Using non-invasive computed tomography (CT) may detect colon cancer as accurately as conventional colonoscopy, according to a new report. Dubbed "virtual colonoscopy," the new technique involves taking multiple CT scans of the patient's colon and then reconstructing the images with a computer to create a three-dimensional picture of the colon. While the study did reveal some limitations of virtual colonoscopy, the researchers say the technique may be useful for patients who are unable or unwilling to undergo conventional colonoscopy. Further research may show that virtual colonoscopy is an effective screening tool for colon cancer.
Conventional colonoscopy involves inserting a flexible tube called an endoscope through a patient's rectum into the lower portion of his or her colon. The tube has a light on its end and is connected to a video camera, which allows the physician to actually see inside the patient's colon to check for abnormalities. Colonoscopy is often performed to check for abnormalities in the colon and rectum. The procedure is also used to remove polyps (growths) or to remove (biopsy) a sample of tissue from the colon or rectum for analysis under a microscope.
By contrast, a virtual colonoscopy is a non-invasive procedure that involves taking CT scans of the colon. The CT scans are then put together into a three dimensional image of the colon on a computer. However, both virtual colonoscopy and conventional colonoscopy require the bowel to be cleansed prior to the exam. In an effort to compare the effectiveness of virtual colonoscopy to conventional colonoscopy, researchers from the University of California performed both procedures on 300 patients, many of whom had symptoms that suggested they could have colon cancer. The results of the study showed that virtual colonoscopy was just as accurate at detecting colon cancer as conventional colonoscopy.
Virtual colonoscopy also detected 90% of large polyps in the colon (10 millimeters or larger). Polyps are growths of tissue in the colon or rectum. Some types of polyps increase the risk of colon cancer, especially if they are large or if a patient has several polyps. In the study, lead researcher Judy Yee, MD and her colleagues found that virtual colonoscopy was less effective than conventional colonoscopy at detecting smaller polyps. Another drawback of virtual colonoscopy is that polyp removal and biopsies of tissue in the colon must be performed using conventional colonoscopy. Therefore, if a polyp or abnormality is seen with virtual colonoscopy, the patient will still have to undergo a conventional colonoscopy to have the abnormality removed or biopsied.
Nevertheless, the researchers say that virtual colonoscopy could be beneficial for patients with conditions that prevent them from undergoing conventional colonoscopy, such as a blood disorder, an intolerance of the required sedative, or other problems such as heart or lung disease. Also, only 40% of eligible patients over age 50 receive colon cancer screenings with colonoscopy. Virtual colonoscopy could potentially provide screening to those patients who refuse to have a conventional colonoscopy out of fear or dislike for the procedure.
However, the American Cancer Society will not recommend virtual colonoscopy as a colon cancer screening exam until larger studies confirm Dr. Yee's results. Other physicians are concerned that because virtual colonoscopy requires highly skilled physicians to accurately interpret the results, the technology may not be appropriate for widespread screening.
While virtual colonoscopy was compared with conventional colonoscopy in the study, virtual colonoscopy is actually more like a double contrast barium enema or abdominal/pelvic CT, two other types of colon cancer detection exams. Double contrast barium enemas or abdominal/pelvic CTs are x-ray exams of the colon which require that patients first be given barium sulfate through a small tube placed in the rectum. The barium sulfate "opens" the colon and enables physicians to obtain clear pictures.
Conventional Colonoscopy
This exam involves placing a flexible, lighted tube through the anus into the colon. The physician can guide the tube up and down the walls of the patient's colon, zooming in on regions of interest. The tube has a video camera on its tip, and an external video monitor displays what the camera records.
Barium Enema
This is real-time x-ray imaging of the colon (called fluoroscopy). Patients are given barium sulfate through a tube inserted in the anus. Air will also be pumped into the colon to make it expand. The barium and air help provide clear x-ray images of the colon.
Abdominal/Pelvic CT
This exam involves taking a series of CT scans of the colon. The result is several separate two dimensional scans of the colon. Patients are given barium sulfate orally and often through the rectum as part of the exam.
CT ("Virtual") Colonoscopy
This exam involves taking a series of CT scans similar to a barium CT (except that barium is not used; only air is pumped in the colon prior to the exam). However, the CT images can be assembled with a computer to create a three dimensional view of the patient's colon. The physician can then "navigate" through the 3D image on the computer to check for abnormalities.
Currently, the American Cancer Society recommends that all people 50 years of age and older have one of the following tests:
- Yearly fecal occult blood test (FOBT)
- Flexible sigmoidoscopy* every 5 years
- Yearly fecal occult blood test plus flexible sigmoidoscopy every 5 years
- Double contrast barium enema every 5 years
- Colonoscopy** every 10 years
*a sigmoidoscopy involves examining the rectum and lower portion of the colon
**a colonoscopy involves examining the entire lining of the colon
- The study, "Colorectal Neoplasia: Performance Characteristics of CT Colonography for Detection in 300 Patients", is published in the June 2001 issue of Radiology
- The June 25, 2001 American Cancer Society News Today report, "Virtual" Colonoscopy Promising, But May Overlook Smaller Polyps", is available on the ACS website along with information about colon cancer screening and diagnosis.
- The May 31, 2001 Diagnostic Imaging online report is entitled, "Virtual Colonoscopy Matches Conventional Screening's Detection Ability".
- Learn more about CT.
