RADT-23. MODALITY-SPECIFIC CNS INJURY FOLLOWING PROTON VS PHOTON BEAM RADIOTHERAPY IN GLIOMA

Abstract BACKGROUND CNS injury following brain-directed radiotherapy (RT) is common and may mimic disease progression. Dosimetric advantages of protons (PRT) over photons (XRT) minimize radiation to healthy brain, potentially limiting radiotoxic sequelae. We characterized radiotoxicity i.e., radiation-induced leukoencephalopathy (RIL), brain tissue necrosis (TN), cerebral microbleeds (CMB), during progression-free survival (PFS) periods in glioma patients irradiated with PRT or XRT. METHODS 34 (19 male; mean age = 40.10y) grade 2/3 gliomas treated by partial cranial RT were stratified modality [XRT(n 17) vs PRT(n 17)] matched on 11 criteria [age, sex, tumor type/location/laterality, mutational status (IDH; 1p19q deletion), concurrent/adjuvant chemotherapy, dose/fractions] for retrospective analysis. Radiotoxicity was longitudinally until 3 years post-RT via analysis serial MRI T2/FLAIR- T1+Contrast- Susceptibility (CMB)-weighted sequences. RIL rated using a novel scoring system embedded Fazekas scale (intraclass correlation coefficient > 0.9), at global (whole-brain) hemispheric levels. RESULTS Analysis delayed identified significant modality-specific differences. While both groups developed moderate-to-severe [62% 72% (PRT)], XRT resulted greater dynamics (i.e., average % change between 1 post-RT) the contralesional hemisphere [F(1, 31) 4.32, p < .05]. Conversely, associated higher TN incidence [6%(XRT) 18%(PRT) z - 2.22, 0.03, two-tailed]. Finally, while CMB [76%(XRT) 71%(PRT)] burden [mean #CMB: 4.3(XRT) 4.2(PRT)] comparable, lesion-to-radiation field (RF)-correlation preferential clustering RF margins [X2(2, N =200)= 8.8, .02]. CONCLUSIONS progressive patients. Injury patterns suggest specificity as RIL, TN, exhibit unique spatiotemporal differences PRT, likely reflecting dosimetric biological photons. The impact such sequelae cognition subject current investigation.