ESTRO 2023 - Abstract Book

S1498

Digital Posters

ESTRO 2023

1 T. The results between the simulations with and without the correct modelling of the non-sensitive air volume of the Farmer chamber differs by 0.14 % at maximum. The detailed simulations demonstrate that the secondary electrons contribute primarily to the magnetic field dependent chamber response. Conclusion Results from this study are consistent with the experimental results presented by Fuchs et al. Overall, the change of chamber response is limited to below 0.66 % for all studied configurations. Detailed simulations have provided useful insights on the chamber behavior in magnetic field. Further studies are necessary to extend the investigations to other measurement conditions, such as depths and field sizes, before the safe implementation of MRgPRT can be guaranteed. M. Caprioli 1 , L. Delombaerde 2 , N. De Keyzer 3 , L. de Freitas Nascimento 4 , R. De Roover 5 , K. Himschoot 6 , B. van der Heyden 4 , D. Vandenbroucke 6 , P. Leblans 6 , W. Crijns 2 1 KU Leuven, Laboratoty of Experimental Radiotherapy, Leuven, Belgium; 2 UZ Leuven, Radiation Oncology, Leuven, Belgium; 3 PTW Freiburg GmbH, PTW benelux, Freiburg, Germany; 4 SCK CEN , Research in Dosimetric Application group, Mol, Belgium; 5 UZ Leuven, Oncology Department, Leuven, Belgium; 6 Agfa N.V., Corporate Innovation Office, Mortsel, Belgium Purpose or Objective The present study evaluates the performance of three multidimensional dosimeters used for radiotherapy patient-specific QA (PSQA). Materials and Methods 10 clinical 6MV flattening filter free (FFF) SBRT plans (Table 1) were selected. The dose was calculated with the ACUROS algorithm (16.1.0) available on Eclipse (Varian Medical Systems). The plans were delivered on a TrueBeam STx linac. The Octavius® 1500 detector (O4D, from PTW) performed a real-time 3D dose integration using its rotational phantom. A longitudinal resolution (res) of 5mm was obtained merging two measurements with a 5mm gun-to-target couch shift. 10 GafchromicTM EBT3 films (Ashland) and an erasable optically stimulated luminescence (OSL) research film (QUARTEL project) were used in a Mcube phantom (IBA). The EBT3s were scanned 48h after the irradiation with a Flatbed CCD scanner (res 0,175mm) and converted to dose via a red channel optical density calibration on 2 additional calibration films as used in our local protocol. The luminescence was digitized with a CR scanner (res 0.1mm, AGFA) 10min after irradiation. After a flood field homogeneity correction on the raw image, the signal was linearly converted to dose with a calibration in the dose range 0.5-15Gy. The measured dose of the three detectors was rescaled using a factor (krescale) calculated as the median local dose difference within the 50% region from the DTPS,max. With DTPS,max as the reference, a 3%/2mm 3D global gamma was calculated with 50% low-dose threshold on the O4D (PTW, VeriSoft®). All 2D global gamma analysis (3%/2mm, 50% low-dose-threshold) were performed in a third party software (MyQA® patient, IBA) on the coronal dose plane measured at the isocenter with the three detectors. Results The krescale are listed in Table 1. The O4D has the lowest average rescaling and 1 σ (1.035±0.019), suggesting a higher accuracy and precision. The EBT3s dose difference might be due to a calibration aging effect (under study). The OSL accuracy can improve with a calibration considering its signal fading and energy dependence. After the rescaling, with the exception of the SBRT sacral treatment, the O4D showed a 3D agreement score >97.5%. When used in a 2D dose plane projection the score is >91%. The mm-intracranial target volume requires a detector with a higher resolution. For EBT3s the 2D agreement score is 100% unless for the adrenal plan, which is 94.8%. The same trend was captured by the OSL film with an agreement score> 92%. PO-1777 Comparison of Diode Array, Radiochromic Film and OSL Film Dosimeters for Patient Specific QA

Fig.1: global gamma (3%/2mm, 50% low-dose-thresold) with DTPS,max as reference.