About one-third of people who have undergone surgery for colorectal cancer face development of additional tumors. But tests to detect cancer recurrence can give contradictory results, forcing doctors to perform exploratory surgeries that may be too late to be useful.
A new study led by researchers at Washington University School of Medicine in St. Louis suggests that positron emission tomography (PET) can detect additional tumors early and reveal the extent to which the colorectal cancer has spread. The researchers evaluated 22 patients who had warning signs of cancer based on elevated blood levels of a factor produced by tumor cells. However, traditional means of imaging tumors, such as computed tomography and magnetic resonance imaging, did not reveal cancer. When PET imaging was used, it confirmed disease in 15 patients out of 17 who had tumors. The five patients whose PET scans were negative remained cancer-free for at least nine months of further observation. The results were published in the March 1998 issue of Annals of Surgery. Farrokh Dehdashti, M.D., lead investigator of the study and an assistant professor at the medical school's Mallinckrodt Institute of Radiology, says the results suggest that PET is a good option for patients with warning signs of cancer spread. "The whole focus is on detecting cancer recurrence early, and PET gives patients this chance," she says.
Other imaging methods failed The patients in the study ranged from 26 to 75 years in age and had elevated blood levels of the tumor marker carcinoembryonic antigen, a signal that cancer may be present. Yet imaging methods such as computed tomography failed to reveal new growths.
PET highlights cells' biological activity and can visualize a tumor months before it is large enough to be detected by other imaging methods. To undergo these scans, patients are injected with a radioactive form of glucose called fluoro-deoxyglucose (FDG). Cancer cells utilize glucose at a higher rate than normal cells.
The FDG-PET images showed that four of the 15 patients with additional cancer had a single tumor. These four underwent further surgery and remained free of cancer for 18 months or more. Eleven others had widespread cancer in sites such as the liver, pelvis and spleen, making them poor candidates for surgery. They received chemotherapy or no additional treatment.
Olagunju A. Ogunbiyi, M.D., from the University of London, England, participated in the study during a two-year fellowship at the School of Medicine's Department of Surgery. Ogunbiyi says FDG-PET helps determine when surgery is useful or inappropriate. "Surgeons need to determine whether a cancer would be operable. You could put a patient's life at risk by performing unnecessary surgery."
Ogunbiyi estimates that 15 percent to 20 percent of patients who have suspicious levels of carcinoembryonic antigen after colorectal surgery show no signs of cancer with other imaging methods. "The only chance for curing a recurrent colorectal cancer is usually surgical, and the earlier you detect recurrence, the better the prognosis," he says.
The finding of negative FDG-PET results in patients who showed no signs of disease within nine months or more of follow-up also was important, Dehdashti says. "We now know that if a PET scan was negative, the chance that a patient will have disease is low. With positive FDG-PET results, you have to be more careful and do further testing."
Dehdashti and Ogunbiyi note that PET imaging can cost $2,000 to $3,000 and that scanners are not available at all cancer-treatment sites. They say, however, that the technique's ability to detect cancer early and its track record compared with other techniques make it an extremely useful option. "In certain clinical settings, FDG-PET appears to be far better than routine radiological tests that we are using everyday," Dehdashti says.
She notes that PET also appears to be better than computed tomography and magnetic resonance imaging at detecting cancer in the liver, a common site for spread of colorectal cancer. It also gives doctors an easier way to look over the entire body for detection of tumors.
by Barbra Rodriguez
Note: For more information, refer to: Flanagan FL, Dehdashti F, Ogunbiyi OA, Kodner IJ, Siegel BA, "Utility of FDG-PET for Investigating Unexplained Plasma CEA Elevation in Patients With Colorectal Cancer," Annals of Surgery, 227 (3), 319-323, March 1998.
Journal
Annals of Surgery