18F-GE-180 TSPO PET in High-Grade Glioma RT Planning

The translocator protein (TSPO) is overexpressed in high-grade glioma and can be visualized in vivo by TSPO PET even in areas without MRI contrast enhancement. Here, we assessed the value of TSPO PET using the high-affinity TSPO ligand 18F-GE-180 in primary irradiation (RT) and re-irradiation (reRT) treatment planning.Radiation treatment plans of high-grade glioma patients, examined with the TSPO PET tracer 18F-GE-180 prior to RT and reRT between 08/2016 and 01/2019, were included. PET based biological tumor volumes (BTV) were built on basis of tumor-to-background activity thresholds of 1.6, 1.8, and 2.0. Consensus GTVs (cGTV) based on GTV volumes delineated on MRI by 4 independent observers were generated using the simultaneous truth and performance level estimation (STAPLE) algorithm. Conformity of cGTV with BTV1.6, BTV1.8 and BTV2.0 were assessed by Sørensen-Dice coefficient (SDC) and conformity index (CI). Minimal clinical target volume (CTV) margins including the whole BTV into the cGTV were calculated.Target volumes of 35 RT and 16 reRT plans were assessed. Volumes of cGTVs were smaller than the corresponding volumes of BTV1.6, BTV1.8 and BTV2.0 in the primary RT plans (median volumes 22.6 vs. 67.4, 50.7, 39.1 cm³; P < 0.001, P < 0.001, P = 0.018; Wilcoxon-Test) and reRT plans (median volumes 22.7 vs. 80.5, 55.0, 41.6 cm³; P = 0.001, P = 0.007, P = 0.134; Wilcoxon-Test). Conformity between BTVs and cGTV calculated by SDC and CI for thresholds 1.6, 1.8 and 2.0 were low in median in the RT (SDC: 0.51, 0.55, 0.58; CI: 0.35, 0.38, 0.41) and reRT setting (SDC: 0.38, 0.40, 0.40; CI 0.24, 0.25, 0.25). Median minimal CTV margins to include the entire BTV in the expanded cGTV volume was smaller in the RT setting (16, 12, 10 mm) than in the reRT setting (> 20, 17.5, 11 mm) for thresholds 1.6, 1.8 and 2.0.TSPO PET based BTVs showed highest conformity and best comparability in terms of size with MRI based GTVs for the threshold 2.0 both in the primary and reRT setting. Recurrence pattern analyses are needed to assess the clinical relevance of TSPO PET in glioma RT and reRT treatment planning.D.F. Fleischmann: None. M. Buettner: None. M. Unterrainer: None. S. Corradini: None. B. Zollner: None. J. Hofmaier: None. R. Bodensohn: None. N. Thon: None. C. Belka: None. P. Bartenstein: None. N. Albert: None. M. Niyazi: None.