ESTRO 2021 Abstract Book

S124

ESTRO 2021

However, availability of CFs is still limited. The aim of this study is to develop and implement a Monte Carlo (MC) based framework for the determination of CFs for a large variety of ionization chambers. Materials and Methods The EGSnrc V2019 MC software package was employed throughout this study. Phase space files for the 10x10cm 2 field size (Q) of the Elekta Unity 7MV flattening filter free were provided by the manufacturer and used as the source model. A total of 15 ionization chambers from 3 manufacturers (Table 1) were modelled in the C++ geometry package based on detector blueprints provided by the corresponding vendor. Each detector’s effective point of measurement was placed in a water phantom (dimensions 30x30x30cm 3 ) at a depth of 5cm. The magnetic field was always perpendicular to the irradiation beam and parallel to the treatment couch. Two detector orientations were considered: parallel and perpendicular to the magnetic field. To calculate the CFs, dose to each detector’s sensitive volume was scored with and without the presence of magnetic field, as well as dose deposited to a small volume of water placed at the same depth. All simulations were carried out by the egs_chamber user code deployed in a 32-core workstation. The EM ESTEPE electron transport parameter in the presence of magnetic field was set equal to 0.01. For validation purposes, results were compared with corresponding published CFs, if available. Results The calculated CFs for both orientations are presented in Table 1. For the parallel configuration, corrections less than 2% are required for the majority of detectors. In the case of perpendicular set-up, changes in the detector response of up to 5% were observed, depending on chamber geometry and materials involved. Compared with corresponding published values (wherever available), reported CFs are in excellent agreement within uncertainties, except for the CFs related to the Exradin chambers in the parallel orientation.

Conclusion A set of CFs for 15 detectors and 2 orientations was determined within the context of a recently proposed formalism, considerably expanding the available dataset. Elekta is acknowledged for providing the phase space source files. Manufactures listed in Table 1 are acknowledged for providing detailed schematics of their detectors. OC-0195 The effective point of measurement of thimble-type chambers in the presence of a magnetic field T. Tekin 1 , I. Blum 1 , B. Delfs 1 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University,, Oldenburg, Germany Purpose or Objective The effective point of measurement (EPOM) of thimble-type air-filled ionization chambers is shifted from the chamber axis towards the source as the results of the cylindrical geometry of the air cavity and the angular spread of the secondary electrons traversing it. In the presence of a magnetic field, the deflection of secondary electrons by the Lorentz force is expected to affect the EPOM. In this study, the EPOM of three chambers with different sizes with and without a magnetic field have been assessed. Materials and Methods Using the EGSnrc code with eemf-macro (EMSTEP = 0.2), the EPOM was simulated for three chambers (Farmer 30013, Semiflex 3D 31021, PinPoint 3D 31022, all from PTW Freiburg, Germany) at a field size of 10 × 10 cm 2 using a 6 MV photon beam. Four geometrical setups were investigated by varying the relative orientation between the magnetic field and the chambers symmetry axis: (i) chambers axis antiparallel to the beam axis (+z) and perpendicular to the magnetic field (+x), (ii) chambers axis (+x) perpendicular to the beam axis and parallel to the magnetic field (+x), (iii and iv) chambers axis perpendicular (+x) to both the beam axis and the magnetic field (+y and -y). The magnetic field was varied up to 1.5 T. To assess the EPOM in each spatial direction (Δx, Δy and Δz), a modified user-code (Looe et al. 2018 45:5608- 5621) was used to register the coordinates of all secondary electrons impinging onto the surface of the sensitive volume. Based on a theoretical approach (Dutreix and Dutreix 1966 Biophysik 3:249-258), the EPOM