MRI and 11C-methyl-L-methionine PET Differentiate Bevacizumab True Responders After Initiating Therapy for Recurrent Glioblastoma.

Purpose 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.