ESTRO 2020 Abstract Book

S110 ESTRO 2020

first commissioning results of IDEAL/GATE-RTion for scanned proton and carbon ion beams. Material and Methods DICOM input files (CT, Plan, Dose and Structure) are imported into IDEAL and run on a dedicated cluster of modular capacity (currently equipped with 48 cores). Commissioning work is divided into 1D beam delivery (depth-dose profiles, spot sizes, and nuclear halo), 2D (spot maps, output factors) and 3D (targets of various shapes and complexity: square, cylinder, L-shape, etc.). TEDD (a Toolkit for the Evaluation of DICOM Doses) was developed to analyze 1D/2D/3D beam delivery parameters. Proton and carbon ion beam models were generated and validated in terms of energy and optical properties and then calibrated in dose. 3D commissioning of IDEAL is performed using TEDD. IDEAL recomputed treatment plan doses are evaluated with TEDD against measurements performed in water using a 3D-block featured with 24 pin-point ionization chambers. Results The proton beam model was found to be within clinical tolerances of 0.5 mm in range and 10% in spot size in the air-gap from the nozzle exit to the isocenter. So far, 26 targets of various shapes were simulated in water for protons. Averaged dose differences were always better than +/-3% and mostly within 1% or 2%. These plans included field sizes from 3 to 20 cm, target volumes from 0.03 to 2 liters and more, with and without range shifter and were centered between 3 and 31 cm depth. Overall, more than 2000 pin-point measurements were analyzed. An example is provided in Figure 1. Conclusion As of today, proton commissioning is on-going and promising preliminary results have been obtained. More complex plans including inhomogeneities and various phantom geometries and compositions will be analyzed. In parallel, carbon ion commissioning will start by the end of 2019 and preliminary results will be obtained early 2020. To our knowledge, this work is the first attempt of transferring general purpose MC codes such as GATE/Geant4 into a CE certified IDC product, making this project very unique. OC-0218 Approximate modeling for dose calculations in Diffusing Alpha-emitters Radiation Therapy L. Arazi 1 , M. Dumančić 1 , Y. Keisari 2 , I. Kelson 3 1 Ben-Gurion University of the Negev, Nuclear Engineering Unit- Faculty of Engineering Sciences, Beer-Sheva, Israel ; 2 Tel Aviv University, Department of Clinical Microbiology & Immunology- Sackler Faculty of Medicine, Tel Aviv, Israel ; 3 Tel Aviv University, School of Physics and Astronomy- Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv, Israel Purpose or Objective Diffusing Alpha-emitters Radiation Therapy (“DaRT”) is a new concept, which enables – for the first time - the treatment of solid tumors by alpha particles. The treatment utilizes implantable seeds, whose surface is embedded with a low activity of radium-224. Each seed continuously emits the short-lived alpha-emitting daughters of radium-224, which spread over several mm around it, creating a “kill region” of high alpha-particle

dose. DaRT is presently tested in clinical trials, starting with locally advanced and recurrent squamous cell carcinoma (SCC) of the skin and head and neck, with promising results with respect to both efficacy and safety. This work aims to provide a simple model which can serve as a zero-order approximation for DaRT dosimetry. Material and Methods The model consists of diffusion equations for radon-220, lead-212 and bismuth-212, with the other short-lived daughters in local secular equilibrium. For simplicity, the medium is assumed to be homogeneous, isotropic and time-independent. Vascular effects are accounted for by effective diffusion and clearance terms. To leading order, the alpha particle dose can be described by simple analytic expressions, which shed light on the underlying physics. The beta and gamma dose is calculated using the EGSnrc Monte Carlo code. Animal studies were used to estimate the values of the key parameters of the model, using a combination of phosphor-imaging-based autoradiography (to record the spatial distribution of lead-212 in treated tumors) and gamma spectroscopy. Results The calculations demonstrate that, for a reasonable choice of model parameters, therapeutic alpha-particle dose levels are obtained over a region measuring 4-7 mm in diameter for sources carrying a few µCi of radium-224. In particular, for SCC tumors the macroscopic alpha particle dose is predicted to exceed 10 Gy over a ~5 mm diameter region. The beta and gamma dose falls below 5 Gy at a radial distance of ~2 mm from the seed. The model predictions served as the basis for treatment planning in the SCC clinical trial, where treatments employing DaRT seeds carrying 2 µCi of radium-224 and spaced 5 mm apart resulted positive response (30-100% shrinkage in volume) in all treated tumors. Conclusion The promising results of the SCC clinical trial indicate that in spite of its approximate nature, the simple diffusion- based dosimetry model provides a quantitative starting point for DaRT treatment planning. The parameters governing the alpha particle dose are expected to vary, to some extent, between different tumor types and should be evaluated in suitable animal models. The predictions concerning the beta and gamma dose are expected to be largely tumor-independent. OC-0219 Development and validation of an EGSnrc accelerator head model for a 1.5 T MR-Linac M. Friedel 1 , M. Nachbar 1 , D. Mönnich 1,2 , O. Dohm 3 , D. Zips 2,3 , D. Thorwarth 1,2 1 University Hospital Tübingen, Department of Radiation Oncology- Section for Biomedical Physics, Tübingen, Germany ; 2 German Cancer Consortium DKTK, partner site Tübingen- and German Cancer Research Center DKFZ Heidelberg, Tübingen, Germany ; 3 University Hospital Tübingen, Department of Radiation Oncology, Tübingen, Germany Purpose or Objective To develop a full EGSnrc accelerator head and cryostat model for a 1.5 T MR-Linac (MRL) to enable independent, high precision dose calculations accounting for magnetic field effects. In this work, an EGSnrc model of the 1.5 T Elekta Unity (Elekta AB, Stockholm, Sweden) was developed, implemented and validated against experimental data and a commercial treatment planning system (TPS). Material and Methods Primary electron beam parameters for the implemented model were adapted to be in accordance with measured dose profiles of the 1.5 T Elekta Unity. Those parameters were the mean electron energy as well as the Gaussian radial intensity and energy distribution. Energy tuning was done comparing percentage depth dose (PDD) curves simulated with monoenergetic beams of varying energies