Abstract BACKGROUND The phase 2 REGOMA trial suggested that glioblastoma patients treated with the multikinase inhibitor regorafenib at first progression have significantly longer overall survival than patients treated with lomustine. Here, we evaluated the value of O-(2-[18F]fluoroethyl)-L-tyrosine (FET) PET and MRI for the evaluation of response to regorafenib in recurrent glioma patients. MATERIAL AND METHODS Twenty patients (age range, 30-72 years) with CNS WHO grade 3 or 4 gliomas (glioblastoma, 85%) at recurrence were treated according to the REGOMA trial with regorafenib at 160 mg/d for the first three weeks of each 4-week cycle, with individual dose adjustment according to toxicity. FET PET and MRI were performed at baseline and follow-up after the second cycle. Static FET PET parameters comprised tumor-to-brain ratios (TBR) and metabolic tumor volumes (MTV). Parameters derived from dynamic FET PET acquisition were time-to-peak and slope. MRI response assessment was based on RANO criteria. Additionally, mean and minimum apparent diffusion coefficients (ADC) were obtained from diffusion-weighted MRI. FET PET parameters and ADC thresholds were defined using ROC analyses to predict an overall survival of ≥ 6 months. The predictive values of FET PET parameters, ADC values, and RANO criteria were subsequently evaluated using univariate and multivariate survival estimates. RESULTS Patients received a median of three regorafenib cycles (range, 2-10 cycles). After treatment initiation, the median follow-up was 8.6 months (range, 3.2-27.3 months). The median number of previous treatment lines was two (range, 1-4 lines). At the time of data analysis, 17 patients (85%) had died. After two cycles of regorafenib, a regorafenib-induced reduction of mean TBR values by ≥ 10% predicted a longer overall survival (9.9 vs. 5.3 months; P=0.023). Interestingly, absolute mean TBR values ≤ 2.0 at follow-up were prognostic and associated with longer OS (10.6 vs. 4.5 months; P=0.007). In contrast, MTV, dynamic PET parameters, RANO criteria, and ADC values were not predictive for response or prognostic (all P>0.05). Multivariate survival analyses revealed that relative mean TBR changes were most potent in predicting response to regorafenib (P=0.038; HR, 0.246) and absolute mean TBR values for prognostication (P<0.001; HR, 0.005). CONCLUSION Data suggest that FET PET parameters are clinically valuable for identifying responders to regorafenib early after treatment initiation and helpful for prognostication. MRI parameters, including ADC values, do not appear to add valuable information in this context.