S368
Clinical - CNS
ESTRO 2026
et de services sociaux de Chaudière-Appalaches, Levis, Quebec, Canada. 4 Medical Oncology, Austin Health, Melbourne, Australia. 5 Diagnostic Imaging, Austin Health, Melbourne, Australia. 6 Medical Radiations, RMIT, Melbourne, Australia. 7 Molecular Imaging and Therapy, Austin Health, Melbourne, Australia Purpose/Objective: Glioblastoma (GBM) changes during chemoradiation are typically only assessed using pre- and post- treatment magnetic resonance imaging (MRI). MRI- linear accelerator (MR-Linac) enables real-time daily monitoring of tumour dynamics throughout treatment. This study aims to quantify daily interfraction variations in tumour volume and position during treatment. Material/Methods: GBM patients suitable for chemoradiation using a 1.5T MR-Linac were enrolled in a prospective study. T2/FLAIR hyperintensity regions were contoured at baseline, every second fractions, and at one month post-radiotherapy. Tumour dynamics were assessed using absolute volume, percentage change in volume (Vrel) and migration distance (dmigrate) representing the linear displacement of volume relative to fraction 0. Tumour dynamics were compared between patients with early progression (within 6 months) and those without. Comparisons were also made between patients with methylated versus unmethylated MGMT promoter status. Results: Thirty-three patients were included, with 11 receiving 15-fraction and 22 receiving 30-fraction regimens. Twenty-eight had ≥ 6 months of follow-up; 12 (36%) were classified as early progressors. Median absolute tumour volumes at key timepoints were 44 cm3 (F0), 65 cm3 (F15), and 71 cm3 (F30). Corresponding median Vrel were 2% (F1), 5% (F10), and 18% (F30). Patients with biopsy-only and subtotal resection (STR) had higher Vrel during treatment – 22.2% at F20 and 49.2% at F30 for biopsy-only; 4.4% at F20 and 13.8% at F30 for STR – whereas those with gross total resection had declining Vrel (-8.2% at F15, -4.4% at F20, -4.4% at F30). Early progressors showed a consistent volume increase, with Vrel of 47% (F15) and 72% (F30), compared to -10%and -14% in non-progressors (p<0.05 across all timepoints). Methylated tumours had stable volumes with Vrel of 4% (F30), while unmethylated tumors increased to 44% (F30). When stratified by both progression and MGMT status, median Vrel at F15 and F30 were 47% and 91% in unmethylated early progressors versus -11% and -30% unmethylated non-progressors (p<0.05). Median dmigrate increased from 0.8 (F1) to 5.6 (F30) mm, with no significant differences by progression or methylation status. At F15 and F30, 9 (27%) and 12 (36%) patients had dmigrate >5mm; five patients (15%)
higher dosimetric values: medianGTV 1.3 vs 0.1 cm ³ (p<0.0001) and median V12Gy 4.9 vs 0.8 cm ³ (p=0.0002). Lesions with V12Gy ≥ 5 cm ³ had a higher rate of symptomatic RN compared to those with V12Gy <5 cm ³ (p = 0.02).No significant difference in the incidence of symptomatic RN was observed according to systemic therapy type (immunotherapy vs TKI, p=0.47) or the timing of systemic therapy relative to GK radiosurgery (p=0.08). Local control was 100% across all lesions. Conclusion: In this lesion-based analysis, symptomatic radionecrosis occurred in 4.9% of treated lesions, despite a 100% local control rate. Both V12Gy (cm ³ ) and GTV (cm ³ )were significantly associated with symptomatic RN, highlighting a dose– volume effect even in the era of modern systemic therapies. Given the excellent local efficacy observed, these findings support the need for prospective dose de-escalation studiesto identify thresholds that balance toxicity risk and tumor controlin patients receiving immunotherapy or targeted therapy. References: - Walker HB, Casey K, Ricci S, et al. A systematic review and meta-analysis of radiation necrosis incidence in brain metastases treated by Gamma Knife and CyberKnife. World Neurosurg. 2025;202:124390. doi:10.1016/j.wneu.2025.124390.- Martin AM, Cagney DN, Catalano PJ, et al. Immunotherapy and symptomatic radiation necrosis in patients with brain metastases treated with stereotactic radiation. JAMA Oncol. 2018;4(8):1123–1124. doi:10.1001/jamaoncol.2017.3993- Vaios EJ, Shenker RF, Hendrickson PG, et al. Symptomatic necrosis with dual immune-checkpoint inhibition and radiosurgery for brain metastases. JAMA Netw Open. 2025;8(4):e254347.
doi:10.1001/jamanetworkopen.2025.4347 Keywords: Radionecrosis - GammaKnife - Immunotherapy
Digital Poster Highlight 1956 Glioblastoma Tumour Dynamics During Chemoradiation on the 1.5T Magnetic Resonance Imaging-Linear Accelerator Sweet Ping Ng 1,2 , Kurl Jamora 1 , Dominique Mathieu 3 , Su Chen Fong 1,2 , Morikatsu Wada 1 , Richard Khor 1 , Mark Tacey 1 , Benjamin Harris 1 , Sandra Fisher 1 , Brayden Geary 1 , Hui Gan 4,2 , Lawrence Cher 4 , Eddie Lau 5 , Daryl Wilding-McBride 6 , Ricky O'Brien 6 , Andrew M Scott 2,7 1 Radiation Oncology, Austin Health, Melbourne, Australia. 2 Tumour Targeting Laboratory, Olivia Newton-John Cancer Research Institute, Melbourne, Australia. 3 Radiaton Oncology, Centre intégré de santé
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