S1787
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
the McNamara vRBE model scripted in Raystation 12A using a α / β value=2 Gy [3]. Results:
Hospital (UKHD), Heidelberg Ion-Beam Therapy Center (HIT), Heidelberg, Germany
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Fifty patients were included in this analysis. Medulloblastoma was the most common histology (76%) and 56% of tumours were located in the fourth ventricle. Median age at time of radiotherapy was 13.8 years (range: 2.9 - 42.7 years). Median prescribed dose was 54 Gy (range: 50.4 - 60 Gy) with craniospinal irradiation in 80% of patients. The crude incidence of TRIA in the brainstem was 20.0%, with a median onset of 4.3 months (range: 0.1 - 11.1 months), with SBI occurring in 5 patients with a median onset of 3.4 months (range: 0.1 - 8.2 months). Patients with TRIA in the brainstem had significantly higher brainstem D10%, D0.1cc and D0.03cc values using a cRBE of 1.1 compared to patients without brainstem toxicity. However, these brainstem dosimetric parameters were not associated with TRIA in the brainstem when using the McNamara vRBE model (Table 1). No correlation was found between any dosimetric parameters and SBI using either RBE model. Conclusion: TRIA was associated with higher brainstem doses calculated using a cRBE of 1.1, but not with the McNamara vRBE model, these findings suggest that variable RBE models may not improve the prediction of TRIA in the brainstem and should be applied cautiously in clinical treatment planning until further validated. References: [1] Wagenaar D, Habraken SJM, Rinaldi I, Eekers DBP, Kramer M, Jaspers JPM, et al. Evaluating and reporting LET and RBE-weighted dose in proton therapy for glioma – The Dutch approach. Radiotherapy and Oncology 2025;202:110653. [2] Indelicato DJ, Flampouri S, Rotondo RL, Bradley JA, Morris CG, Aldana PR, et al. Incidence and dosimetric parameters of pediatric brainstem toxicity following proton therapy. Acta Oncol (Madr) 2014;53:1298–304. [3] McNamara AL, Schuemann J, Paganetti H. A phenomenological relative biological effectiveness
Proton therapy for posterior fossa tumours: do variable RBE models improve brainstem toxicity prediction? Lieselotte Lauwens 1,2 , Dario Di Perri 3,4 , Karen Van Beek 3,1 , Robin De Roover 1,3 , Maarten Lambrecht 1,3 1 Department of Radiation Oncology, University hospitals Leuven, Leuven, Belgium. 2 Department of Oncology Laboratory of experimental Radiotherapy, University of Leuven, Leuven, Belgium. 3 Particle Therapy Interuniversity Centre Leuven (PARTICLE) Proton Therapy Centre, University hospitals Leuven, Leuven, Belgium. 4 Department of Radiation Oncology, Cliniques universitaires Saint-Luc, Brussels, Belgium Purpose/Objective: Proton therapy (PT) is widely used for the treatment of posterior fossa tumours, however brainstem toxicity remains a significant concern. While dose constraints are currently evaluated using a constant relative biological effectiveness (RBE) of 1.1, variable RBE models have been proposed to better reflect biological dose distribution. In the Netherlands, the McNamara variable RBE (vRBE) model was recently recommended for evaluation and reporting purposes [1]. This study aimed to investigate the association between brainstem dose and toxicity using both a constant RBE (cRBE) and the McNamara vRBE model. Material/Methods: Following ethical approval, patients treated with PT for posterior fossa tumours, with a minimum follow-up of one year were retrospectively included. Patients who received more than three backup fractions of photon therapy were excluded. Clinical and treatment data were collected. Post-radiotherapy MRIs were reviewed to identify treatment-related imaging abnormalities (TRIA) in the brainstem. Symptomatic brainstem injury (SBI) was assessed according to the criteria defined by Indelicato et al. [2]. Dosimetric parameters were recorded using a cRBE model and recalculated using
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