Abstract Book

S954

ESTRO 37

under the Australian MRI-Linac program, explores the configuration in which the linac is oriented parallel to the magnetic field (inline configuration), which has the potential to minimise or even exploit some of these effects. The purpose of this work was to dosimetrically characterise and commission this system for an upcoming application in patient treatment. Material and Methods The system consists of (1) a dedicated 1.0 T split-bore magnet (Agilent) integrated with the Avanto control system (Siemens Healthcare), Tesla gradient coils, Magentica radiofrequency coils and (2) a portable 6 MV FFF linear accelerator (Linatron-MP, Varex) combined with a 120-leaf MLC (Millennium, Varian). To minimise the beam output loss and the deflection of electrons in the linac, a twofold strategy was employed, guided by investigative measurements of the beam properties: a precise physical alignment of the radiation head to the magnetic field was complemented by an optimised design of the magnetic shielding of the target. Subsequently, mechanical parameters and dosimetric data characterising the radiation beam were acquired in line with the relevant national and international standards and recommendations (i.a. IEC, AAPM). Results Alignment of radiation beam and imaging isocentre within 2 mm was obtained through iterative adjustments, using a dedicated phantom (Leeds Test Objects), ball-bearing crosshair phantoms and a stand-alone EPID panel (Perkin Elmer). Reproducibility, proportionality and calibration of the dose monitoring system fulfilled the IEC criteria. Absolute dose, TPR 20,10 and output factor measurements were performed in a 1D water tank using a Farmer-type ionisation chamber (Scanditronix Wellhofer). Absorbed dose depth and lateral profiles were acquired using EBT3 (Ashland) films placed in solid water. In the depth dose profiles, focusing of the contamination electrons was visible. Penumbra of the lateral profiles did not show asymmetry inherent to perpendicular configurations. The profile symmetry was measured using the Starcheck MR maxi ionisation chamber array (PTW) and remained under 103% for the entire range of field sizes. Conclusion A comprehensive set of commissioning measurements has been conducted to characterise the unique high-field split-bore inline MRI-linac. The key parameters were within tolerances specified by the applicable standards and will form a baseline for future clinical quality assurance protocols. EP-1778 Long-term dose-response of Gafchromic EBT 2 film irradiated by proton beam and γ- rays. D. Borowicz 1,2 , J. Malicki 1,3 , G. Mitsyn 2 1 Greater Poland Cancer Centre, Dept. of Medical Physics, Poznań, Poland 2 Joint Institute for Nuclear Research, DLNP, Dubna, Russian Federation 3 Poznan University of Medical Sciences, Department Electroradiology, Poznań, Poland Purpose or Objective This study presents the long-term dose-response of dosimetric film after irradiation by γ-rays and proton beam with different energies. Material and Methods The EBT 2 film produced by Gafchromic was used in this work. The detector was irradiated by proton beam produced by a synchrocyclotron and by γ- rays. We used four energies of proton beam to our study: 110 MeV, 145 MeV, 180 MeV and 215 MeV. The sheet of the EBT 2 film was cut into small pieces-squares of 20mm x 20mm with perpetuated orientation for each square. The small films were irradiated perpendicularly to the central axis beam (CAX) with different doses from 0.0 Gy to 9.0 Gy at

intervals of 1.0 Gy. After 24 h, the small film were scanned for the first time by a flatbed Epson Perfection V750-M Pro Scanner (transparent 48-bit mode with 300 dpi resolution). ImageJ software was used to analyse the darkening of the film after irradiation. For each small film the pixel value (PV) was read at the red canal of the RGB bit map The response of the EBT 2 film to irradiation was articulated by net optical density (netOD) calculated from the PV. The small pieces of film were scanned several times over 5 months. Results The results of this study show that the time after irradiation affects on the dose-response of the EBT 2 film. The darkening of irradiated film changes and the netOD increases with time after irradiation. The observed changes are not big. However, we observed that boost of netOD is more evident for high doses of irradiation, e.g. 8 Gy, than for low doses, e.g. 2 Gy (Fig.1).

Figure. 1 Long-term dose-response of the EBT2 film to irradiation: 180 MeV proton beam and γ-rays. Conclusion There is no significant difference in netOD between γ-ray and proton beam irradiation. The energy of the proton beam also does not affect to the response of the film detector. The length of time after irradiation does not change the correlation between different types of irradiation and energies. EP-1779 Comparison between different acquisition protocols in cone beam CT-based dose calculation D. Sánchez-Artuñedo 1 , C. Delgado-Soler 1 1 Hospital Universitari Vall d'Hebron, Servei de Física i Protecció Radiològica, Barcelona, Spain Purpose or Objective Cone beam CT (CBCT) images in radiotherapy are mainly used to assess patient setup during treatment. Varian On- Board Imager (OBI) includes default imaging protocols for head, thorax and pelvis to ease its implementation. Adaptive radiotherapy and recently developed concepts as “dose of the day” try to exploit the potential for CBCT-based dose calculation. In any case, variability in HU due to using a CBCT can lead to differences in calculations. This study aims to evaluate differences in CT-based and CBCT-based dose calculation for the default Varian protocols.