ESTRO 36 Abstract Book

S779 ESTRO 36 2017 _______________________________________________________________________________________________

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, considering all the techniques, only 4.4% of mean g mean tests resulted out of tolerance. In addition, removing class-2 errors, this percentage decreases to approximately 3%. Actually the workload of IVD procedures on 9 patients is 1 hour per day. Conclusion

IVD performed using SOFTDISO assures: (i) a rapid response of dose delivery alert with a reduced workload; (ii) a large number of patients tested daily and (iii) for out- of- tolerance tests repeating IVD in the subsequent day, the possibility to verify the efficacy of the adopted corrections. EP-1477 Evaluating gamma-index quality assurance methods for Nasopharynx Volumetric Arc Therapy (VMAT) E.M. Pogson 1,2,3 , S. Arumugam 2 , S. Blake 1 , N. Roberts 4 , C. Hansen 5,6 , M. Currie 7 , M. Carolan 7 , P. Vial 2 , J. Juresic 2 , C. Ochoa 2 , J. Yakobi 2 , A. Haman 2 , A. Trtovac 2 , L. Holloway 1,2,3,4,8 , D.I. Thwaites 1 1 University of Sydney, Institute of Medical Physics- School of Physics- Faculty of Science, Sydney NSW, Australia 2 South Western Sydney Local Health District, Liverpool and Macarthur Cancer Therapy Centres, Liverpool, Australia 3 Ingham Institute, Medical Physics, Liverpool, Australia 4 University of Wollongong, Centre for Medical Radiation Physics- School of Physics, Wollongong, Australia 5 Odense University Hospital, Laboratory of Radiation Physics, Odense, Denmark 6 University of Southern Denmark, Faculty of Health Sciences- University of Southern Denmark- Denmark, Odense, Denmark 7 Illawarra and Shoalhaven Local Health District, Illawarra Cancer Care Centre, Wollongong, Australia 8 University of New South Wales, South Western Sydney Clinical School, Sydney, Australia Purpose or Objective Pre-treatment dose verification is often performed on dose measuring phantoms with some form of gamma evaluation. However it has been shown that the clinical relevance of a 3% and 3mm pass rate tolerance is questionable. The purpose of this study is to simulate machine errors of clinical significance for nasopharynx patients and test if these errors can be detected on a standard commercial phantom. In this study systematic errors including collimator rotation, gantry rotation, MLC shifts, and MLC field sizes are investigated. Material and Methods Ten retrospective VMAT patients were planned with a department protocol. Machine errors were deliberately introduced to all plans. Plans were modified by increments using Python to create simulated error plans; -5 to 5° for gantry and collimator angles and -5 to +5mm for MLC shift and MLC field size, considering each parameter separately. Simulated error plans (Dose error ) were compared to the original non-error plan (Dose Baseline ) utilising equation (1). (1) All error plans doses were then recalculated in Pinnacle 3 . Plans were reviewed against acceptable tolerance limits. Plans were above tolerance and considered unacceptable if PTV D95%, Brainstem D1cc or spinal cord D1cc were beyond a ±5% deviation in dose. Additionally if either of the left or right parotid mean doses were beyond ±10%, this was also considered an unacceptable plan. The smallest unacceptable error plan for each error type (including the Gantry (G), Collimator (C), MLC Shift (S), and MLC Field Size (F) error was delivered on an Elekta Linac and dose was measured using an ArcCheck. Gamma analysis was performed in SNCpatient version 6.6 utilising a global 3%/3 mm (10% threshold with correction off) gamma pass rate. Before measurement, the Linacs were tested for MLC, gantry and dose accuracy. Only one