ESTRO 2025 - Abstract Book

S3554

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

ESTRO 2025

References: Santo, R. et al., Pencil-beam Delivery Pattern Optimization Increases Dose Rate for Stereotactic FLASH Proton Therapy, International Journ. of Radiation Oncology*Biology*Physics 2023. Zhao, X. et al., A Novel Dose Rate Optimization Method to Maximize Ultrahigh-Dose-Rate Coverage of Critical Organs at Risk Without Compromising Dosimetry Metrics in Proton Pencil Beam Scanning FLASH Radiation Therapy, International Journ. of Radiation Oncology*Biology*Physics 2024. Deffet, S. et al., Definition of dose rate for FLASH pencil-beam scanning proton therapy: A comparative study, Med Phys 2023. Nilsson, R et al. A framework for creating clinically realizable robust proton conformal FLASH plans, FLASH Radiotherapy and Particle Therapy Conference 2023.

4046

Poster Discussion Equivalent constant dose rate as a unifying dose rate concept for FLASH across all beam time structures Per R Poulsen 1,2 , Brita S Sørensen 1,2,3 , Line Kristensen 1,2,3 , Cai Grau 1,2 , Jacob G Johansen 1,2 1 Danish Centre for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark. 2 Department of Clinical Medicine, Aarhus University, Aarhus, Denmark. 3 Department of Experimental Clinical Oncology, Aarhus University Hospital, Aarhus, Denmark Purpose/Objective: FLASH studies have used both pulsed electron beams and pencil beam scanning (PBS) proton beams with various time structures. Beams with identical dose and mean dose-rate (DR mean ), but different time structures, can have very different FLASH sparing effects [1], demonstrating that DR mean is insufficient to describe the FLASH effect. A biologically founded dose-rate concept that is universally applicable across all beam time structures is highly needed. For any beam with time-varying dose rate, we define the Equivalent Constant Dose-Rate (DR eq,const ) as the dose-rate that would give the same FLASH sparing effect if the same dose had been delivered with constant dose rate. This study investigates DR eq,const and its ability to describe acute skin toxicity in mice across many beam time structures. Material/Methods: Using a published oxygen enhancement model [2], DR eq,const was calculated for a range of beam time structures that has previously been used in 40Gy mice irradiations with varying dose-rates, repainting schemes, inlaid beam pauses and modalities (transmission proton beams, spread-out Bragg peak (SOBP) proton beams, pulsed electron beams) [1,3]. The ability of DR eq,const to describe acute skin toxicity occurrence in these experiments was investigated.