ESTRO 2024 - Abstract Book

S3368

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Conclusion:

We have developed and commissioned the first pMBRT system for single-gantry proton facility, which has demonstrated accuracy in benchmark with MC simulations, and allows for efficient plug-and-play setup.

Keywords: SFRT, proton minibeam RT

2919

Digital Poster

A semi-analytical calculation of the recombination correction factor in high dose-per-pulse beams.

Julien Bancheri 1 , Jan Seuntjens 2

1 McGill University, Department of Physics and Medical Physics Unit, Montreal, Canada. 2 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada

Purpose/Objective:

The proliferation of FLASH and high dose per pulse (DPP) beams has demonstrated that the conventional pulsed Boag theory and the related Jaffe plot extrapolation and two voltage method (TVM) are unable to accurately determine the ion recombination correction factor above 10 mGy/pulse [1]. This is due to a large free electron fraction that does not re-attach to air molecules and are directly collected. Additionally, the large free electron fraction induces a space charge effect that causes the electric field to deviate significantly from a uniform value [2]. The current methods of determining the ion recombination correction factor in high DPP such as numerical solutions and physics simulations are not suitable for quick evaluation of the correction factor in a clinical environment and very thin parallel plate chambers are not widely available. Analytical expressions, on the other hand, are quick to evaluate, can be used to develop procedures such as Jaffe plot extrapolation and the TVM, and all parameters carry physical meaning. The purpose of this work is to use a semi-analytical solution method to develop an analytical expression for the ion recombination correction factor. The voltage dependence of the expression is also established so a clinically practical extrapolation procedure can be proposed. The results of this procedure are compared to previously published experimental data.

Material/Methods:

The homotopy perturbation method (HPM), a semi-analytical solution method, is used to solve the partial differential equations (PDEs) describing the charge transport equations of positive ions, negative ions and free electrons. Space charge is modelled with Gauss' law. The electron velocity and electron attachment rate are fully modelled as function of the electric field strength. In this study, only a plane parallel chamber geometry is considered. It is argued that neither a delta function pulse nor a constant dose rate is suitable to model a high DPP beam. To approximate a square pulse (a non-analytic function that cannot be used in the HPM), the dose rate is modelled as DPP/s , where s is an