ESTRO 2020 Abstract book

S385 ESTRO 2020

simulations have been performed to understand the 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 at ultra-high dose rates. Material and Methods

points. The data points were also fitted using the general theoretical Boag model of ion recombination. The experiment was repeated with increased polarizing voltages (-640 V and -960 V), to investigate any possible improvements of the chamber’s ion collection efficiency at ultra-high dose rates. Results The ion collection efficiency of the transmission chamber decreased with increasing DPP values. However, by increasing the voltage applied over the chamber from 320 V to 640 V and 960 V (negative polarity), the ion collection efficiency at the highest dose rate was increased by a factor 1.8 and 2.7, respectively. The logistic model accurately described the measured data, with R 2 -values of 0.995, 0.998 and 0.996 for -320 V, -640 V and -960 V, respectively (Figure 1a). In comparison, the Boag model was not as good at describing the drop in ion collection efficiency with increasing DPP, with corresponding R 2 - values of 0.989, 0.972 and 0.971 (Figure 1b).

Conclusion In this study, the drop in ion collection efficiency with increasing DPP of a linear accelerator’s built-in transmission chamber has been measured and modelled. By increasing the polarizing voltage, the drop in ion collection efficiency could be reduced, improving the response of the chamber at FLASH dose rates by a factor 2.7 when compared to the standard operational mode. The suggested model allows correction for ion recombination, making the transmission chamber useful for online dosimetry at ultra-high dose rates. OC-0635 ImageDosis: 2D real-time dosimetry system L. De Freitas Nascimento 1 , D. Verellen 2 , J. Goossens 2 , F. Vanhavere 1 , M. Akselrod 3 1 Belgian Nuclear Research Centre - SCK-CEN, Radiation Protection Dosimetry and Calibration, Mol, Belgium ; 2 Iridium Kankernetwerk, Department of Radiotherapy, Antwerpen, Belgium ; 3 Landauer Inc., Stillwater Crystal Growth Division, Stillwater, USA Purpose or Objective Radiotherapy (RT) is a highly complex, multi-step process that requires the input of different experts in planning and delivering the treatment. It is important to consider in- vivo dosimetry (IVD) as part of RT safety and quality