Coincidence Detection Allows Wider Use of FDG Scans

GLASGOW—Gamma camera molecular coincidence detection (MCD) technology allows imaging of positron-emitting radiotracers such as FDG without the need for a dedicated PET scanner, and thus should make FDG imaging much more widely available, Henry N. Wagner, Jr., MD, professor of radiation health sciences at Johns Hopkins Medical Institutions, told Oncology News International.

In lung cancer, the technique may enable physicians to better identify patients with inoperable disease, thus sparing them unnecessary surgery, Dr. Wagner said at the European Association of Nuclear Medicine Congress. He reported interim results of a multicenter trial looking at the clinical utility and cost-effectiveness of the method in non-small-cell lung cancer patients.

In this study, he said, patients scheduled for lung surgery undergo a preoperative MCD scan. Before the MCD scan is interpreted (by two blinded independent reviewers), the patient’s disease is staged by other methods and a management plan is determined. Then, the surgeon reviews the MCD scan and records any proposed management changes based on the findings.

Later, the accuracy of the original plan and the proposed modification is evaluated against the actual surgical result. The data will ultimately be used to develop a model comparing the cost of incorporating MCD imaging into the workup with the reduction in cost associated with elimination of unnecessary surgery.

In the first 35 patients, Dr. Wagner said, MCD has shown high sensitivity (96%), picking up 24 of 25 lung cancers, and high specificity (80%), identifying 8 of the 10 patients who did not have lung cancer. He told Oncology News International that another 30 or 40 patients have been enrolled in the study, but their scans have not yet been interpreted.

One of the reasons why the specificity is not 100%, he commented, is that there are some infections that have the same increased utilization of glucose as does cancer [FDG is glucose based]. “Two of the 10 patients who did not have lung cancer did have infections as evidenced by the fact that they got better with antibiotics,” he said. “That’s why their lesions were read as positive by MCD.”

The most immediate clear-cut application of the technique, he said, is in disease staging. “If a person has suspected distant metastases with MCD, then you can biopsy one of the metastases, which is a lot easier on the patient than doing a thoracotomy,” he said.

The other potential use is in distinguishing benign from malignant tumors. Dr. Wagner stressed, however, that the technique is not yet at the point where a physician would base a decision not to operate solely on a negative MDC study. “If a lesion doesn’t accumulate FDG,” he said, “this provides strong evidence that it’s not malignant, but it’s not absolute. You would combine this finding with other information to decide on surgery or observation.”

Understanding the Technology

The MCD technology involves injection of fluorodeoxyglucose (FDG) prior to imaging with an MCD gamma camera (manufactured by ADAC Laboratories, Milpitas, Calif). Uptake of FDG by cancer cells reveals the location and extent of disease. (Other gamma camera manufacturers refer to the technique as positron coincidence detection.)

Dr. Wagner explained that dedicated PET systems are limited to measuring the 511 keV photons that come from positron-emitting tracers. Single-photon tracers have lower energies than 511 keV.

MCD is a hybrid system in which a single-photon emission computed tomography (SPECT) scanner, designed for lower energies, has been modified to perform coincidence imaging of positron-emitting radiotracers while retaining the ability to do single-photon imaging.

“It is basically a multienergy system,” he said, “and that is its biggest advantage—that it can measure positrons and single photons.”

In the United States, there are only 73 dedicated PET centers, but there are 3,500 or more nuclear medicine departments, all of which have SPECT systems. “In the past, many people didn’t want to bite the bullet of PET,” he said, “since it is relatively complex and expensive. Combining PET capability with SPECT makes positron imaging more widely available.”