ESTRO 2021 Abstract Book

S1313

ESTRO 2021

However, no phantom rotations were taken into account.

PO-1588 Is the Octavius4D able to detect some pre-fixed errors during pre-treatment quality controls? V. Silvestri 1,2 , M.G. Guerrisi 1 , D. Genovesi 2 , M.D. Falco 2 1 University of Rome Tor Vergata, Department of Medicine, Medical Physics Section, Rome, Italy; 2 University of Chieti, SS. Annunziata Hospital, Department of Radiation Oncology ‘‘G. D’Annunzio”, Chieti, Italy Purpose or Objective The presence of small errors during the delivery of complex radiotherapy treatments (IMRT and VMAT) can cause an under/over dosage on both the target and organ at risk (OAR). We investigated the ability of the Octavius4D phantom to detect some pre-fixed errors introduced in the TPS and their impact on DVH. Materials and Methods MLC bank shift errors (±0.25 mm and ±1 mm), collimator angle errors (±1°) and gantry angle errors (±1°) were intentionally introduced into the Pinnacle TPS using appropriate scripts. A total of 27 plans were generated from Head&Neck (H&N), prostate (Pro) and breast treatments. All measurements were conducted with the Elekta Synergy Agility linac and Octavius4D phantom equipped with the Octavius 1500 detector. The comparison between TPS plan and measurements was performed using the gamma index method, using customized protocols: 3%/3 mm/Local for H&N; 2%/2 mm/Local for Pro; 2.5%/2.5 mm/Local for breast. In the TPS, the error-free plans were accepted considering, for the PTV coverage, the V95 greater than 95% and for all OAR, compliance with all constraints. Results Only for H&N case and MLC bank shift of ±1mm, a significant reduction in PTV coverage was observed (from 96% to 84%). No relevant effects were found on OAR. A ±1mm MLC bank shift caused a reduction in the gamma index pass rate which dropped to 78.6%, 80% and 89.7% for H&N, Pro and breast, respectively. Regarding collimator errors, the gamma test failed for H&N e Pro (88.2% and 88%, respectively). Finally, for the gantry errors, the reduction in pass rate was only observed for Pro plans (84.3%). Conclusion Octavius4D phantom was able to detect MLC bank shift errors of ±1 mm with a significant impact on DVH only in the case of H&N; depending on the complexity of the plan, it can detect deviations of the collimator and gantry angle from the prescribed ones. Purpose or Objective Optically stimulated luminescence dosimeters (OSLDs) have properties that make them so important to radiation dosimetry for in vivo dosimetry. The aim of the study is to figure out the physical and the dosimetric characteristics of the Beryllium Oxide (BeO) OSL dosimetric system before using it routinely in clinical practice in radiotherapy. Materials and Methods In this study, OSLs were irradiated by using 6 MV, 10 MV, and 15 MV photon beams with the Elekta Versa HD linear accelerator. The output dose of the linear accelerator was calibrated by using the absorbed dose calibration protocol of the IAEA TRS-398 in the water at the depth of d max . Monitor units per cGy for a 10x10cm 2 field size at a source-to-surface distance (SSD) of 100cm was calibrated 1cGy for 1MU. Absolute dose measurements were made with a 0.6cc cylindrical ion chamber, PTW 30013 Farmer. PTW 30x30cm2 solid water slab phantoms and PTW Octavius 4D phantom was used at the measurements. The build-up thicknesses varied depending on the irradiated photon energy, back scatter thickness was 10 cm for all measurements. BeO OSLs were first annealed at 700 o C for 3 hours. They were placed in a plastic mold and given an ID number. In the optical eraser section, the eraser process was performed for 30 minutes and the base level was determined in the reader. The OSL decay curve was obtained by irradiating at a known dose with the irradiation system with Cs-137. Calibration curves were defined by matching each curve with the ID numbers of the OSLs. Bolus was used to filling the cavity around the OSL dosimeters. The measurements were taken by placing BeO OSLs at the maximum dose depth and ion chamber at the reference depth for each energy at the source surface distance (SSD) at 100 cm. For the angular/directional dependence test, the setup for the cylindrical phantom was the same as the slab phantom, but a SAD of 100 cm was set to the isocenter of the phantom and center of the OSL dosimeter. The tests performed were reproducibility/sensitivity, dose- response linearity, energy dependence, angular dependence and field size dependence. As a result of the tests, correction factors and functions were obtained, and the OSL measurement accuracy was increased by using these factors and functions. Results Reproducibility/sensitivity and angular dependence are smaller than %2, dose-response linearity and energy dependence are smaller than %1. After making SSD corrections recommends in IAEA Human Health Reports No:8, the dependency of SSD is %3 for SSD 80 and smaller than %2 for other SSDs. The field size correction factor should be used if a field size other than 10x10 cm 2 during irradiation. Conclusion By obtaining factors as mentioned in the AAPM TG 191 the accuracy of OSLD system could be increase. The use of BeO OSL in radiotherapy applications is promising with advanced measurements. PO-1590 Accuracy of brain radiosurgery: an in phantom study for Gamma Knife treatments S. Calusi 1,3 , C. Arilli 2 , M. Casati 2 , A. Compagnucci 2 , L. Marrazzo 2 , C. Talamonti 1,3,2 , M. Zani 2 , I. Desideri 1,4 , D. Greto 4 , G. Pecchioli 5 , S. Pallotta 1,3,2 1 University of Florence, Department of Biomedical, Experimental and Clinical Sciences “Mario Serio”, PO-1589 Study on OSL for BeO: Performance Tests for High Energy Photons E. Kara 1 , A. Hicsonmez 1 1 Onko Ankara Oncology Center, Radiation Oncology, Ankara, Turkey