ESTRO 2024 - Abstract Book

S339

Beachytherapy - Physics

ESTRO 2024

998

Poster Discussion

Development of a new software package for dosimetry of the Xoft electronic brachytherapy source

Azin Esmaelbeigi 1 , Jonathan Kalinowski 1 , Nada Tomic 2 , Slobodan Devic 1,2 , Mark J Rivard 3 , Te Vuong 2 , Shirin Enger 1,2

1 McGill University, Medical Physics, Montreal, Canada. 2 Jewish General Hospital, Radiation Oncology, Montreal, Canada. 3 Brown University, Radiation Oncology, Providence, USA

Purpose/Objective:

Xoft electronic brachytherapy sources are operational for approximately hours or up to 10 treatments before reaching the end of their usable lifespan. Variationsin the manufacturing process may result in inter-source differences in x-ray production that can impact the dose rate [1]. This study aimed to develop a software package called e-Brachy to characterize the Xoft electronic brachytherapy source.

Material/Methods:

A Monte Carlo package called e-Brachy based on the Geant4.10.02 Monte Carlo toolkit [3] was developed. The software consists of two parts. In the first part, x-rays are generated by simulating electron collisions with the anode. The generated x-rays are saved in a phase space file at the boundary of the source and surrounding medium. This is done by implementing the source in detail, using the design of the source model provided by the vendor in computer-aided design and converting it to the geometry design markup language [2]. The material and mass densities are assigned according to the vendor's specifications. To increase the efficiency of the x-ray generation, bremsstrahlung splitting is implemented. The second part of the code, samples x-rays from this phase space file for further investigation. Currently, in the software, the Xoft source model S7500 is implemented. There is a large tolerance for some of the important materials in the source composition, such as a silver (Ag) layer in the source wall and composition for a silver epoxy layer around the anode coating. The details of the epoxy material composition are proprietary to the vendor. The x-ray spectra for different Ag thicknesses and epoxy compositions were calculated 178 cm from the origin of the source in air and compared with the NIST measurements [5]. The software can calculate the parameters for the modified report of task group 43 of the American Association of Physicists in Medicine (TG-43) [6,7]. The TG-43 parameters are calculated analytically after obtaining the necessary values such as the generated air kerma rate in air at 50 cm from the source and dose rate values around the source in the water medium using Monte Carlo simulations.

Results:

Figure 1 shows the characteristic x-rays originating from the Cu, Y, and Ag elements present in the source components. The spectrum generated without the epoxy agrees with the the correlation coefficient of 0.99 and a calculated Euclidean distance of 0.06 ± 2% with the NIST measurements. In contrast, the presence of a thicker silver layer and silver epoxy, agrees the least with the respective values of the correlation coefficient and the Euclidean distance to be 0.92 and 0.12 ± 2%. The radial dose functions calculated for model S7500 showed the expected fall-off moving further away from the source. Figure 2 shows the values for the anisotropy function. Overall, the anisotropy decreases with increasing polar angle from 10º to 150º at distances 1-10 cm from the source. The anisotropy values increase at the higher polar angles (150 º -175 º).