RADT-17. FRACTIONATED REIRRADIATION OF RECURRENT IDH WILDTYPE GLIOBLASTOMA: HIGHER REIRRADIAITON DOSE AND SAFETY

Abstract The optimal regimen, normal tissue tolerances, and appropriate indications for reirradiation for recurrent glioblastoma (GBM) are uncertain. We evaluate outcomes and toxicity after fractionated reirradiation. Patients with recurrent IDH wildtype GBM after prior standard radiotherapy during 2007-2022 were retrospectively reviewed. All patients initially received conventionally fractionated radiotherapy. Radiation necrosis (RN) was determined by imaging or pathology. Overall survival (OS) was estimated using Kaplan-Meier methodology. Univariate and multivariate Cox proportional hazard models for OS were constructed. Bivariable associations with RN were estimated using fisher exact tests. 99 patients (median age 56 yrs, median KPS 80) were treated with reirradiation (median followup 7.4 mo). Median time between initial and reirradiation was 20 months (range, 2 – 214 mo). Median received dose at reirradiation was 41.4 Gy (30.6 – 60 Gy) to a median planning target volume (PTV) of 183 cc (16 – 825 cc). Median cumulative maximum dose to the optics and brainstem were 54 Gy (4 – 103 Gy) and 76 Gy (5 – 146 Gy), respectively. Median OS was 9 mo (95% CI 6.2 – 10.6 mo). On univariate analysis, time between initial and reirradiation ≥24 mo (HR 0.58, CI 0.36 – 0.94) and reirradiation dose ≥41.4 Gy (HR 0.56, CI 0.35 – 0.88) were associated with longer OS. On multivariate analysis, interval between radiation sessions (HR 0.56, CI 0.34 – 0.91) and higher reirradiation dose (HR 0.55, CI 0.35 – 0.87) remained associated with OS. No cases of radiation injury to the optics or brainstem were reported. 10% of patients experienced RN. Reirradiation dose ≥41.4 Gy, reirradiation PTV ≥200 cc and interval between radiation sessions < 24 mo were not significantly associated with RN. These data support the safety and efficacy of fractionated reirradiation for recurrent GBM including for large tumors. This study suggests that higher reirradiation dose may be feasible, including for large treatment volumes.