ESTRO 2020 Abstract Book

S353 ESTRO 2020

linac daily ART must be proven, with solid clinical evidences to distinguish patients who need this approach from patients who will not derive a significant benefit. I believe that there is a future for ART, but we need to generate evidences on its use and to define frequency and clinical scenarios in which this approach is more valuable. In every case, this can be done with and without a MR- linac. SP-0631 For the motion rebuttal (there is no future for ART without an MR-linac) C. Gani 1 1 University Hospital Tübingen, Radiation Oncology Department, Tübingen, Germany Abstract text In my rebuttal I will present arguments for the motion, that there is no future for adaptive RT without an MR- Linac. SP-0632 Against the motion rebuttal (there is a future for ART without an MR-linac) S. Korreman Aarhus University Hospital. Aarhus, Denmark OC-0633 The dose response of high-resolution diode detectors in magnetic field T. Tekin 1 , B. Delfs 1 , I. Büsing 1 , A. Schönfeld 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 clinical establishment of MR linacs for magnetic resonance guided radiotherapy (MRgRT) with online soft tissue contrast imaging introduces new challenges in dosimetry due to the modification of secondary electrons’ trajectories in static magnetic field by the Lorentz force. Alteration of dose response of air-filled ionization chambers has been frequently reported. Nevertheless, similar studies on the change of dose response of semiconductor detectors are scarce and the related mechanism is not fully understood yet. Since these detectors are used at MR Linacs whenever high spatial resolution is required, such as during device commissioning, the magnetic field dependent dose response of semiconductor detectors has been studied in this work. Material and Methods The measurements were carried out with five commercially available semiconductor detectors from three manufacturers (PTW 60017, PTW 60023, PTW 60019, IBA Razor Diode and Sun Nuclear Edge detector) at a linac equipped with an electromagnet using 6 MV photon beam. The strength of the magnetic field that is oriented orthogonal to the chamber and beam axis was varied from 0 to 1.4 T. Thereby, the change of the detector´s response in a magnetic field has been evaluated by the magnetic field correction factor, k B,Q . The experimental magnetic field correction factors were validated by Monte Carlo simulations using the EGSnrc code. Detailed step-wise simulations have been performed to understand the Abstract not received Proffered Papers: Proffered papers 33.2: Dosimetry and QA

underlying mechanism and the role of detector components. Results Both the measured and simulated dose responses of all investigated semiconductor detectors decrease continuously in the presence of magnetic field by up to 12 %. The effect has been shown to be highly dependent on the detector’s design and the magnetic field strength. Measurement and simulation results agree within 1.7 % at all investigated magnetic field strength. The results from the detailed Monte Carlo analysis demonstrated that the sensitive volume itself, unlike the case of air-filled ionization chambers, does not contribute to the observed magnetic field dependence. Conclusion Detailed Monte Carlo analysis showed that the alteration of the secondary electrons fluence within the sensitive volume of semiconductor detectors in magnetic field is mainly attributed to the presence of other high density non-water equivalent components within the detectors. Due to the strong magnetic field dependent dose response observed for all investigated semiconductor detectors, cross calibration must be performed in the same magnetic field as is present during the actual measurements. The findings of this work also demonstrated the importance to further study the influence of magnetic field on semiconductor detector’s dose response due to changing measurement conditions such as field size, measurement depth or detector’s orientation. OC-0634 Correction for ion recombination in a built-in monitor chamber at ultra-high dose rates E. Konradsson 1 , M. Lempart 2 , B. Blad 2 , C. Ceberg 1 , K. Petersson 2 1 Medical Radiation Physics, Lund University, Lund, Sweden ; 2 Department of Hematology- Oncology and Radiation Physics, Skåne University Hospital, Lund, Sweden Purpose or Objective When moving towards clinical implementation of the novel FLASH radiotherapy technique, it would be favorable to use the linear accelerator’s built-in transmission (monitor) chamber to monitor the dose delivery. However, the drop in ion collection efficiency caused by ion recombination at high dose-per-pulse (DPP) values makes online dosimetry using conventional chambers at ultra-high dose rates challenging. The aim of this study was to model the change in ion collection efficiency of a transmission chamber with increasing DPP, so that the effect can be taken into account, making the chamber useful for online dosimetry All measurements in this study were performed on a clinical linear accelerator modified for FLASH delivery, using a 10 MeV electron beam. The raw transmission chamber signal from one of the two existing monitor channels (with a polarizing voltage of -320 V) was extracted and measured at varying dose rates. The corresponding DPP values were determined using Gafchromic EBT3 film at 2 cm depth in a solid water phantom with a source-to-surface distance of 100 cm. The pulse repetition frequency was set to 200 Hz and the mean dose rates at the point of measurement ranged from conventional (≈6 Gy/min) to ultra-high (≈160 Gy/s). An empirical model of the ion collection efficiency of the transmission chamber (1/k s ) with increased DPP was created by fitting a logistic function to the measured data points. The data points were also fitted using the general at ultra-high dose rates. Material and Methods