Normalization of tumor vasculature after administering bevacizumab (BEV) makes assessment of therapeutic response using MRI difficult. The aim of this study was to clarify whether PET with 11C-methyl-L-methionine (MET-PET) would supplement MRI assessing of response after initiating BEV in glioblastoma.Methods
Twenty patients with recurrent glioblastoma were treated with biweekly BEV plus temozolomide. Both MRI and MET-PET were performed before treatment (baseline) and at 4 and 8 weeks after starting treatment. Results on MRI (response or nonresponse) were compared with those on MET-PET, with response defined as a tumor-to-normal brain ratio of SUV (SUVT/N) of less than 1.6. Progression-free survival (PFS) was compared between responders and nonresponders on MRI alone and MET-PET alone. Progression-free survival was also compared between patients showing response on both MRI and MET-PET and patients showing response on MRI but nonresponse on MET-PET at each time point.Results
PFS was significantly longer in responders than nonresponders on both MRI at 4 and 8 weeks and MET-PET at 8 weeks, whereas MET-PET at 4 weeks provided no information regarding outcomes. Combined assessment with MRI and MET-PET at 4 weeks was not provide predictive of PFS, whereas patients showing response on both modalities (true responders) at 8 weeks exhibited significantly longer PFS than did patients showing response on MRI but nonresponse on MET-PET (pseudoresponders).Conclusions
Combined assessment with MRI and MET-PET at 8 weeks can differentiate true responders who are predicted to show more favorable prognosis from pseudoresponders.