ESTRO 2025 - Abstract Book

S693

Clinical - CNS

ESTRO 2025

Results: In total, in 13 primary studies, 6 studies were interpreted as radionecrosis (TBRmax up to 2.4), 2 as disease progression (TBRmax 3.2) , 5 could not be interpreted unambiguously according to current guidelines and required dynamic follow-up. In two cases, we encountered cases of necrosis outside the original high volume (D95%). In both cases two fields were used, in one case coplanar. In both cases, radionecrosis occurred in the paraventricular regions of the lobar lobes (TBRmax 2.3).

Conclusion: 1. TBRmax in cases diagnosed as radionecrosis is higher than in guidelines 2. Comparison with the radiation plan is important. 3. PET/CT before treatment to assess tumor metabolic activity - highly desirable

Keywords: Proton therapy, PET, glioblastoma

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Poster Discussion Tumor Dynamics during MR-guided Online Adaptive Radiotherapy for Glioblastoma and the Effect on Survival Yuanyuan Chen, Shouliang Ding, Youmao Yin, Rui Li, Hongdong Liu, Xiaoyan Huang, Yonggao Mou Radiation Oncology, Sun Yat-sen University Cancer Center, Guangzhou, China Purpose/Objective: This study aimed to assess the feasibility and potential benefits of MR-guided online adaptive radiotherapy (MRgOART) for patients with glioblastoma. Material/Methods: There were 95 consecutive patients with glioblastoma treated with MRgOART of 60 Gy in 30 fractions by the 1.5 T MR-Linac. The MRgOART fractions employed daily MR scans and the contours were utilized to create each adapted plan by “adapt to shape” workflow. The gross tumor volume (GTV) and clinical target volume (CTV) were delineated on MRI of pre-treatment simulation (Fx0) and all fractions (Fx1, Fx2, Fx3 … Fx30) to evaluate the inter-fractional changes for 22 patients. Tumor dynamics assessed included absolute/relative volume (∆V), Dice similarity coefficient (DSC) and Hausdorff distance (HD) metrics. Covariables evaluated included: diagnostic type, extent of surgery, O6 methylguanine-DNA-methyltransferase methylation (MGMT), and isocitrate dehydrogenase mutation (IDH) status. Additionally, the pattern of failure as primary endpoint was evaluated in this study. The secondary endpoint included progression-free survival (PFS) and overall survival (OS) according to treatment schedule (long versus short course). Results: The tumor had obvious pre-treatment changes between simulation and first treatment (GTV: DSC: 0.67~0.94, HD: 3.76~22.24mm, ∆V: -42.27%~72.25%; CTV: DSC: 0.84~0.99, HD: 1.93~20.28mm, ∆V: -20.71%~15.93%). The changes were related to gap time between simulation and first treatment and the more obvious beyond 8 days. The results showed that large inter-fractional changes for GTV limited the efficacy of radiation therapy (DSC: 0.78~0.90, HD: 5.9~19.3mm, ∆V: -26.9%~9.71%). The inter-fractional CTV changes were smaller (DSC: 0.88~0.94, HD:4.9~9.6mm, ∆V: -2.43%~1.29%). The patients who had gross total resection, MGMT promoter methylated, IDH wild-type had significant tumor dynamics. GTV coverage of non-adaptive plans was below the prescribed coverage in 38% fractions, with 15% failing by more than 10%. For CTV coverage of non-adaptive plans, the changes were less than 5%. Online adaptative plans improved GTV and CTV coverage significantly (p ＜ 0.001) to 99%. The adaptive plans