P03.05.A Radiation-induced leukoencephalopathy (RIL) in glioma: unique injury dynamics following proton vs photon beam radiotherapy

Abstract Background White matter injury after brain-directed radiotherapy (RT), aka radiation-induced leukoencephalopathy (RIL), is common in brain tumor patients. Differentiation from progressive disease can be challenging. Dosimetric advantages of protons (PRT) over photons (XRT) minimize radiation to healthy brain tissue, potentially limiting radiotoxic sequelae including RIL. We characterized RIL during periods of progression-free survival (PFS) in glioma patients irradiated with either PRT or XRT, hypothesizing that PRT would result in reduced RIL outside of the target field. Material and Methods 34 patients (19 male; mean age = 40.10y) with grade 2/3 gliomas and a history of partial cranial RT were stratified by RT modality [XRT (n=17) vs PRT (n=17)] and matched on 11 criteria [age, sex, tumor type/location/laterality, mutational status (IDH; 1p19q deletion), concurrent/adjuvant chemotherapy, radiation dose/fractions] for retrospective analysis. RIL development was characterized longitudinally for up to 3 years post-RT via analysis of serial MRI T2/FLAIR sequences. A novel RIL scoring system with embedded Fazekas scale was designed to quantify injury severity at both global (whole brain) and hemispheric levels. Results Matched groups did not differ significantly on any demographic or clinical characteristics. Median PFS post-RT was 4.7 (XRT) and 5.1 (PRT) years. The novel RIL scoring system was reliable (intraclass correlation coefficient >0.9). There was a significant increase in global RIL in both XRT [F(3, 57)=8.63, p< .001] and PRT [F(3, 61)=4.69, p< .005] groups over time, relative to baseline (1-month post-RT). A majority [62% (XRT) and 72% (PRT)] developed moderate or severe RIL within 3 years, with the ipsilesional hemisphere more severely affected. Analysis of RIL injury dynamics (i.e., average % change between 1 and 3 years post-RT) at hemispheric level identified radiation modality-specific differences: XRT resulted in greater contralesional hemispheric injury than PRT [F(1, 31)=4.32, p