ESTRO 2025 - Abstract Book

S3459

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

ESTRO 2025

Purpose/Objective: Unexpected toxicity can be a problem with proton therapy. There is a current debate about the origin of this complication, ranging from specific radiosensitivity to the potential role of physical quantities not currently considered. One of these quantities is the dose rate [1,2], whose derivation is a complex task as it evolves on very short time scales (milliseconds) and depends on how the different spots deliver dose in a specific spatial location. This work presents a novel approach to get a fast but accurate estimate of the dose rate in Pencil Beam Scanning Proton Systems. Material/Methods: Dose rate distributions have been obtained for a small set of five patients in a Hitachi Synchrotron facility located in our University. Two approaches have been used: a complete approach, where the dose rate distribution on each voxel is obtained from dose distributions calculated for each individual spot; and a model, where the dose rate is obtained from separated Monte Carlo simulations for each energy layer. In this later case, the 3D dose distributions per spot are derived from this energy layer dose distribution using the virtual source model, using the individual trajectories of virtual protons to determine the weight of each spot on the dose deposed on a specific voxel. In both cases, the irradiation times are derived from a model of the temporal structure of the plan delivery [3]. The second approach has the advantage of significantly reducing (by an average factor of one hundredth) the computing time and the memory requirements. It makes affordable the derivation of dose rate distributions even for plans with more than twenty thousand spots. Results: There is an agreement between the irradiation rates obtained in both approaches (see figure). The differences between both approaches are below 5% in 95% of the voxels.

A histogram of dose rates is computed for each voxel. From these data we have obtained the spatial distribution of the maximum (upper panel) and mean (bottom panel) dose rates. The dose rate can reach a maximum value between 20 to 25 Gy/s when computed in the spot characteristic time (1 to 2 milliseconds), but can increase by a factor 1.2 in an intraspot scale (between 0.5 to 1ms).