Colonoscopy to detect polyps that are the precursors of colorectal tumors can be uncomfortable, and perhaps even slightly dangerous for older people whose intestinal walls have become more fragile. But most people agree that it is the preparation for the procedure that is the biggest pain in the...well, you know where. The bowel has to be thoroughly cleaned out before the procedure so that it is possible to see the polyps. That requires drinking as much as a gallon of laxative and spending hours sitting on the commode playing hand-held video games. Researchers are studying new laxatives that may not require drinking as much unpleasant liquid, but the second half of the equation remains unchanged.
Now it may be possible to avoid the laxatives altogether by combining CT imaging with another form of imaging called positron emission tomography, or PET scanning, British researchers reported Tuesday in the June issue of the Journal of Nuclear Medicine. CT scanning is already being used to perform virtual colonoscopies that are noninvasive, but those still require cleansing of the bowels beforehand. Adding PET scans eliminates that need.
In PET scans, sugars labeled with slightly radioactive fluorine are injected into the bloodstream and the radiolabels accumulate in tissues that are using a lot of energy -- that is, that are burning a lot of sugar. Sensitive detectors then use their emissions to form an image. Previous studies have shown that polyps are more metabolically active than intestinal tissues and tend to accumulate the radiolabel. Previous studies have shown that PET scanning can be used to determine which suspicious spots on a CT scan of the intestines are polyps, but those first studies were small and required patients to cleanse their intestines.
In the new study, Dr. Stuart A. Taylor of University College London and his colleagues enrolled 56 patients who were at high risk of colorectal cancer and who were scheduled for conventional colonoscopy. The patients were asked to submit to the new procedure within a two-week window before their conventional test. Their only dietary restriction was to eat a low-fiber diet the day before the test. No laxatives were used. The researchers found that the test identified all polyps that were at least 10 millimeters (0.4 inches) in diameter -- that is, the polyps that were most likely to progress to tumors -- and nearly all of those that were smaller.
The team wrote in their report that they "were probably overcautious" in requiring a low-fiber diet the day before the test, and would like to try it with a normal diet. Among the limitations of the study were its relatively small size and the need to choose patients who were healthy enough to undergo a conventional colonoscopy. And the biggest drawback is the cost of the procedure, which is expected to be much higher than that of conventional colonoscopy. For that reason, they said, the procedure will probably be reserved for those who are unable to undergo a colonoscopy or who are not able to withstand the bowel cleansing.
-- Thomas H. Maugh II
A CT scan of the cross section of a colon (A) showing unidentifiable spots, a PET scan (B) and the combination of the two (C), revealing a polyp. Credit: Journal of Nuclear Medicine