To evaluate whether response assessment of newly diagnosed glioblastoma at 3 months using 11 C-methionine-positron emission tomography (MET-PET) is better associated with patient outcome compared with baseline MET-PET or anatomic magnetic resonance imaging alone. Methods and Materials: Patients included were participants in a phase I/II trial of dose-escalated chemoradiation based on anatomic magnetic resonance imaging. Automated segmentation of metabolic tumor volume (MTV) was performed at a threshold of 1.5 times mean cerebellar uptake. Progression-free (PFS) and overall survival were estimated with the Kaplan-Meier method and compared with log-rank tests. Multivariate analysis for PFS and overall survival was performed using Cox proportional hazards, and spatial overlap between imaging and recurrence volumes were analyzed. Results: Among 37 patients, 15 had gross total resection, of whom 10 (67%) had residual MTV, 16 subtotal resection, and 6 biopsy alone. Median radiation therapy dose was 75 Gy (range, 66-81). Median baseline T1 Gd-enhanced tumor volume (GTV-Gd) was 38.0 cm 3 (range, 8.0-81.5). Median pre-CRT MTV was 4.9 cm 3 (range, 0-43.8). Among 25 patients with 3-month MET-PET, MTV was only 2.4 cm 3 (range, 0.004-18.0) in patients with uptake. Patients with MTV Z 0 cm 3 at 3 months had superior PFS (18.2 vs 10.1 months,

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