Doctors recommend all those 50 year of age or older to be screened preventatively for colon cancer. There are three distinct ways of doing this: colonoscopy, sigmoidoscopy and ‘virtual colonoscopy.’ The first method is the most involved and still appears to be the most sensitive approach. Sigmoidoscopy is simpler but only surveys the last one-third of the colon (large intestine); however, over half of all colon cancers are in this area, so it is still considered a valuable exam. Both these methods involve the insertion of a flexible viewing scope past the anus and into the colon.
The ‘virtual colonoscopy,’ however, is non-invasive: pictures are taken from the outside. It is actually the technique called computed tomography (CT) applied to the colon. Computed tomography – sometimes called computer assisted tomography, CAT – can be described simply as a more accurate version of traditional X-rays, though there are various technical differences. So far, the CT method seems to be somewhat less sensitive than a real colonoscopy, yet can be a partial substitute for it in patients who are too sick or weak to tolerate a colonoscopy and the sedation it requires.
Both colonoscopy and it’s virtual counterpart require a patient to not have solid food the day before the procedure, and also to drink literally a gallon of a laxative solution. This thoroughly cleans out the intestines and makes for the best visibility of any potential tumors or pre-tumors, which are called polyps. Unfortunately, a minority yet substantial portion of people find they just can’t stand getting through this preparation routine. The sigmoidoscopy, on the other hand, is usually done without any laxative – but isn’t as thorough as the other two techniques.
Now a team of doctors primarily from the Massachusetts General Hospital has tested the accuracy of a virtual colonoscopy performed without any laxative preparation ahead of time. They have found that these are almost as good as standard colonoscopy for detecting the largest polyps. These are defined as one centimeter (cm)(10 millimeters) or more in width, half the diameter of a penny. They did both types of exams on over 600 patients undergoing screening, and calculated that for these largest polyps the sensitivity of CT was 91% while that of the traditional colonoscopy was 96%. They also looked at the number of false positives each technique gave, called the specificity of the test; fewer false positives means a higher number for the specificity. Here too, colonoscopy was just a little bit better than the virtual type: 89% versus 85%. On the other hand, the experiences of the patients were rated much more positively by them for virtual than for real colonoscopy, as might be expected.
That was the good news. The less good news? Both the sensitivity and specificity of the laxative-free virtual colonoscopies were significantly lower with smaller and smaller polyps. For example, for polyps 0.8 cm wide (8 millimeters), colonoscopy was 88% sensitive but CT sensitivity was only 70%. For polyps 0.6 centimeters wide (6 millimeters) the sensitivity of true colonoscopy was 76% while the virtual variety was only 59% sensitive. Differences in specificity were very similar, namely always better with true colonoscopy. These results indicate that those who absolutely cannot tolerate the large volume of laxative – for either personal or medical reasons – can still receive screening that has some value to it, even if not the full value of true colonoscopy.
As doctors continue to improve their technologies and their techniques, preparing for colon cancer screening may become somewhat easier. In the meantime, those who are able to prepare for a full colonoscopy are still encouraged to do so. It’s a little unpleasant to prepare for, but in exchange for that, a patient can be assured they’re getting the most sensitive and accurate exam possible.