ESTRO 36 Abstract Book

S788 ESTRO 36 _______________________________________________________________________________________________

dose by the accelerator directly and evaluates the dose delivery error to the plan. We have used True-Beam (Varian) with 10 MV photon beams and a QA plan which underwent prostate VMAT. For the QA plan, we prepared seven modified MLC files in which MLC positions were manually shifted between 0 mm to 3 mm, respectively. Then, the dose delivery errors were measured and analyzed dose deviation, distance-to-agreement (DTA) and gamma index by the Delta4 Discover system as well as to the Delta 4 3D dosimetry system. All measurements were compared against the points that received less than appropriate percentage dose (<10%) were excluded from the gamma index calculation. Results The results of the Delta4 Discover system and the Delta 4 3D dosimetry system using all available points for the gamma calculation, 95.7±5.9% and 97.8±3.5% of them passed the criteria (3%/2mmDTA/Th10%), respectively. The two systems also had high correlations of dose deviation and DTA, permitting the routine verification of VMAT patient-specific QA plan as well as permanent in- vivo dosimetry during the patient’s treatment course. Conclusion In this study, we have found that there was high correlation between the pass rate and the intentional dose delivery error, as respect to dose deviation, DTA, and gamma index of the Delta 4 Discover system and the Delta 4 3D dosimetry system. It was suggested that the dosimetric verification system under investigation could be useful for routine patient specific QA. EP-1475 RBE estimation of different Brachytherapy sources based on micro- and nanodosimetry M. Bug 1 , T. Schneider 2 1 Phys. Techn. Bundesanstalt PTB, 6.5 Radiation Effects, Braunschweig, Germany 2 Phys. Techn. Bundesanstalt PTB, 6.3 Radiation Protection Dosimetry, Braunschweig, Germany Purpose or Objective Depth-dependent RBE values of typical photon-emitting Brachytherapy (BT)-sources were determined by a microdosimetric and a nanodosimetric approach. The microdosimetric approach considers a biological endpoint while the nanodosimetric approach is entirely based on the track structure, given by the interactions of the photons and secondary electrons. The track structure characterizes the radiation quality on the nanometric scale. Material and Methods Within a cylindrical water phantom, isotropically emitting BT-sources were positioned 4 cm below the surface. Studied were Co-60 and Ir-192 representing high-energy photon-emitting sources, I-125 being a low-energy photon- emitting source, and Intrabeam ® - and Axxent ® -devices as examples for electronic BT X-ray sources (EBX). Resulting photon spectra were calculated at several points along the cylindrical axis within cylindrical voxels of 0.5 mm depth and 2 mm radius up to a depth of 10 cm. The microdosimetric calculations of RBE are based on yield coefficients α dic, representing the linear component of the dose-effect relationship for the dicentric chromosome aberration yield after an irradiation with monoenergetic photons. The RBE for a given source in a given point was determined by convoluting the respective spectrum with the function α dic (E), obtained previously by microdosimetric calculations. The same depth-dependent photon spectra were used to determine nanodosimetric quantities by Geant4-DNA calculations. For each initial photon track, target volumes in size of one DNA convolution which experienced at least 4 ionizations (F4) were identified. Such quantities were previously shown to describe the DSB yield. Based on the

average minimum distance between these volumes within each photon track, the RBE was estimated by normalization to the distance for Co-60 at 0.125 mm depth. Results Relative depth-dependent RBE based on nanodosimetric quantities are similar to the microdosimetric RBE. For Co- 60 and Ir-192, the RBE increases with depth due to an increasing contribution of low-energy photons in the spectra. For the denser ionizing sources, nanodosimetric RBE values were divided by 1.9. Apart from this factor, the constant RBE-dependence up to 10 cm for I-125 and the decrease of RBE for the two EBX sources due to beam hardening are in good agreement with the microdosimetric RBE. Conclusion RBE based on track structure (nanodoismetric approach) shows that the average intra-track distance between DNA- modelling volumes potentially suffering severe damage is well related to the microdosimetric RBE, based on the formation of dicentric chromosomes, for several BT- sources. Apart from a constant normalization factor for the denser ionizing sources, the depth-dependence is in excellent agreement. This indicates that the nanodosimetric photon track characterization performed in this study is a good descriptor for the radiation quality. Furthermore, the proposed target volume appears realistic. Note, that neither the photon fluence nor biological endpoints were taken into account for this approach. EP-1476 Preliminary results of in-vivo dosimetry by EPID S. Giancaterino 1 , M. Falco 2 , A. De Nicola 2 , N. Adorante 2 , M. Di Tommaso 2 , M. Trignani 2 , A. Allajbej 2 , F. Perrotti 2 , D. Genovesi 2 , F. Greco 3 , M. Grusio 3 , A. Piermattei 3 1 Ospedale Clinicizzato S.S. Annunziata, Radioterapia, Chieti, Italy 2 University of Chieti SS. Annunziata Hospital, Department of Radiation Oncology “G. D’Annunzio”-, Chieti, Italy 3 Università Cattolica del Sacro Cuore, Medical Physics Institute - Fondazione Policlinico Universitario A. Gemelli-, Rome, Italy Purpose or Objective This study reports in-vivo dose verification (IVD) results elaborated with SOFTDISO software on 300 cancer patients treated with 3D-CRT, IMRT and VMAT techniques. SOFTDISO uses the integral EPID image referred to each single static or dynamic beam providing a quasi- real-time test elaboration. Material and Methods The selected patients for this study were treated with an Elekta Synergy Agility LINAC at SS. Annunziata Hospital. 3D-CRT, IMRT and VMAT treatment plans of 300 patients were randomly selected. IVD tests were processed with the SOFTDISO software who provides two type of tests: (i) R ratio between the reconstructed isocenter dose and the planned one; (ii) transit dosimetry based on γ- analysis of EPID imaging (P g (%) and g mean ). Results We identified class-1 errors, derived from inadequate QCs, and class-2 errors due to patient morphological changes. Considering overall (6697) tests, we found out that only 5% of them showed out-of-tolerance mean R values. For gamma index analysis, in 13% of the overall tests were found to be out of tolerance. Ignoring class-2 errors, 100% of patients treated with different radiotherapy techniques (except 3DCRT breast treatment, for which no class-2 errors were observed) reported mean P g (%) values within tolerance levels. Thus, the percentage of out- of- tolerance tests decreases from 13% to 7%. However,