An Intra-Individual Comparison of MRI, 18F-FET and 18F-FLT PET in Patients with High-Grade Gliomas

OBJECTIVES: Intra-individual spatial overlap analysis of tumor volumes assessed by MRI, the amino acid PET tracer 18F-FET and the nucleoside PET tracer 18F-FLT in high-grade gliomas (HGG). METHODS: MRI, 18F-FET and 18F-FLT PET data sets were retrospectively analyzed in 23 HGG patients. Morphologic tumor volumes on MRI (post-contrast T1 (cT1) and T2 images) were calculated using a semi-automatic image segmentation method. Metabolic tumor volumes for 18F-FET and 18F-FLT PETs were determined by image segmentation using a threshold-based volume of interest analysis. After co-registration with MRI the morphologic and metabolic tumor volumes were compared on an intra-individual basis in order to estimate spatial overlaps using the Spearman's rank correlation coefficient and the Mann-Whitney U test. RESULTS: 18F-FLT uptake was negative in tumors with no or only moderate contrast enhancement on MRI, detecting only 21 of 23 (91%) HGG. In addition, 18F-FLT uptake was mainly restricted to cT1 tumor areas on MRI and 18F-FLT volumes strongly correlated with cT1 volumes (r = 0.841, p<0.001). In contrast, 18F-FET PET detected 22 of 23 (96%) HGG. 18F-FET uptake beyond areas of cT1 was found in 61% of cases and 18F-FET volumes showed only a moderate correlation with cT1 volumes (r = 0.573, p<0.001). Metabolic tumor volumes beyond cT1 tumor areas were significantly larger for 18F-FET compared to 18F-FLT tracer uptake (8.3 vs. 2.7 cm3, p<0.001). CONCLUSION: In HGG 18F-FET but not 18F-FLT PET was able to detect metabolic active tumor tissue beyond contrast enhancing tumor on MRI. In contrast to 18F-FET, blood-brain barrier breakdown seems to be a prerequisite for 18F-FLT tracer uptake.