For many people, the thought of getting a colonoscopy can be so intimidating that they don't. Now, in a New England Journal of Medicine study published today, researchers at the University of Wisconsin School of Medicine report that an alternative screening procedure for colorectal cancer, known as CT colonography, proves to be just as effective at detecting dangerous polyps as traditional colonoscopy. The noninvasive procedure allows a radiologist to view a 3D image of the colon and to determine, based on size, if a polyp appears to be precancerous and should be surgically removed. With traditional optical colonoscopy, polyps are removed during the procedure, which can sometimes result in complications. What role might CT colonography play as a screening option for detecting and preventing colorectal cancer? U.S.News & World Report spoke with the study's lead author, David Kim, an assistant professor of radiology at Wisconsin, about the risks and benefits.
Why do you think there was a similar detection rate between the two groups?
CT colonography and optical colonoscopy are equivalent at detecting polyps—they both do very well. The difference is that CT colonography uses a screening practice called selective polypectomy. That is, we set a threshold at a certain size at which people get the polyps removed. And then for a smaller-size group, 6 to 9 millimeters, we offer the possibility of just following that lesion. And then anything less than 5 mm, we actually ignore.
The optical colonoscopy uses a practice called universal polypectomy. That is, a polyp of any size is removed. But most dangerous lesions are large—above that size threshold where we send people to polypectomy. There's really a tiny fraction of small polyps that are dangerous.
We can use CT colonography as a filter and only send the really high-risk patients for a colonoscopy. For the patients who are at much lower risk, but not completely zero, you follow the polyps. The beauty is that the cancer biology of colon cancer is such that these target lesions are benign and they take eight to 10 to 12 years before they turn into cancer. So you can follow these things, and if they get bigger you remove them, and it's still not cancer.
Why is there universal polypectomy with the optical colonoscopy? Why not just do selective polypectomy?
The main reason is that optical colonoscopy is a fairly invasive procedure. It is a great procedure to detect polyps and to remove polyps. But in order to do it, you've got to sedate the person and put a fiberoptic scope into their colon, which is really uncomfortable and carries a risk of perforation. If you have a patient that has already accepted all these risks to go ahead and look for polyps, you might as well take them all out and cast a wide net so you're not leaving anything behind. If you leave a polyp in the colon and try to follow it, you may again have to accept the risks of another procedure.
The second aspect of optical colonoscopy is that it's really hard to localize where you are when you're looking through a scope. You're looking through a long tube, so you don't know exactly where you are. So if you see a polyp, you're not sure if you're looking at the same polyp you saw two years ago if you're trying to follow it. CT colonography is fundamentally different. It is a minimally invasive exam where you just put in a little catheter to distend the colon with CO2, and you scan the person so you don't need sedation, and the complication risks are orders of magnitude less than with optical colonoscopy. It's a very safe procedure that you can repeat, and because it's a CT scan, you can see the extracolonic environment to use as landmarks to know exactly where in the colon you are. It works very well to follow lesions, whereas optical colonoscopy does not.
How do your findings inform the use of CT colonography as a primary screening technique?
I think that in the near term, we need both modalities because there are a very large number of people that are currently not screened. The estimates are somewhere in the order of 40 million adults over the age of 50 that are not screened by any method. The most highly sensitive and the most accurate method is to use colonoscopy. But huge advances in computer hardware and software have made it possible to reach the same effectiveness at detection with CT colonography. So it can serve as a filter for the more involved optical colonoscopy, which is a really big procedure.
For patients 50 and older going in for screening, can they request one or the other, or is it up to the doctor to decide?
At the University of Wisconsin, you can request. It's your choice. But currently there is not national reimbursement for CT colonography. We were fortunate at Wisconsin to get local third-party reimbursement through HMOs. Elsewhere in the country, people currently don't get reimbursed, though this is hopefully going to change in the next couple of years with all this new data coming out. If you did want to do a CT colonography, first you'd have to find a place that can do it, because it is a fairly new technique and not all centers do it. Secondly, the patient would have to pay out of pocket.