes.

During a traditional colonoscopy, a gastroenterologist looks at the entire length of the colon and rectum with a colonoscope, a long flexible tube with a small video camera on the end, which is inserted through the rectum into the colon. (For more information, see "Can Colorectal Polyps and Cancer Be Found Early?")

CT colonography has some possible advantages over conventional colonoscopy. For starters, some people are uncomfortable with the idea of a colonoscopy, and will put off having the test, or even avoid it altogether. These people might be good candidates for CT colonography.

There are also higher (although still very small) risks of complications with standard colonoscopy. "With the conventional method, there's a higher risk of perforation –– poking a hole in the colon wall –– and infection," said study co-author Judy Yee, MD, chief of radiology at San Francisco VA Medical Center and professor and vice-chair of radiology at the University of California, San Francisco. "In addition, the patient must undergo sedation, which takes them out of action for an entire day, along with the person who must accompany them to the procedure and drive them home. This adds significantly to cost. With CTC, the patient can be in and out within 30 minutes, and back to work that day, with no sedation and no loss of function."

Study: Virtual Colonoscopy Finds Most Polyps

The National CT Colonography Trial is the largest study to date comparing the two screening tests. Fifteen research centers collected data for more than 2500 participants. Each person in the study underwent CT colonography, followed by a standard colonoscopy on the same day. CT colonography correctly found 90% of polyps measuring 10mm or more in diameter. (These polyps are more likely to be pre-cancerous than smaller ones.) The test was less sensitive for lesions measuring 6 to 9mm – it found 65%.

"These patients had no cancer symptoms," said Yee. "They were simply healthy older adults, typical of people who should be screened for colon cancer according to current screening standards."

"The results of the current study validate the high level of sensitivity of CT colonography for large polyps and cancers documented in earlier studies and provide additional evidence supporting the reasonableness of its inclusion in colorectal cancer screening guidelines," said Brooks.

The American Cancer Society added CT colonography to the list of recommended colon cancer screening and prevention tests in March 2008. According to those guidelines, men and women at average risk for developing colorectal cancer should start screening for colorectal cancer at age 50, or earlier if they have a personal history of colorectal cancer or polyps, a history of chronic inflammatory bowel disease, a strong family history of colorectal cancer or polyps, or a known family history of hereditary colorectal cancer syndromes. (For a list of recommended tests, see the American Cancer Society's Guidelines for the Prevention and Early Detection of Cancer.)

Cautions with CT Colonography

While CT colonography provides average-risk patients with a colorectal cancer screening option that is less invasive and less expensive than colonoscopy, it’s important that patients understand its drawbacks before deciding to have this type of test.

First, CT colonography requires the same amount of bowel preparation, such as liquid diets and strong laxatives on the day before the test, as traditional colonoscopy. People who have standard colonoscopy often claim that the bowel preparation is more bothersome than the actual test.

CT colonography also does not allow doctors to remove any polyps or biopsy any suspicious areas that are seen during the test, as can be done with standard colonoscopy. If anything abnormal is found, the patient will still need to come back for a colonoscopy later, which means they’ll need yet another bowel preparation.

The test may also miss some smaller, possibly pre-cancerous polyps that would likely be caught and removed during colonoscopy. And while the risk is hard to assess, the test does expose patients to a low level of radiation. It is not yet known if this might cause problems over many years of regular screening at 5 year intervals (as recommended by current guidelines).

"As long as they understand these limitations and are comfortable with the fact that they'll need to have a colonoscopy if abnormalities are found, CT colonography is a good screening option for average-risk patients," said Brooks. "But colonoscopy is still the only recommended approach for preventing colorectal cancer among high-risk patients."

Colorectal cancer is the third most common cancer diagnosed among both men and women in the United States. The American Cancer Society estimates that about 108,070 new cases of colon cancer (53,760 in men and 54,310 in women) and 40,740 new cases of rectal cancer (23,490 in men and 17,250 in women) will be diagnosed in 2008. Regular colorectal cancer screening can find colorectal cancer early, when it is highly curable. For more information, see Detailed Guide: Colon and Rectum Cancer.

Citation: "Accuracy of CT Colonography for Detection of Large Adenomas and Cancers ." Published in the Sep. 18, 2008 New England Journal of Medicine (Vol. 359, No. 12: 1207-1217). First author: C. Daniel Johnson, MD, MMM, Mayo Clinic Arizona.

Reviewed by: Members of the ACS Medical Content Staff

ACS News Center stories are provided as a source of cancer-related news and are not intended to be used as press releases.