While traditional colonoscopy has been the method of choice for decades, virtual colonoscopy (VC), also called CT colonography, is a relatively new alternative that offers a less-invasive option for some people.
What exactly is VC? How is it performed?
In virtual colonoscopy, a CT (computed tomography) scanner is used to examine the abdomen and pelvis. Then the images are analyzed with sophisticated two- and three-dimensional viewing techniques.
We can literally fly inside the computer-generated model of a patient's colon, looking for polyps (growths on the wall of the colon) that are the precursors to cancer.
The process has four steps:
Cleansing the bowel with laxatives the day before the procedure
Inserting a small tube into the rectum to inflate the colon with carbon dioxide gas; room air may also be used, but it makes the procedure less comfortable
CT scanning of the abdomen and pelvis, which takes less than 10 seconds
Image analysis using specialized computers
What are some of the advantages of VC compared to traditional colonoscopy?
No sedation is required for VC, and that is a huge advantage. Another plus is that we can look beyond lesions that might be obstructing the colon or examine a tortuous colon (a longer than normal colon that may become twisted or coiled).
Since VC is less invasive, the risk of bleeding and tearing of the colon is significantly less.
What are some of the disadvantages and criticisms of VC?
The main disadvantage is that, while VC screens for precancerous polyps and cancers, they cannot be removed during the exam. So a follow-up colonoscopy might be required. Some physicians discourage VC on those grounds, saying, "If you get a VC and a polyp is found, you will have to have a colonoscopy. So why not have a colonoscopy in the first place?"
That is partly true, but only 20% of people have significant lesions that warrant biopsy or removal. So, if we screen everyone with colonoscopy, 80% will undergo colonoscopy -- with its inherent risks of bowel perforation and the need for anesthesia -- unnecessarily.
Since the CT scan covers all the organs in the abdomen and pelvis, VC often can detect disease outside the colon. Significant lesions of this type are found in about 10% of cases. This can be a blessing and a curse due to the potential for unnecessary workups of benign lesions. Rather than discarding VC, I think we need better practice management guidelines on what to do with these findings outside the colon.
Another criticism has been that the process gives the patient a low dose of radiation. However, most VCs use ultra-low dose techniques with an extremely small radiation risk.
Who might be a candidate for virtual colonoscopy?
I think average-risk asymptomatic adults age 50 and older should consider VC. It's a good idea to discuss the alternatives with your doctor.
How common is the technology?
Even though VC is endorsed by the American Cancer Society and other national groups, its adoption has been slow for several reasons including:
limited insurance coverage,
few well-trained providers and
the public's general reluctance to comply with colon cancer screening recommendations
However, as insurance coverage increases, availability should follow.
The best VC exams generally are performed at large academic medical centers. It's a matter of the volume of cases a facility performs and the radiologist's experience in performing and interpreting VC examinations. The technology is available at most medical centers, but the devil is in the details in how to conduct the examinations.
What should people look for in choosing a facility?
Ask how many cases the facility has performed and the radiologist has interpreted. The American College of Radiology and the American Gastroenterological Association recommend that physicians who interpret VC exams have done at least 75, but I suggest that the more the better.
What types of research is MD Anderson doing?
Since bowel preparation is the largest barrier to most people seeking colon cancer screening, my research involves the development of new bowel cleansing approaches. These include solutions to dissolve feces and remove it from below without the need to drink the usual cleansing agents.
MD Anderson resources: MD Anderson Colorectal Screening Guidelines