ESTRO 37 Abstract book

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ESTRO 37

an Elekta Synergy linacs. The dosimetric properties of the MDs were assessed in terms of absorbed dose to water determination in reference conditions and OF measurements down to field sizes of about 5 mm. A detailed modelling of the MD detector was performed, and both beam quality and output correction factors were obtained for each accelerator by Monte Carlo calculation. The obtained MD output correction factors were also analysed in terms of two main effects, namely the volume averaging and detector material effects. MC simulations were thus performed in order to separately evaluate their contribution to the output correction factors. Results The experimental beam quality correction factors measured for the ten MDs were found to be in agreement within 1% with the MC calculated ones. A high reproducibility of the OF determination was observed among the ten MDs with a maximum standard deviation of the OF values of about 0.4 × 10 −2 at the smallest fields. Such a value was demonstrated to be due to a relatively poor reproducibility of the field sizes defined by jaws and multi-leaf collimators, affecting the output of the accelerators at very small fields. Indeed, a standard deviation of 0.14 × 10 −2 was obtained when fixed collimators were used. Output correction factors within +0.7% and −1.4% were obtained in all cases for field sizes down 5 mm. An empirical function was also derived, providing output correction factors within 0.5% from the MC values calculated for each of the three linacs. The very small values of the output correction factors, as well as their trend as a function of the field size were found to result from a compensation between the volume averaging and detector material effects. Conclusion The results of the present study represent a validation of the Monte Carlo modelling of the PTW 60019 dosimeter. The MD evidenced a high reproducibility of its response and very small output correction factors down to beam sizes as narrow as 5 mm, making it a suitable candidate for reference small field dosimetry. EP-1731 A comparative study of passive detectors in active scanning proton beams A. Thummerer 1 , P. Kuess 1,2 , D. Georg 1,2 , M. Clausen 1,2 1 Medical University of Vienna, Department of Radiotherapy, Vienna, Austria 2 Medical University of Vienna, Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria Purpose or Objective Radiochromic films and thermo-luminescent detectors (TLDs) have smaller size and higher spatial resolution than ionization chambers. This makes them suitable for use in custom or commercial phantoms and allows obtaining a complete depth-dose curve in a single irradiation for particle beams. Performing depth-dose measurements in this manner saves valuable beam time which is especially beneficial in dose verification with active scanning beam delivery. The purpose of this study is to compare the response of three different types of passive detectors: radiochromic EBT3 and EBT-XD films (GAFchromic, NJ, USA) and TLDs (TLD-100, Thermo Scientific, MA, USA) in proton beam spread-out Bragg peaks (SOBP) delivered by active scanning. Additionally, considerable effort was put into TLD characterization and individual correction factor determination in order to reduce the uncertainties for advanced dosimetric applications, i.e. in anthropo- morphic phantoms. Material and Methods The RayStation treatment planning system (RaySearch, Sweden) was utilized to generate a SOBP of 8 cm in depth. An RW3 slab phantom (PTW, Germany) was used

Results Two plans in different depth of the phantom were evaluated. The first irradiation setting was designed in the proximal part of the phantom (SOBP 8cm, energies: 66-135 MeV). The second one was designed in the middle of the phantom (SOBP 8cm, energies: 127-180 MeV). The depth dose measurements with IC showed a very good agreement with the TPS prediction (within 3%) for the first plan (see figure). The differences up to 6% between TPS data and IC were measured for the second plan. Dose values measured with EBT3 films agreed well with IC measurements in the area before the SOBP. The under- response in SOBP of up to 15% for EBT3 films indicates signal saturation due to linear energy transfer dependence. The dose homogeneity of up to 5% was obtained for both plans. Conclusion The presented phantom is a valuable tool for biological cell studies of RBE values. A dosimetric characterization was performed in non-reference conditions and all possible errors from the setup and measurements were determined in order to realistically estimate dosimetric uncertainties for biological experiments. EP-1730 Small field dosimetry by the PTW microDiamond: multicenter experimental study and MC simulations G. Verona-Rinati 1 , P. Francescon 2 , M. Marinelli 1 , L. Masi 3 , L. Paganini 4 , M. Pimpinella 5 , G. Prestopino 1 , S. Russo 6 , A. Stravato 4 , C. Verona 1 1 Tor Vergata University, Industrial Engineering, Roma, Italy 2 Ospedale di Vicenza, Department of Radiation Oncology, Vicenza, Italy 3 IFCA, Department of Medical Physics and Radiation Oncology, Firenze, Italy 4 Humanitas Research Hospital, Medical Physics Unit of Radiation Oncology, Milano, Italy 5 ENEA-INMRI C R Casaccia, Istituto Nazionale di Metrologia delle Radiazioni Ionizzanti, Roma, Italy 6 Azienda Sanitaria Firenze, Medical Physics Unit of Radiation Oncology, Firenze, Italy Purpose or Objective A great effort was recently devoted to the charac- terization of several commercial detectors for small field relative dosimetry application. Output correction factors were evaluated for such devices, either experimentally or by Monte Carlo calculations. In the case of the PTW 60019 microDiamond (MD), some discrepancies can be noticed in very small fields (below 1 cm) among the results reported in the recent literature. In the present work, a systematic study of the MD output factors (OFs) is reported, aimed at clarifying its response in very small fields and investigating the main effects that influence the detector response in such irradiation conditions. Material and Methods Ten MDs, calibrated under 60 Co source, were investigated in the present work. Measurements were performed in 6 MV photon beams by a CyberKnife M6, a Varian DHX and