ESTRO 2022 - Abstract Book

S1310

Abstract book

ESTRO 2022

Figure 2: Calculated and measured dose area ratio as a function of the beam quality specifier %dd(10) x for the Elekta Precise 6 MV linac. Monte Carlo data calculated for different electron beam spot sizes (FWHM) and electron energies. Conclusion A Monte Carlo based model of a detector array with quad wedges was created to investigate the possibility of determining the radiation quality from dose profile measurements. The measurements have confirmed the Monte Carlo calculation data. According to the results from measurements as well as from Monte Carlo simulations, a linear relationship between DAR and the beam quality specifier %dd(10) x was confirmed with r ² =0.90 for a Elekta 6 MV photon beam.

PO-1529 Monte Carlo calculation of magnetic field correction factors for two ionization chambers

M. Alissa 1,2 , K. Zink 1,3 , A.A. Schoenfeld 4 , D. Czarneck 1

1 University of Applied Sciences Mittelhessen, Institute for Medical Physics and Radiation Protection, Giessen, Germany; 2 Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Giessen, Germany; 3 Department of Radiotherapy and Radiation Oncology , University Medical Center Giessen and Marburg, Marburg, Germany; 4 Sun Nuclear Corporation , Research, Melbourne, USA Purpose or Objective Integration of linear accelerator and magnetic resonance tomography is a great advantage for radiotherapy treatment, the MR-linacs provide a high-contrast imaging in real-time irradiation without exposing extra doses to the patient, but the strong magnetic fields have an influence of the trajectories of the secondary electrons. Due to the Lorentz force the trajectories of these secondary electrons become helical. As a result, the dose distribution in water and the dose response of ionization chambers will change. Therefore, new correction factors are required in clinical dosimetry for MR-linacs. In this work, the Monte Carlo code EGSnrc was applied to calculate the correction factors k B of two ionization chambers (SNC 125c and SNC 600c, Sun Nuclear Corp., Melbourne, USA) for different strengths and directions of the magnetic field B. Materials and Methods The cylindrical chambers were modeled in detail according to the information given by the manufacturer and placed in a water phantom. They were irradiated under reference conditions according to the TRS-398 and DIN 6800-2 Codes of Practice. Both codes differ regarding the positioning of the detector. A 6 MV spectrum of an ELEKTA linac was used as photon source. In this study the magnetic field correction factors were calculated in different directions of magnetic field relative to the beam axis and the chamber’s symmetry axis and was varied between 0 and 2 T in steps of 0.2 T. Additionally, the effective point of measurement for the chamber SNC 125c was determined, by comparing the depth dose curve in a small water voxel and the depth dose curve in the chamber in absence and presence of magnetic field.

Results