Assessment of Treatment Response in Patients with Glioblastoma Using O-(2-18F-Fluoroethyl)-l-Tyrosine PET in Comparison to MRI

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. Methods: 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 (TBRmax and TBRmean, 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 (Tvol 1.6) were calculated using threshold-based volume-of-interest analyses. The patients were grouped into responders and nonresponders according to the changes of these parameters at different cutoffs, and the influence on progression-free survival and overall survival was tested using univariate and multivariate survival analyses and by receiver-operating-characteristic analyses. Results: Early after completion of RCX, a decrease of both TBRmax and TBRmean was a highly significant and independent statistical predictor for progression-free survival and overall survival. Receiver-operating-characteristic analysis showed that a decrease of the TBRmax between FET-1 and FET-2 of more than 20% predicted poor survival, with a sensitivity of 83% and a specificity of 67% (area under the curve, 0.75). Six to eight weeks later, the predictive value of TBRmax and TBRmean was less significant, but an association between a decrease of Tvol 1.6 and PFS was noted. In contrast, Gd-volume changes had no significant predictive value for survival. Conclusion: In contrast to Gd-volumes on MRI, changes in 18F-FET PET may be a valuable parameter to assess treatment response in glioblastoma and to predict survival time.