ESTRO 2024 - Abstract Book

S4607

Physics - Optimisation, algorithms and applications for ion beam treatment planning

ESTR0 2024

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Comparing RBE models based on LET vs. a track structure quantity in pediatric brain proton therapy

Maria CW Heisel 1 , Peter MT Lægdsmand 1,2 , Villads L Jacobsen 1,2 , Rasmus Klitgaard 1,2 , Niels Bassler 1,2 , Anne Vestergaard 1,2 , Yasmin Lassen-Ramshad 1 , Ludvig P Muren 1,2 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark

Purpose/Objective:

For proton therapy, a constant relative biological effectiveness (RBE) of 1.1 is currently applied in clinical treatment planning. However, studies have shown a variable RBE along the beam, but the exact dependencies are uncertain. The physical quantity linear energy transfer (LET) has been widely explored to predict RBE. A recent study has proposed RBE models based on the track structure quantity effective Q. Effective Q is defined as

,

where z * is the effective charge and ß is the velocity of the particle relative to the speed of light in vacuum. In this work, RBE models based on Q eff were investigated and compared to RBE models based on LET in the setting of proton therapy for pediatric brain tumors.

Material/Methods:

Monte Carlo simulations were performed with TOPAS MC (and validated with an independent Monte Carlo code) to calculate dose, LET, and Q eff distributions in both a water phantom and a clinically delivered treatment plan for one pediatric brain tumor case. The water phantom was constructed as 40x40x50 cm 3 cuboid with water and was irradiated with a single proton beam with energy 170MeV. The clinical treatment plan for the pediatric brain tumor case consisted of three pencil beam scanning fields of which two were non-coplanar. The brain tumor was localized in the left hemisphere and the prescription dose was 54Gy(RBE=1.1). RBE-weighted dose metrics were calculated for two organs at risk (brainstem and chiasm) and the clinical target volume (CTV). These calculations were performed using two RBE models based on LET and two RBE models based on Q eff . All four RBE models were derived from the same dataset, with two of the models having linear relationships and the other two having non-linear relationships with either quantity LET d or Q eff,d (these are the dose-weighted quantities). The results were compared to using a constant RBE=1.1 as well as the established McNamara et al RBE model.

Results: