We evaluated the amino acid and glucose metabolism of brain tumors by using PET with 3,4-dihydroxy-6-(18)F-fluoro-l-phenylalanine ((18)F-FDOPA) and (18)F-FDG.
Methods: Eighty-one patients undergoing evaluation for brain tumors were studied. Initially, 30 patients underwent PET with (18)F-FDOPA and (18)F-FDG within the same week. Tracer kinetics in normal brain and tumor tissues were estimated. PET uptake was quantified by use of standardized uptake values and the ratio of tumor uptake to normal hemispheric tissue uptake (T/N). In addition, PET uptake with (18)F-FDOPA was quantified by use of ratios of tumor uptake to striatum uptake (T/S) and of tumor uptake to white matter uptake. The accuracies of (18)F-FDOPA and (18)F-FDG PET were determined by comparing imaging data with histologic findings and findings of clinical follow-up of up to 31 mo (mean, 20 mo). To further validate the accuracy of (18)F-FDOPA PET, (18)F-FDOPA PET was performed with an additional 51 patients undergoing brain tumor evaluation.
Results: Tracer uptake in tumors on (18)F-FDOPA scans was rapid, peaking at approximately 15 min after intravenous injection. Tumor uptake could be distinguished from that of the striatum by the difference in peak times. Both high-grade and low-grade tumors were well visualized with (18)F-FDOPA. The sensitivity for identifying tumors was substantially higher with (18)F-FDOPA PET than with (18)F-FDG PET at comparable specificities, as determined by simple visual inspection, especially for the assessment of low-grade tumors. Using receiver-operating-characteristic curve analysis, we found the optimal threshold for (18)F-FDOPA to be a T/S of greater than 1.0 (sensitivity, 96%; specificity, 100%) or a T/N of greater than 1.3 (sensitivity, 96%; specificity, 86%). The high diagnostic accuracy of (18)F-FDOPA PET at these thresholds was confirmed with the additional 51 patients (a total of 81 patients: sensitivity, 98%; specificity, 86%; positive predictive value, 95%; negative predictive value, 95%). No significant difference in tumor uptake on (18)F-FDOPA scans was seen between low-grade and high-grade tumors (P = 0.40) or between contrast-enhancing and nonenhancing tumors (P = 0.97). Radiation necrosis was generally distinguishable from tumors on (18)F-FDOPA scans (P < 0.00001).
Conclusion: (18)F-FDOPA PET was more accurate than (18)F-FDG PET for imaging of low-grade tumors and evaluating recurrent tumors. (18)F-FDOPA PET may prove especially useful for imaging of recurrent low-grade tumors and for distinguishing tumor recurrence from radiation necrosis.