ESTRO 38 Abstract book

S968 ESTRO 38

different measurement days (sets) are shown. The time between the six repetitions ranged from 1-3 hours. TLD signal was normalized to the mean of a measurement set. Calibration of TLDs was performed in a 60 Co beam. The precision of measurements was improved from 3% to 1.7% by applying the individual TLD sensitivity correction factors. By using the approach of repetitive measurements of the same dose (50 mGy) without annealing, the standard deviation was improved, in average from 1.7% to 1% compared to a readout process with prior annealing. Doses higher than 120 mGy led to an accumulated signal, for which the signal reset by readout was not sufficient. The number of repetitions, alternation of doses and the dose limits for the repetitive measurements need to be further investigated.

(square, circular and personalized). The results of eMC and GATE dose distribution are compared using the GIPR 3D. Results When comparing GATE and measurements, GIPR 2%/2mm for PDD and LDP in homogenous water phantom is always superior to 96% for all energies. Field size difference is always inferior to 1%. GIPR 3%/3mm for most of complexes cases (lung, bone and step) is superior to 98.5% for GATE (6, 9, 12 MeV). In all configurations GIPR of eMC are always inferior to GATE except for two cases: 12 MeV at 20 mm for lung (100% and 96.1% for eMC and GATE respectively) and bone (68.9% and 66.2%). First results of clinical cases show discrepancies between GATE and eMC dose distribution calculation (especially in high density regions). GIPR 3%/3mm ranged from 74.63 % to 77.25% for first tested patients. Conclusion GATE model for electron beams is validated in reference conditions. When comparing to measurements for heterogeneous media and irregular surfaces, eMC was limited while GATE was satisfactory in almost all configurations. First dosimetric results of clinical cases show also relevant discrepancies with eMC. These differences must be confirmed by experimental investigations. The study shows that GATE could be proposed as a 3D dose check solution for complex electron beams. Using our regional meso-computer (>13000 cores) GATE computation takes less than 4 minutes. EP-1789 Repetitive use of TLD-100 without annealing for imaging doses in radiotherapy M. Clausen 1,2 , P. Kuess 1,2 , D. Georg 1,2 , H. Palmans 3,4 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria ; 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Physics for Radiation Oncology, Vienna, Austria ; 3 MedAustron GmbH, EBG MedAustron, Wiener Neustadt, Austria ; 4 National Physical Laboratory, National Physical Laboratory, Teddington, United Kingdom Purpose or Objective Due to their small size and reusability thermoluminescence detectors (TLDs) are very practical and commonly used in various fields of radiation dosimetry. Before reusing the detectors, it is generally recommended to heat them in a TLD oven to achieve a complete reset via thermal annealing. This study investigates the repetitive use of TLDs, type TLD-100 (LiF:Mg,Ti, Thermo-Fischer Scientific), without intermediate annealing to improve the efficiency of measurements for the imaging doses (~ 50 mGy) in radiotherapy. Using the TLDs without the annealing process is time efficient. Moreover, since the consistency in the process of treating TLDs is a very crucial aspect, this can simplify the use and avoid possible errors coming from annealing. Material and Methods 20 TLDs were repetitively irradiated and read in a special- purpose reader (Risø, DTU Nutech, Denmark), with an inbuilt radiation ( 90 Sr/ 90 Y beta irradiator) source. For readout the temperature was increased at a rate of 5°C/s up to the final temperature of 400°C, in 400 steps. For the analysis, a region of interest (20 measurement points) around the dominant peak of the glow curve (5 th peak, 250°C) was evaluated. Individual sensitivity factors were applied for each specific TLD. The reproducibility of the dose measurements were studied as a function of irradiation time, and comparisons were made between TLD measurements with or without initial annealing. Results As a first step, TLDs were readout without irradiation. The TLD responses after the repetitive measurements without intermediate annealing are shown in figure below. Two

Conclusion The results indicate that the heating of the TLDs inside the reader for signal readout suffices to reset the TLDs to an acceptable level in the low dose range, which is typical for kV imaging procedures. Utilizing TLDs without annealing for this dose range can be beneficial for the dose assessment of imaging doses during X-rays, CT or microCT imaging, as this increases the workload efficiency and precision of measurements and simplifies the use of TLDs. EP-1790 TPS out of field dose accuracy: impact on dose volume histogram calculation of pacemaker devices A. Delana 1 , A. Barbareschi 2 , F. Maria Daniela 3 1 S. Chiara Hospital- Azienda Provinciale Per I Servizi Sanitari-, Department of Medica Physics, Trento, Italy ; 2 University of Roma Tor Vergata, Scuola di Specializzazione in Fisica Medica, Trento, Italy ; 3 SS. Annunziata Hospital, Department of Radiation Oncology, Chieti, Italy Purpose or Objective As always more people carrying an electronical device such as pacemakers (PMK) undergo radiotherapy, an evaluation to the dose received by PMKs has to be made. As stated in AAPM TG 158, Treatment Planning Systems (TPS) show large error in the out of field region where PMK is typically placed, and the magnitude of dose calculation error should be known to ensure the proper categorization and clinical patients management. Aim of this multicenter study is to evaluate the behavior of three TPSs in the of out of field dose region, comparing the dose volume histogram (DVH) of PMK placed in in-house built phantoms. Material and Methods Two phantoms were created with a PMK embedded in a PMMA slab with 2 cm or 1 cm of PTW Real Water 3 (RW3) over and 10 cm of RW3 under, to account for PMK depth inside the body and to ensure backscatter. CT datasets were acquired, PMK structure delineated, density corrections for the CT artifact performed; the CT datasets were then sent to the centers. TPSs considered are: Elekta-Monaco v.5.11 (C1), Philips-Pinnacle3 v.8 (C2) and Elekta–Oncentra Masterplan (C3). Linac commissioned is an Elekta Sinergy Agility; dose calculation algorithms are