ESTRO 2024 - Abstract Book

S3168

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

16

Proffered Paper

MC simulations of detector type specific output correction factors in the presence of magnetic field

Ilias Billas 1 , David R Shipley 1 , Michael Homer 2 , Sonja Surla 3 , Simon Duane 2

1 National Physical Laboratory, Medical Radiation Physics, Teddington, United Kingdom. 2 National Physical Laboratory, Medical Radiation Sience, Teddington, United Kingdom. 3 German Cancer Research Center (DKFZ), Medical Physics in Radiation Oncology, Heidelberg, Germany

Purpose/Objective:

MRI guided radiotherapy (MRgRT), is revolutionising radiotherapy by allowing for more precise and personalised treatment. The metrology of the radiation dose in such systems, however, is challenging due to the influence of the magnetic field on the dose distribution and the detector’s response. Dosimetry in reference conditions in MRgRT, for conventional field sizes of 10x10 cm2, has been established and detectors well characterised. These conditions, however, are far from being representative of clinical beams. Small radiation fields, that are typically smaller than 4x4 cm², are the main ingredients for optimising clinical treatments. In such conditions, standardised protocols for small field dosimetry in the presence of magnetic fields do not exist. Therefore, there is a strong need to develop a measurement methodology for small fields in MRgRT to support existing dosimetry protocols (i.e. TRS 483). This will assure optimal treatment outcomes and patient safety. The aim of this work is to obtain Monte Carlo (MC) calculated datasets of output correction factors for commercial MRI-linac systems and for two different detector types to support small field dosimetry in MRgRT. The EGSnrc system was used for the MC simulations to obtain output correction factors in the presence of magnetic fields, which are defined as the ratio of the calibration coefficient in clinical filed size divided by the calibration coefficient in machine specific reference field size. A Fano test, was performed to benchmark the egs_chamber usercode, and experiments to validate the MC models. The experimental setup involves the irradiation of a PTW/31022 pinpoint 3D and a PTW/60019 microDiamond detectors, using 6MV Elekta Synergy x-ray beam, in an electromagnet. A water phantom was designed to place the detectors between the 7cm gap of the magnetic poles, which gives a maximum strength of 1.6T. The detectors were irradiated both in perpendicular and parallel orientation to the x-ray beam (the microdiamond only parallel) and always perpendicular to B-field. A selection of square and rectangular field sizes were used. For the pinpoint 3D, finite element modelling (COMSOL) was also used to determine the true collecting volume as opposed to the actual (geometrical) volume of the ion-chamber in the detector reponse simulations. A ViweRay MRIdian MRI-linac beam model as developed and validated by Billas et al (2021) together with the detector models validated in this study were used to calculate output correction factors, which are compared with experimental data. Material/Methods: