ESTRO 2023 - Abstract Book

S1476

Digital Posters

ESTRO 2023

The positive ion, negative ion, free electron concentration and electric field are considered as perturbation series. A FLASH beam is considered in the system of PDEs by taking into account a time-dependent radiation source, space charge (through Gauss's law) and free electrons. It has been shown that space charge and free electrons must be accounted for when describing ion recombination in FLASH beams. Gauss's law and the recombination coefficient are treated as perturbations. Each term of the perturbation series is subsequently obtained by solving the system of PDEs at every order. A series representation of the ion recombination correction factor is then calculated from these solutions. An initial study was performed considering a delta function source and a parallel plate geometry. Only positive ions and electrons are considered. As the velocity of electrons in air is much higher than that of ions, the positive ion mobility was set to zero. All of the computations were done with Wolfram Mathematica V13. Results The initial study yielded the following expression for the ion recombination correction factor ks:

where T(n,m) is a constant and k_ is a space charge parameter. In equation 1, alpha is the recombination coefficient, N0 is the total amount of charge density initially created by the pulse, V is the applied voltage, mu_ is the electron mobility, d is the plate spacing, e is the electronic charge and epsilon is the permittivity of air. The first term is a well known result from Boag theory while the second term accounts for space charge. A comparison of equation 1 to the Boag theory (pulsed) is shown in Figure 1. It should be noted that equation 1 diverges at higher charge densities and an expression at these higher values needs to be established. Subsequent studies are now being performed that include all three charge species.

Conclusion A perturbation series procedure is employed to solve the system of charge recombination-transport PDEs in FLASH conditions and obtain an analytical expression for the ion recombination correction factor. An initial study considering a delta function source and certain assumptions yielded a novel expression that takes into account space charge. In the future, this procedure can be used to interpret a range of experimental data by adjusting the physical parameters present in the PDEs. Additionally, a simplified analytical expression of the ion recombination correction factor along with ionization chamber measurements can be used to determine the correction factor in clinical settings, in a method similar to the two-voltage method.

PO-1757 Characterization of the Exradin W2 in FLASH Radiation therapy

K. Liu 1 , S. Holmes 2 , E. Schueler 1 , S. Beddar 1

1 MD Anderson Cancer Center, Radiation Physics, Houston, USA; 2 Standard Imaging, Medical Physics, Middleton, USA

Purpose or Objective In FLASH radiation therapy, the ability to monitor the beam output and measure the physical beam parameters are not a trivial task due to extremely high dose-rate and dose-per-pulse (DPP) conditions. In this study, we are characterizing the response of a water-equivalent detector that provides real-time dose measurements: the W2 Exradin Scintillator (Standard Imaging. Inc). Materials and Methods The W2 Exradin plastic scintillator detector (PSD) was exposed to different parameters relevant to FLASH by modifying the pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings for an IntraOp electron FLASH Mobetron accelerator. The response of the W2 Exradin PSD was evaluated as a function of dose, DPP, and mean and instantaneous dose-rate using the blue and green signal channels as well as the net signal from subtracting the green signal from the blue.