ESTRO 2020 Abstract Book

S16 ESTRO 2020

chambers’ reference points were positioned in PMMA slabs at water equivalent depths of 21.3 and 21.1mm for the Farmer and Roos chamber, respectively. Both chambers were oriented orthogonal to the beam direction; the Farmer chamber orthogonal, the Roos chamber in-line with respect to the magnetic field (see Fig. 1). Measurements used a scanned 10x8cm² field of 252.7MeV protons at three magnetic field strengths of 0, 0.5 and 1T, respectively. Cross-calibrations in terms of absorbed dose to water were performed without magnetic field following the IAEA TRS 398 protocol adapted to scanned proton beams. Data evaluation was done after correcting for temperature and pressure. Statistical evaluation was based on the unpaired Student’s T-test.

A small cavity ion chamber (models: PTW31010, PTW31021, PTW31016 and PTW3012) is irradiated in water by a conventional 6 MV Elekta linac beam for various B- field strengths at two polarities: 0 T, ±0.35 T, ±0.5 T, ±1 T, ±1.5 T. The B-field is always perpendicular to the irradiation beam and the chamber axis. Two orientations of the chamber axis are studied: parallel and perpendicular to the photon beam. The experimental setup is simulated using egs_chamber (EGSnrc) and the enhanced EMF macro. The sensitive volume is reduced to account for the inefficiency adjacent to the guard electrode (i.e. dead volume) based on COMSOL simulations of electric potentials. Results For small chambers, the dead volume represents a large volume (15%-23%) of the sensitive volume. The B-field affects the chamber response by up to 4.1% and 4.5% in the parallel and perpendicular orientations, respectively. In the parallel orientation, the maximal percentage difference in relative response was reduced from 4.37%, 6.06% and 2.81% to 0.60% for PTW31010, PTW31021, PTW31016 and PTW30122 and from 1.91 % to 1.57% for PTW31016 when the dead volume is removed. In the perpendicular orientation, for B > 0 T, the maximal difference was reduced from 2.10%, 3.57%. 2.24% and 1.73% to 0.31%, 0.67%, 2.15% and 0.70% for PTW31010, PTW31021, PTW31016 and PTW30122, respectively. Experimental and simulated relative response of PTW31022 considering the complete sensitive volume and removing the dead volume are shown in the attached figure. Conclusion Results suggest that B-fields highlights chamber geometry imperfections in simulations and in measurements, discrepancies between them increases with B-field strength. The reliability of design specifications plays an essential role in dose response characterization, including the dead volume considerations, especially in the presence of magnetic fields. PH-0048 Recombination effects on the liquid-filled ionization chamber array used in VMAT quality control N. Solís Preciado 1 , B.C. Portas Ferradás 1 , A. Niebla Piñero 1 , J.Á. Merino Gestoso 1 , D. Jiménez Vegas 1 , G.F. Alberto 1 1 Hospital Universitario Nuestra Señora de Candelaria, Radiofísica, Santa Cruz de Tenerife, Spain Purpose or Objective VMAT quality assurance measurements are performed with a liquid-filled ionization chambers array because it allows a higher resolution than air-filled devices. However, it is accompanied by reduced collection efficiencies because of ion recombination effects. Ion recombination depends on the dose per pulse and pulse repetition frequency. This work is aimed to characterize the variability in PTW Octavius 1000 SRS array efficiencies with pulse dose and pulse frequency, and to correct subsequently quality assurance measurements. Material and Methods This study was made in a Versa HD (Elekta) linear accelerator with 6 MV FFF and 10 MV FFF beams with Octavius 1000 SRS array and a 0.125 PTW 31010 air-filled ionization chamber as reference. They both were exposed to different dose rates through the variation of solid water depth and source to detector distance. Python software was developed to perform a simple correction method using the measurements files from SRS and DICOM RT Plan files from TPS. From dose and time at each interval from SRS file, together with MU that were found from TPS file as a function of gantry angle, MU/min and pulse dose were calculated. Results A linear relationship was obtained between pulse dose and collection efficiencies, the latter increasing as the pulse

Figure 1: Sketch of the experimental set-up. Results

For the Roos chamber virtually no difference were found between measurements at 0 and 1T (differences on average 0.01%, STDev 0.2). Chamber behavior at 0.5T showed on average differences (w.r.t. 0T) of -0.72% (STDev 0.29), which was statistically significant (p>0.001). For the Farmer chamber, a similar behavior was observed. Measurements at 1T were not found to differ from measurements at 0T (differences on average 0.01%, STDev 0.26). Again, for 0.5T statistically significant differences (p>0.001) were found, with average deviations of -0.19% (STDev 0.25). Conclusion Differences in chamber readings were found to be magnetic field dependent. For 1T no noticeable influence of the magnetic field on the chamber reading was detected. However, the encountered differences were more pronounced for the intermediate magnetic field strength. In addition, the effect seems to be dependent on the chamber type. Especially for the plane-parallel Roos chamber the observed differences need to be corrected for. PH-0047 Small-cavity chamber dose response to megavoltage photon beams in the presence of magnetic fields Y. Cervantes 1 , I. Billas 2 , D. Shipley 2 , S. Duane 2 , H. Bouchard 1 1 Université de Montréal, Département de physique, Montreal, Canada ; 2 National Physical Laboratory, Purpose or Objective With the advent of magnetic resonance guided radiation therapy (MRgRT), dosimetry measurements must be performed in the presence of magnetic fields (B-fields). Monte Carlo (MC) simulations play an essential role in determining dosimetry correction factors. In particular, high-resolution measurements require small cavity ionization chambers, MC models require careful validation with experiments. The objective of this study is to characterize small cavity ion chamber response in the presence of B-fields via experimental measurements and MC simulations. Material and Methods Chemical- Medical and Environmental Science Department, Teddington, United Kingdom