ESTRO 2025 - Abstract Book

S694

Clinical - CNS

ESTRO 2025

also had lower dose to whole brain than non-adaptive plans (p ＜ 0.001). Mean PFS was 7.4 mo and the central failure events were 50%.

Conclusion: Significant inter-fractional tumor changes could be found during radiotherapy in patients with glioblastoma treated by 1.5 T MR-Linac. The changes of target were related to the extent of surgery, MGMT and IDH. Daily MR-guided re optimization of treatment plans corrected for day-to-day anatomical variations and resulted in adequate target coverage in all fractions. MRgOART had low risk of central failure without compromising PFS.

Keywords: Glioblastoma ， Adaptive Radiotherapy ， MR-Linac

2261

Digital Poster Randomized Phase II Trial: Hippocampal Avoidance Whole Brain Radiotherapy With or Without Simultaneous Integrated Boost in Brain Metastases Brendan Seng Hup Chia 1,2 , Jing Yun Leong 3 , Siqin Zhou 4 , Amitayus Lim 2 , Kevin Lee Min Chua 2 , Eu Tiong Chua 2 , Fuh Yong Wong 2 , Melvin Lee Kiang Chua 2 , Tih Shih Lee 5 1 Raffles Cancer Centre, Raffles Hospital, Singapore, Singapore. 2 Division of Radiation Oncology, National Cancer Centre Singapore, Singapore, Singapore. 3 Department of Psychiatry, Changi General Hospital, Singapore, Singapore. 4 Division of Clinical Trials and Epidemiological Sciences, National Cancer Centre Singapore, Singapore, Singapore. 5 Signature Research Programme in Neuroscience & Behavioural Disorders, Duke-NUS Medical School, Singapore, Singapore Purpose/Objective: Hippocampal avoidance during whole brain radiotherapy (HA-WBRT) is a recommended treatment option for patients with multiple brain metastases and a good prognosis. The reported intracranial progression-free survival (PFS) with this approach is approximately 5–5.3 months 1 . This study aimed to assess the role of delivering a simultaneous integrated boost (SIB) to brain metastases in improving intracranial PFS. Material/Methods: We conducted a prospective, randomized phase II trial in patients with multiple brain metastases (4–25 lesions) who had a physician-assessed life expectancy of >6 months and were unsuitable for radiosurgery 2 . Patients were randomized to receive HA-WBRT at 30 Gy, with or without an SIB of 40–45 Gy to brain metastatic lesions. The target recruitment was 100 patients from the National Cancer Centre Singapore. The primary endpoint was the control of target brain metastatic lesions. Patients were followed up for one year, and outcome data were collected at predefined intervals. Results: Between June 2020 and August 2023, 36 patients were randomized: 18 to each arm. The study was closed prematurely due to poor accrual. The mean number of brain metastases treated was 9.4 (range: 4–25), with a total mean tumor volume of 11.97 cc (range: 0.33–91.1 cc). The mean follow-up duration was 10.9 months. At the last follow-up, 5 patients (13.9%) in the control arm and 3 patients (8.33%) in the experimental arm experienced target lesion progression, though this difference was not statistically significant (p=0.42). The median intracranial PFS was 3.4 months in the control arm compared to 6.6 months in the experimental arm, but this difference was not significant (p=0.16). Overall PFS and overall survival were not significantly different between the treatment arms, with a combined median of 2.6 months and 4.3 months, respectively. On follow-up, 16 events of Grade 3 toxicities were observed in 5 patients in the experimental arm and 4 patients in the control arm. No grade 4 or grade 5 toxicities were reported.