ESTRO 2021 Abstract Book

S125

ESTRO 2021

shift was calculated as the energy-weighted sum of the shifts of all entry points (x e , y e and z e ) from the chamber reference point. Additional shift caused by the chamber wall was considered by computing its water- equivalent thickness. For comparison, the EPOM shift along the depth direction Δz was derived from the comparisons of the relative depth dose curves. Results In the absence of a magnetic field, the EPOM was found to depend on the chamber inner radius with a Δz between -0.34r to -0.48r. In the presence of a magnetic field, the magnitude of Δz is reduced by up to 54% in a 1.5 T magnetic field. This indicates a reduced upstream shift of the EPOM, that is, the EPOM lies closer to the chamber axis. Additional to the Δz shift, the results also demonstrated a lateral displacement of the EPOM (Δx and Δy) in the direction opposite to the prevalent direction of the Lorentz force, which increases with the magnetic field strength. Conclusion The influence of a magnetic field on the EPOM of three ionization chambers have been investigated up to 1.5 T. The prevalent lateral deflection of the secondary electrons by the Lorentz force displaces their entry points into the air cavity towards one side of the chamber, hence moving the EPOM closer to the chamber axis along the depth axis and introducing additionally a magnetic field dependent lateral displacement of the EPOM. OC-0196 Determination of the beam-quality correction factors kQ for the PTW PinPoint 3D 31022 chamber B. Delfs 1 , R. Kapsch 2 , J. Kretschmer 3 , I. Blum 3 , T. Tekin 4 , L. Brodbek 3 , B. Poppe 1 , R. Kranzer 1,5 , K. Burzlaff 6 , D. Poppinga 7 , J. Würfel 7 , H.K. Looe 1 1 Carl von Ossietzky University, University Clinic of Medical Radiation Physics, Oldenburg, Germany; 2 Physikalisch-Technische Bundesanstalt, Hochenergetische Photonen- und Elektronenstrahlung , Braunschweig, Germany; 3 University Clinic of Medical Radiation Physics, Carl von Ossietzky University, Oldenburg, Germany; 4 Carl von Ossietzky University, University Clinic of Medical Radiation Physics, , Oldenburg, Germany; 5 PTW Freiburg, Detector Development, Freiburg, Germany; 6 PTW Freiburg, Product Management, Freiburg, Germany; 7 PTW Freiburg, Research, Freiburg, Germany for numerous chambers, including some smaller thimble chambers, have been published for the update of IAEA TRS-398 protocol (Andreo et al . 2020 Phys. Med. Biol. 65 095011). These factors, along with the calibration coefficient provided by the secondary standard dosimetry laboratories (SSDL), would facilitate direct dose measurement using these chambers, as required for the verification of treatment plans or the so-called end-to-end testing, especially when small targets are concerned. Although the k Q values for the discontinued PTW PinPoint 3D 31016 have been published for the update, no values are available for the successor model PTW PinPoint 3D 31022. This work reports the k Q values for the new model following the methodology adopted in the update of IAEA TRS-398. Materials and Methods Measurements have been performed at the National Metrology Institute of Germany (PTB) using three samples of the PTW Pinpoint 3D 31022 chamber. In the first step, calibration coefficients were obtained for all chambers using the PTB 60 Co calibration beam. Subsequently, the k Q values were determined based on cross- calibration against two secondary-standard chambers, IBA FC65G-771 and NE2561-297, which have been previously calibrated using the primary-standard water calorimeter at each megavoltage photon energy. The average k Q values were computed from the three chamber samples and the cross-calibrations with the two reference chambers. All measurements have been carried out under reference condition as defined in TRS- 398. Monte Carlo simulations were performed with EGSnrc (v2019a) using the same parameters as tabulated in Kretschmer et al . 2020 (Med. Phys. 47:5890-5905). Materials were defined considering the I -values in the ICRU Report 90. Photon spectra from Mora et al . ( 60 Co) and Mohan et al. (TPR 20,10 between 0.623 and 0.805) were used. The PinPoint 3D 31022 chamber was modelled based on manufacturer’s blueprints, including a detailed chamber stem and the consideration of its effective sensitive volume. For benchmarking purpose k Q values were simulated for the NE-2571 chamber, which was modelled based on the same detector information available for the TRS update. Results The k Q values for the NE-2571 chamber lie within the 95% prediction limit of the fit provided in Andreo et al . 2020. Both the experimental and Monte Carlo simulated k Q values for the PinPoint 3D 31022 chamber are shown in figure 1. The maximum deviation between the three chamber samples for one beam quality was 0.58%. The data was fitted according to Eq. (6) in Andreo et al . 2020 by considering both the experimental and Monte Carlo results with a = 1.12 and b = -0.095. Purpose or Objective Recently, beam-quality correction factors k Q