NEW YORK, Nov 10 (Reuters Health) -- An experimental screening test that uses x-ray images to create a three-dimensional picture of the intestine is nearly as effective at detecting polyps as uncomfortable, invasive colonoscopy screening, where a flexible scope is used to examine the colon lining, Boston researchers report.
But the experimental test, known as virtual colonoscopy, is not as effective as conventional colonoscopy at detecting very small polyps, the researchers at Boston University School of Medicine in Massachusetts note in the November 11th issue of The New England Journal of Medicine.
In 1999, colon cancer is expected to kill 56,600 people in the United States, making it the second leading cause of cancer deaths. Since the survival rate is high if the disease is detected early, the American Cancer Society recommends regular colon cancer screening beginning at age 50 -- and starting even younger for people who have a family history of colon cancer.
Several types of tests are used to detect colon cancer, including colonoscopy, in which a flexible, lighted tube is inserted into the rectum to examine the colon lining. A video camera connected to the tube allows a doctor to examine the inside of the intestine for any unusual growths.
In contrast, during virtual colonoscopy the intestine is inflated with air, and then scanned by a computer-assisted x-ray, known as a CT scan.
"The data from the CT scanner is processed by specialized computer software to create a simulated view of the appearance of the walls of the colon," one of the study's authors, Dr. Matthew A. Barish, told Reuters Health. He characterized the images taken by the scanner as two-dimensional slices that can be "stacked up" to form a three-dimensional image.
However, its effectiveness at detecting colon cancer and polyps, which may become cancerous, has been uncertain.
To compare the two types of screens, the researchers studied 100 people who were at high risk for colorectal polyps. After having a virtual colonoscopy, all participants immediately underwent conventional colonoscopy.
Based on the results of the conventional test, 49 people had abnormal findings: 115 noncancerous polyps and three cancers. Virtual colonoscopy also detected all three cancers.
Overall, virtual colonoscopy detected 71% of the polyps. However, the accuracy of the test depended on the size of the polyps. Virtual colonoscopy detected more than 80% of larger polyps, but just 55% of smaller ones, the study authors report.
But, according to senior author Dr. Joseph T. Ferrucci, the fact that virtual colonoscopy does not detect the smallest polyps should be viewed as an advantage, because the vast majority of these small growths are harmless.
Virtual colonoscopy "has certain new advantages," Ferrucci said. "It's very safe, it's fast (and) it doesn't require a whole day."
Early screening with colonoscopy or one of the other tests can prevent cancer deaths, but many people are reluctant to get tested, noted the researcher.
"The problem is that people don't like to talk about their bowels," he told Reuters Health in an interview.
"Virtual colonoscopy has come a long way in a short time, and its future appears bright," Dr. John H. Bond, of the Minneapolis Veterans Affairs Medical Center, writes in an editorial that accompanies the study.
Bond notes, however, that the cost of virtual colonoscopy, difficulty in learning to perform the procedure, as well as questions about its accuracy mean that more testing is needed before the test is used widely.
