BACKGROUND AND PURPOSE: To evaluate the patterns of failure following clinical introduction of amino-acid O-(2-[18F]fluoroethyl)-L-tyrosine (FET)-PET-guided target definition for radiotherapy (RT) of glioblastoma patients.

MATERIALS AND METHODS: The first 66 consecutive patients with confirmed histology, scanned using FET-PET/CT and MRI were selected for evaluation. Chemo-radiotherapy was delivered to a volume based on both MRI and FET-PET (PETvol). The volume of recurrence (RV) was defined on MRI data collected at the time of progression according to RANO criteria.

RESULTS: Fifty patients were evaluable, with median follow-up of 45months. Central, in-field, marginal and distant recurrences were observed for 82%, 10%, 2%, and 6% of the patients, respectively. We found a volumetric overlap of 26%, 31% and 39% of the RV with the contrast-enhancing MR volume, PETvol and the composite MRPETvol, respectively. MGMT-methylation (p=0.03), larger PETvol (p<0.001), and less extensive surgery (p<0.001), were associated with larger PETvol overlap.

CONCLUSION: The combined MRPETvol had a stronger association with the recurrence volume than either of the modalities alone. Larger overlap of PETvol and RV was observed for patients with MGMT-methylation, less extensive surgery, and large PETvol on the RT-planning scans.