18F-FET PET prior to recurrent high-grade glioma re-irradiation-additional prognostic value of dynamic time-to-peak analysis and early static summation images?

Most high-grade gliomas (HGG) recur after initial multimodal therapy and re-irradiation (Re-RT) has been shown to be a valuable re-treatment option in selected patients. We evaluated the prognostic value of dynamic time-to-peak analysis and early static summation images in O-(2-18F-fluoroethyl)-l-tyrosine (18F-FET) PET for patients treated with Re-RT ± concomitant bevacizumab. We retrospectively analyzed 72 patients suffering from recurrent HGG with 18F-FET PET prior to Re-RT. PET analysis revealed the maximal tumor-to-background-ratio (TBRmax), the biological tumor volume, the number of PET-foci and pattern of time-activity-curves (TACs; increasing vs. decreasing). Furthermore, the novel PET parameters early TBRmax (at 5-15 min post-injection) and minimal time-to-peak (TTPmin) were evaluated. Additional analysis was performed for gender, age, KPS, O6-methylguanine-DNA methyltransferase methylation status, isocitrate dehydrogenase 1 mutational status, WHO grade and concomitant bevacizumab therapy. The influence of PET and clinical parameters on post-recurrence survival (PRS) was investigated. Shorter TTPmin was related to shorter PRS after Re-RT with 6 months for TTPmin < 12.5 min, 7 months for TTPmin 12.5-25 min and 11 months for TTPmin >25 min (p = 0.027). TTPmin had a significant impact on PRS both on univariate (p = 0.027; continuous) and multivariate analysis (p = 0.011; continuous). Other factors significantly related to PRS on multivariate analysis were increasing vs. decreasing TACs (p = 0.008) and Karnofsky Performance Score (p = 0.015; <70 vs. ≥70). Early TBRmax as well as the other conventional PET parameters were not significantly related to PRS on univariate analysis. Dynamic 18F-FET PET with TTPmin provides a high prognostic value for recurrent HGG prior to Re-RT, whereas early TBRmax does not. Dynamic 18F-FET PET using TTPmin might help to personalize Re-RT treatment regimens in future through voxelwise TTPmin analysis for dose painting purposes and PET-guided dose escalation.