The imaging substudy of the randomized ARTE trial: MRI and 18FET PET associations with overall survival benefit from bevacizumab in elderly patients with newly diagnosed IDH wildtype glioblastoma.

2520 Background: Bevacizumab failed to demonstrate overall survival benefit despite markedly prolonged progression-free survival in glioblastoma patients. Reasons for this divergence may include suboptimal patient selection and delayed diagnosis of progression on MRI scans under bevacizumab. Imaging analyses of retrospective and uncontrolled clinical trial cohorts suggest MRI diffusion mapping as a predictor of benefit from bevacizumab. Moreover, amino acid PET has been proposed by the RANO working group for the differentiation of tumor versus edema or gliosis based on proof-of-principle studies demonstrating earlier detection of progression with PET compared to MRI. Methods: ARTE (NCT01443676) was a 2:1 randomized, multi-center, open-label trial of hypofractionated radiotherapy in combination with intravenous bevacizumab every 2 weeks (BEV/RT) versus RT alone in patients with newly diagnosed glioblastoma aged 65 years or older. Patients with histologically and molecularly confirmed IDH wildtype glioblastoma aged 65 years or older were analyzed. MRI was available from 67 and serial 18FET PET from 30 patients in this post hoc analysis. 18FET PET intensity ratios and herein reported MRI parameters including tumor volumetric analyses and ADC were analyzed blinded for outcome and study arm. Results: Demographic, clinical and molecular parameters were balanced between treatment arms. Overall survival benefit from bevacizumab was observed for larger contrast-enhancing tumor volumes (hazard ratio [HR] per cm3 0.94, 95% CI 0.89-0.99, p = 0.032) and higher ADC (HR 0.18, 95% CI 0.05-0.66, p = 0.025) on pre-treatment MRI. Response in the BEV/RT arm by the standard MRI-based RANO criteria was associated with overall survival by trend (HR 0.56, 95% CI 0.30-1.10, time-dependent p = 0.094). In a multivariate model controlling for established risk factors, 18FET tumor-to-brain uptake ratios (TBR) of non-contrast-enhancing tumor portions predicted inferior overall survival specifically in the BEV/RT arm (HR [per 0.1 18FET TBR] 1.50, 95% CI 1.05-2.13, time-dependent p = 0.025). Controlling for 18FET TBR at first follow-up identified benefit from BEV/RT by trend (HR 0.41, 95% CI 0.16-1.07, p = 0.069). Conclusions: Large contrast-enhancing tumor mass and high ADC identify patients with overall survival benefit from bevacizumab. Under bevacizumab, non-contrast enhancing tumor portions can be adequately monitored by amino acid PET.