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The assessment of treatment response in glioblastoma is difficult with MRI because reactive blood–brain barrier alterations with contrast enhancement can mimic tumor progression. In this study, we investigated the predictive value of PET using O-(2-¹⁸F-fluoroethyl)-l-tyrosine (¹⁸F-FET PET) during treatment. In a prospective study, 25 patients with glioblastoma were investigated by MRI and ¹⁸F-FET PET after surgery (MRI-/FET-1), early (7–10 d) after completion of radiochemotherapy with temozolomide (RCX) (MRI-/FET-2), and 6–8 wk later (MRI-/FET-3). Maximum and mean tumor-to-brain ratios (TBR_max and TBR_mean, respectively) were determined by region-of-interest analyses. Furthermore, gadolinium contrast-enhancement volumes on MRI (Gd-volume) and tumor volumes in ¹⁸F-FET PET images with a tumor-to-brain ratio greater than 1.6 (T_{vol 1.6}) were calculated using threshold-based volume-of …