ESTRO 36 Abstract Book

S218 ESTRO 36 2017 _______________________________________________________________________________________________

2 Bahawalpur Institute of Nuclear Medicine and Oncology BINO, Department of Medical Physics, BAHAWALPUR, Pakistan 3 The Islamia University of Bahawalpur, Medical Physics Research Group- Department of Physics, BAHAWALPUR, Pakistan Purpose or Objective Novel treatment units are becoming available that combine a magnetic resonance (MR) imager with a mega- voltage radiation beam. The magnetic field of the MR Imaging affects the trajectories of secondary electrons and influences the performance of several types of radiation detectors, including ion chambers and diodes. A dosimeter that is not influenced by the magnetic field would be valuable for commissioning and quality assurance (QA) of an MR-guided treatment unit. The purpose of this work was to characterise the performance of a PSD in the magnetic field of an MR-LINAC system. Material and Methods The MR-LINAC system consists of a 1.5 T Achieva MRI system (Philips, Netherlands) and a 7 MV linear accelerator (Elekta, UK). The feasibility of using the prototype PSD (Standard Imaging, USA) in MR-LINAC radiation therapy system was evaluated by investigating possible effects of the strong magnetic field on the performance characteristics of the PSD. The effects of orientation, axial rotation symmetry, and optical connectivity of the PSD and that of the photo-diode position were measured in the presence and absence of the magnetic field. Results The mean percent differences in the PSD signals for different orientation of the PSD and for various axial rotations of the PSD in the transverse magnetic field between magnet ramped up and down were 1.33% (±0.92%) and 1.37% (±1.01%), respectively. The effects of optical connectivity and photo-diode position were insignificant on the signal. Conclusion We conclude that the PSD can be used for dosimetry of the MR-LINAC radiation therapy system as the effect of a strong magnet field was insignificant on the characteristics of the PSD investigated. It would be a good detector for commissioning and QA of an MR-guided system. PV-0419 The impact that geometric variability in ionization chamber construction has on kQQo J. Puxeu Vaqué 1,2,3 , M. Duch Guillen 4 , M.C. Lizuain Arroyo 3 , W.H. Nailon 2 1 Hospital Universitari Sant Joan de Reus, Servei de Protecció Radiològica i Física Mèdica, Reus, Spain 2 Edinburgh Cancer Centre, Department of Oncology Physics, Edinburgh, United Kingdom 3 Institut Català D'Oncologia, Servei de Física Mèdica i Protecció Radiològica, L'hospitalet de Llobregat, Spain 4 Universitat politècnica de Catalunya, Institut de Tècniques Energètiques, Barcelona, Spain Purpose or Objective To investigate the influence that geometric uncertainties in the manufacturing process of three different ionization chambers has on the beam quality correction factor k QQo. Ionization chambers (IC) have been used as reference detectors in clinical practice for decades. In 2000 a new code of practice (TRS-398) was introduced based on the calibration of the ionization chambers in terms of absorbed dose to water instead of the previous code based on air kerma determination (TRS-277). Not all standard laboratories have beams with the same user beam qualities. One common approach is for the Standard Dosimetry Laboratory (SDL) to perform a calibration of the user’s ionization chamber in the beam quality of the Co-60 source. They may also provide

M.T. G arcia-Hernandez 1 , B. Sanchez-Nieto 2 , J. Roselló Ferrando 1 , L. Irazola 3 , J.A. Terrón 3 , F. Sanchez-Doblado 3,4 1 Hospital General Universitario de Vale ncia. ERESA, Servicio de Radiofísica., Valencia, Spain 2 Instituto Física Pontificia Universidad Católica de Chile, Departamento Física Médica, Santiago de Chile, Chile 3 Universidad de Sevilla, Departamento de Fisiología Médica y Biofísica, Sevilla, Spain 4 Hospital Universitario Virgen Macarena, Servicio de Radiofísica, Sevilla, Spain Purpose or Objective The study of Secondary Malignant Neopl asms, as a consequence of the peripheral doses received by photon radiotherapy patients, is becoming a topic of growing interest due to the higher healing rates and life expectancy accomplished nowadays. Two models have been developed to estimate peripheral doses due to photon and neutron dose deposition (1,2). The aim of this work is the validation of a generic peripheral photon dose model (1) for the flattening filter free (FFF) modality, which was not originally considered. Material and Methods Measurements were carried out in a Varian Truebeam linac for FF and FFF beams (with 6 and 10 MV) for two different field sizes (3x3 and 10x10 cm 2 ) with both, single (gantry 0º) and multiple incidences (0°, 45°, 90°, 135°, 180°, 225°, 270°, 315º). A CC13 (Iba Dosimetry) ionization chamber was placed at a range of out-of-field distances (10 to 60 cm from the field-edge) in a water-equivalent phantom and irradiated with 1000 MU. The obtained results for FF and FFF were compared to estimations with the original model (1). Results Experimental measurements, together with model predictions for all combinations described were collected. By way of example, results the 10x10 cm 2 field using 6 and 10 MV FFF multiple incidences have been depicted in Fig 1a and 1b, respectively. The uncertainty range (UR) of the model (95% confidence interval) (1) as well as the 5% uncertainty estimated for the experimental measurements, are shown. Conclusion The original photon model tends to overestimate peripheral doses, especially for the high energy. This is due to the fact that FFF beams, in comparison with FF beams, are associated to lesser scatter in the linac hea. This effect is enhanced at higher energies. Thus, despite that the original model could be used for peripheral photon dose assessment in FFF modality (experimental data are almost included within the UR of the model), further investigation should be conducted to better model the effect of the absence of the flattening filter. References (1) Sánchez-Nieto et al ., Biomed Phys Eng Express 2015;1:045205. (2) Sánchez-Doblado et al.,Phys Med Biol 2012;57:6167– 6191. PV-0418 Characterisation of the prototype plastic scintillation detector (PSD) in a strong magnetic field M. Asghar 1,2,3 , W. Zhifei 1 , Y. Suh 1 , D. O'Brien 1 , S. Beddar 1 , S.A. Buzdar 3 , G.S. Ibbott 1 1 The University of Texas MD Anderson Cancer Center, Department of Radiation Physics, Houston, USA