RT-07 * APPLICATION OF A GROWTH-RATE BASED RESPONSE METRIC TO RECURRENT MALIGNANT GLIOMAS TREATED WITH LARGE-VOLUME RE-IRRADIATION USING PROTON BEAM THERAPY

Abstract

BACKGROUND: The Days Gained (DG) response metric, which incorporates knowledge of the tumor growth rate, has been shown to correlate with progression-free (PFS) and overall survival (OS) following concurrent chemoradiotherapy for newly diagnosed glioblastoma (Neal et al., 2013 PLOS One; Neal et al., 2013 Cancer Research). We describe the first use of this metric to analyze radiographic response of recurrent malignant gliomas to large-volume re-irradiation using proton beam therapy (PBT). METHODS: The equation described in Neal, et al., 2013 was utilized to derive the DG score for 8 patients with histologically confirmed, contrast-enhancing recurrent gliomas. For each patient, the DG response metric was calculated from the enhancing T1 + Gad sequences of 2 MRIs immediately prior to PBT and the first MRI following completion of PBT. OS time from PT start was estimated with Kaplan-Meier analysis; comparisons used log-rank statistic. RESULTS: Median age was 43, median KPS score was 90, and median number of salvage treatments was 2. The mean DG score was 78.8 (range: -32.3-276.8). Concomitant use of temozolomide and bevacizumab during PBT was associated with a higher mean DG score (191.2 vs. 11.4; p = 0.03). In addition, association between higher DG score and superior OS trended to statistical significance (HR = 0.986, p = 0.08). CONCLUSIONS: First analysis of radiographic response of recurrent malignant gliomas to re-irradiation using PBT supports a possible association between DG and OS and the prolongation of DG with the concomitant use of temozolomide and bevacizumab during re-irradiation. Further analysis with larger sample size remains ongoing.