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Key Words
Colorectal cancer, colonoscopy, computed tomography, screening, virtual colonoscopy. (Definitions of many terms related to cancer can be found in the Cancer.gov Dictionary ³.)

Summary
In a study of people at average risk for colorectal cancer, a screening technique known as virtual colonoscopy was just as accurate as traditional colonoscopy at detecting potential signs of colon or rectal cancer, and was less invasive.

Source
The New England Journal of Medicine, December 4, 2003 (see the journal abstract).

Background
Colorectal cancer is the second leading cause of cancer deaths in the United States. Most colorectal cancers start as polyps - benign (non-cancerous) growths on the inner wall of the rectum and colon. Polyps develop slowly over many years. Currently, doctors recommend that everyone be screened for colon cancer starting at age 50.

There are several screening methods now in use, ranging from a simple test of a patient’s stool sample to a colonoscopy. In a colonoscopy, the patient is sedated and the doctor views the walls of the rectum and colon by inserting a thin, lighted tube and studying the projected images on a video monitor. Often, if polyps are detected, they can be removed during the same procedure.

Virtual colonoscopy, also known as CT colonography, uses an x-ray machine linked to a computer to generate unique 2- and 3-D images of the colon and rectum that can be manipulated for better viewing angles and stored for later study. The procedure does call for the insertion of a rectal tube to push air into the colon for a better view. No sedation is required, but as with traditional colonoscopy, patients must take laxatives and fast 24 hours prior to screening.

The Study
A total of 1,233 adults with no symptoms of colorectal cancer participated in the clinical trial. They took laxatives, fasted for 24 hours and, just prior to examination, drank a “contrast medium” beverage that allowed for better viewing of any polyps.

All the participants first underwent a virtual colonoscopy that produced three-dimensional images. (Previous studies of the method used a technique that produced only two-dimensional images.) A flexible tube inserted into the rectum expanded the colon with air while the CT scans were taken.

Later the same day, participants were given a traditional colonoscopy by experienced colonoscopists, but only afterwards did the colonoscopists see the results of the person’s virtual colonoscopy. Those segments of the colon showing polyps in the virtual exam but not the traditional test were then reexamined by the traditional method.

At the end of all the screening, participants filled out a one-page questionnaire on the level of discomfort and overall convenience of each procedure, as well as which one they’d prefer to use in the future.

Results
Overall, virtual colonoscopy detected more than 90 percent of clinically significant (possibly cancerous) polyps. Two malignant (cancerous) polyps were found, and both were picked up by virtual colonoscopy. Traditional colonoscopy detected one of the malignant polyps, but only detected the second one after its location was revealed by the virtual technique.

When comparing the two techniques, 54.3 percent of participants said virtual colonoscopy was more uncomfortable; 38.1 percent said the traditional method was more uncomfortable (the rest were undecided). According to the study’s authors, this may reflect the fact that participants were sedated only for the traditional colonoscopy. Nevertheless, most participants found virtual colonoscopy to be more acceptable in terms of overall convenience (68.3 percent versus 24.1 percent).

The researchers concluded that virtual colonoscopy is a promising screening tool for colorectal cancer and “compares favorably with conventional colonoscopy for detection of clinically relevant lesions.”

Limitations
In an accompanying editorial, Martina M. Morrin, M.B., and J. Thomas LaMont, M.D., of Harvard Medical School in Cambridge, Mass., noted that “the performance of virtual colonoscopy in this asymptomatic screening population is impressive.” If the technique is shown to be equally effective for persons at high risk of colorectal cancer, they said, virtual colonoscopy could be a useful alternative.

Ernest Hawk, M.D., M.P.H., of the National Cancer Institute’s Division of Cancer Prevention, said that this is an “extremely promising and well-done study.” Only half of all at-risk individuals are getting screened for colorectal cancer. The introduction of an alternate screening tool that is convenient, faster, and less invasive would likely help increase the number of individuals who are screened.

Hawk cautions, however, that it is difficult to recommend changes in practice on the basis of one study. The technologies employed in this study, and its results, are very different from earlier studies evaluating CT colonoscopy as a screening tool. If these results are reproducible on a broader scale, he said, virtual colonoscopy could expand the menu of options for colorectal cancer screening.

Glossary Terms

Having no signs or symptoms of disease.

Examination of the inside of the colon using a colonoscope, inserted into the rectum. A colonoscope is a thin, tube-like instrument with a light and a lens for viewing. It may also have a tool to remove tissue to be checked under a microscope for signs of disease.

An area of abnormal tissue. A lesion may be benign (not cancer) or malignant (cancer).

Checking for disease when there are no symptoms. Since screening may find diseases at an early stage, there may be a better chance of curing the disease. Examples of cancer screening tests are the mammogram (breast), colonoscopy (colon), Pap smear (cervix), and PSA blood level and digital rectal exam (prostate). Screening can also include checking for a person’s risk of developing an inherited disease by doing a genetic test.

A method to examine the inside of the colon by taking a series of x-rays. A computer is used to make 2-D and 3-D pictures of the colon from these x-rays. The pictures can be saved, changed to better viewing angles, and reviewed after the procedure, even years later. Also called computed tomographic colonography, computed tomography colonography, CT colonography, and CTC.