ESTRO 2025 - Abstract Book

S2703

Physics - Dose calculation algorithms

ESTRO 2025

Conclusion: We developed a framework to investigate VHEE interactions using EGSnrc. By selectively enabling/disabling specific interactions, we can systematically examine each interaction's contribution, enhancing our understanding of VHEE behavior in matter. This helps characterizing VHEE interactions, providing insights into improving dose calculation efficiency, such as by omitting unnecessary simulations of specific interactions. Supported by the Swiss Cancer Research Foundation (KFS-5948-08-2023).

Keywords: Monte Carlo simulation, very high energy electron

References: [1] I. Kawrakow, E. Mainegra-Hing, D. W. O. Rogers, F. Tessier, and B. R. B. Walters, “The EGSnrc code system: Monte Carlo simulation of electron and photon transport”, Tech. Rep. PIRS-701, National Research Council Canada, 2019.

3498

Digital Poster GammaTile permanent brain implants – current clinical dosimetry and the role of Monte Carlo simulations Abdullah Alshreef 1,2 , Mustapha Assalmi 3 , Timothy J Allen 4 , Brent Rogers 5 , Courtney Oare 4 , Farhad Jafari 6 , Clara Ferreira 4 1 Department of Radiation Medicine, Loma Linda University, Loma Linda, USA. 2 Department of Biomedical Physics, King Faisal Specialist Hospital & Research Centre, Riyadh, Saudi Arabia. 3 Radiometric Measurements, National Energy Center of Nuclear Science and Technology, Rabat, Morocco. 4 Department of Radiation Oncology, University of Minnesota, Minneapolis, USA. 5 Department of Radiation Oncology, University of Kansas Medical Center, Kansas, USA. 6 Department of Radiology, University of Minnesota, Minneapolis, USA Purpose/Objective: In 2018 the FDA cleared GammaTile permanent brain implants, in which low-energy brachytherapy seeds are implanted in the tumor bed immediately after tumor resection. It is hypothesized that the immediate start of therapy and high localized doses may improve local tumor control. GammaTile has been used to treat meningiomas, glioblastomas, and brain metastases. The purpose of this study was to quantify the dose differences between dose calculation to water and simulated doses considering tissue heterogeneities for GammaTile implanted patients. Material/Methods: A retrospective study of 18 patients treated with GammaTile was conducted at our institution (IRB STUDY00010486). Patients received a variable number of GammaTiles, depending on the size of the resection cavity. Each GammaTile consists of four Cesium-131 seeds embedded in a collagen tile. The collagen tile provides a 3 mm space between seeds and the brain parenchyma. The clinical post-implant dosimetry was performed using Eclipse treatment planning system (TPS) based on the TG-43 dose to water formalism. Additionally, GAMOS Monte Carlo simulation was used to independently perform dose simulations for the same cohort of patients, using CT DICOM images and accounting for the effective atomic number of the head structures. Dose delivered to the high-risk clinical target volume (HR-CTV) and doses delivered to the skull were compared between TPS calculated doses to water versus simulated doses accounting for heterogeneities. Results: Dose volume histograms were generated, and the dose received by 90% (D 90 ) of the HR-CTV was on average 23.6% lower than the dose to water, while D 90 to skull was 3.2 times higher than dose to water. The accurate dose delivered can be substantially different from the doses calculated by the treatment planning system. The dose differences depended considerably on the medium, i.e. bone, and brain and the distances from the implant.