Early static 18F-FET-PET scans have a higher accuracy for glioma grading than the standard 20–40 min scans

Abstract

Purpose: Current guidelines for glioma imaging by positron emission tomography (PET) using the amino acid analogue O-(2-[18F]fluoroethyl)-L-tyrosine (18F-FET) recommend image acquisition from 20–40 min post injection (p.i.). The maximal tumour-to-background evaluation (TBRmax) obtained in these summation images does not enable reliable differentiation between low and high grade glioma (LGG and HGG), which, however, can be achieved by dynamic18F-FET-PET. We investigated the accuracy of tumour grading using TBRmax values at different earlier time points after tracer injection. Methods: Three hundred and fourteen patients with histologically proven primary diagnosis of glioma (131 LGG, 183 HGG) who had undergone 40-min dynamic 18F-FET-PET scans were retrospectively evaluated. TBRmax was assessed in the standard 20–40 min summation images, as well as in summation images from 0–10 min, 5–15 min, 5–20 min, and 15–30 min p.i., and kinetic analysis was performed. TBRmax values and kinetic analysis were correlated with histological classification. ROC analyses were performed for each time frame and sensitivity, specificity, and accuracy were assessed. Results: TBRmax values in the earlier summation images were significantly better for tumour grading (P < 0.001) when compared to standard 20–40 min scans, with best results for the early 5–15 min scan. This was due to higher TBRmax in the HGG (3.9 vs. 3.3; p < 0.001), while TBRmax remained nearly stable in the LGG (2.2 vs. 2.1). Overall, accuracy increased from 70 % in the 20–40 min analysis to 77 % in the 5–15 min images, but did not reach the accuracy of dynamic analysis (80 %). Conclusions: Early TBRmax assessment (5–15 min p.i.) is more accurate for the differentiation between LGG and HGG than the standard static scan (20–40 min p.i.) mainly caused by the characteristic high 18F-FET uptake of HGG in the initial phase. Therefore, when dynamic 18F-FET-PET cannot be performed, early TBRmax assessment can be considered as an alternative for tumour grading.