ESTRO 2025 - Abstract Book

S710

Clinical - CNS

ESTRO 2025

Keywords: Glioblastoma, Retinoic Acid, Radiation Therapy

References: 1. Yalamarty SSK, Filipczak N, Li X, Subhan MA, Parveen F, Ataide JA, et al. Mechanisms of resistance and current treatment options for glioblastoma multiforme (GBM). Cancers (Basel). 2023;15(7):2116. 2. Pepper NB, Stummer W, Eich HT. The use of radiosensitizing agents in the therapy of glioblastoma multiforme-a comprehensive review. Strahlenther Onkol. 2022;198(6):507–26. 3. Lavudi K, Nuguri SM, Olverson Z, Dhanabalan AK, Patnaik S, Kokkanti RR. Targeting the retinoic acid signaling pathway as a modern precision therapy against cancers. Front Cell Dev Biol. 2023; 11:1254612.

2851

Digital Poster Dose-Related Vestibular Toxicity and Optimization in Stereotactic Radiosurgery for Vestibular Schwannomas Dimitrios Daskalou 1 , Edouard Romano 2,3 , Sophie Neveu 4 , Pelagia Tsoutsou 2 , Nikolaos Koutsouvelis 2 , Nils Guinand 1 , Minerva Becker 4 , Pascal Senn 1 , Sebastien Tran 2 1 Otorhinolaryngology-Head and Neck Surgery, Geneva University Hospitals, Geneva, Switzerland. 2 Radiation Oncology, Geneva University Hospitals, Geneva, Switzerland. 3 Radiation Oncology, Vaud University Hospital Center, Lausanne, Switzerland. 4 Radiology, Geneva University Hospitals, Geneva, Switzerland Purpose/Objective: While stereotactic radiosurgery (SRS) achieves high long-term tumor control rates for small and medium-sized vestibular schwannomas (VS), its impact on the vestibular system remains unclear. This study investigates the effects of SRS on subjective vestibular symptoms, measured vestibular function, and potential predictors of symptom worsening post-treatment. Material/Methods: This retrospective analysis includes consecutive non-neurofibromatosis type-2 adult patients treated with SRS over an 8-year period. Vestibular symptoms were graded per CTCAEv5.0 by expert radiation oncologists at baseline and 6 months post-treatment. For dosimetric analysis, we defined the vestibular system as the combined volume of the saccule, utricle, and the three ampullae (anterior, lateral, and posterior). Tumor characteristics were extracted from baseline imaging. Vestibular function was tested via bithermal, bilateral caloric test, video head impulse test, and vestibular evoked myogenic potentials. In an exploratory assessment, we generated hypothetical SRS dosimetric plans for two randomly selected schwannomas (one abutting the cochlear fossa, the other distant) to determine whether vestibular dose reduction could be achieved without compromising planning target volume (PTV) coverage or cochlear dose. Results: Forty-five patients were included (median age, 61.4 years; IQR, 15.45). Fourteen patients (31%) reported worsened vestibular symptoms six months after SRS. In these patients, the vestibular system received a higher mean dose (Dmean) than patients with stable or improved symptoms (median Dmean, 6.45 Gy versus 2.92 Gy; p<0.0001). Individuals receiving a Dmean greater than 4 Gy to the vestibular system had a significantly higher likelihood of symptom worsening (OR=27.3, 95% CI [3.4, 301.8], p=0.0002). Similar results were observed for the maximum dose (Dmax), with a cutoff value of 8 Gy. Age, gender, pre-SRS vestibular symptoms, MRI characteristics (tumor volume and Koos grade), baseline hearing status, and baseline vestibular battery tests did not predict subjective vestibular exacerbation. Regarding measured vestibular function, we found a positive correlation between the percentage shift in caloric weakness and the Dmean received by the lateral ampulla (R²=0.38, CI [0.3, 15.3], p=0.04). Dosimetric optimization in two randomly selected cases achieved an average reduction of 40% in Dmean and 26% in Dmax to the vestibular system, without compromising PTV coverage or increasing the cochlear dose.