ESTRO 36 Abstract Book

S406 ESTRO 36 _______________________________________________________________________________________________

PO-0767 Revisiting EPID design for modern radiotherapy requirements P. Vial 1,2 , S. Blake 2,3 , Z. Cheng 2,3 , S. Deshpande 1,4 , S. Atakaramians 5 , M. Lu 6 , S. Meikle 7 , P. Greer 8,9 , Z. Kuncic 2 1 Liverpool and Macarthur Cancer Therapy Centres and Ingham Institute, Department of Medical Physics, Liverpool BC, Australia 2 University of Sydney, Institute of Medical Physics- School of Physics, Sydney, Australia 3 Ingham Institute, Medical Physics, Liverpool, Australia 4 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia 5 University of Sydney, Institute of Photonics and Optical Science- School of Physics, Sydney, Australia 6 Perkin Elmer, Medical Imaging, Santa Clara, USA 7 University of Sydney, Faculty of Health Sciences & Brain and Mind Centre, Sydney, Australia 8 University of Newcastle, School of Mathematical and Physicsal Sciences, Newcastle, Australia 9 Calvary Mater Newcastle Hospital, Radiation Oncology, Newcastle, Australia Purpose or Objective New methods of treatment verification that are in keeping with advances in radiotherapy technology are desirable. The availability of kilovoltage in-room imaging for example has led to a general trend away from the poorer contrast megavoltage (MV) imaging for patient-set-up. The widespread use of intensity-modulated radiotherapy (IMRT) also reduces the utility of treatment beams as a source of imaging for treatment verification. At the same time there has been a steady increase in the use of electronic portal imaging devices (EPIDs) for dose verification. There is however emerging evidence of new roles for MV imaging in real-time target tracking. In this work we address the issue of EPID detector specifications in light of changing clinical requirements. We present a general overview of the detector development work our group has undertaken to design an EPID that better supports applications relevant to current and future clinical practice. Material and Methods Prototype EPID technologies developed by our group include: a direct detector EPID where the metal/phosphor screen has been replaced by a water equivalent build-up material [1]; a dual detector combining a standard EPID and an array dosimeter [2]; and an EPID comprising a plastic scintillator fibre array (PSFA) in place of the metal/phosphor screen [3]. Our performance specifications were to achieve imaging performance equivalent to standard EPIDs, and a dose response equivalent to standard clinical dosimeters. Quantitative metrics such as detective quantum efficiency (DQE) for imaging and field size response for dosimetry were used in both experimental and Monte Carlo (MC) studies. There are three arms to this project that shall be described; i) MC simulations to characterise and design scintillators, ii) Prototype construction and experimental evaluation, iii) clinical implementation. Results All prototype detectors exhibited near equivalent dose response with ionisation chambers in both non-transit and transit geometries (± 2%), including 2D clinical dosimetry of IMRT fields. The X-ray quantum efficiency of the direct and PSFA detectors is approximately 9% compared to 2% for the standard EPID and dual detector. The imaging performance of the standard EPID and dual detector remains superior to the other prototypes because of the greater efficiency of optical photons detected per incident X-ray and better spatial resolution. MC simulations demonstrate potential improvements in imaging with the PSFA. A model for clinical implementation has been developed that exploits the water equivalence of the detectors. A water equivalent EPID provides more direct and robust verification than can

be achieved with current EPID dosimetry. A water equivalent EPID that retains imaging capability is better suited than current EPIDs for modern radiotherapy. Conclusion This work demonstrates the feasibility and advantages of alternative EPID designs that better meet the needs of modern radiotherapy. PO-0768 Electron Paramagnetic Resonance signal from a new solid polymer material aimed for 3D dosimetry M.R. Bernal-Zamorano 1 , N.H. Sanders 1 , L. Lindvold 1 , C.E. Andersen 1 1 DTU, Nutech, Roskilde, Denmark Purpose or Objective We have developed a water-equivalent solid polymer dosimeter material aimed for 3D dosimetry in radiotherapy beams. The material responds to ionizing radiation by changes in its optical absorbance and by generation of fluorescence centers. The latter signal is of particular interest as the fluorescence centers facilitate detailed mapping the 3D dose distribution us ing laser stimulation. However, in addition to the optical si gnals we also expect that the material could have an electron paramagnetic resonance (EPR) dose response related to the production of stable free radicals. To test this hypothesis, point detector experiments were therefore performed where the material was casted into 5 mm diameter pellets identical in size to the alanine dosimeters that we routinely use for reference EPR dosimetry in our laboratory. The pellets of the new material and alanine were irradiated in 60 Co beams and EPR signals were The dosimeter is based in pararosaniline leuco dye, which is chemically transformed into its dye-form by the effect of radiation. The leuco dye is dissolved in a poly(ethylene glycol) diacrylate matrix (PEGDA-575 g/mol) that contains diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide (TPO) used for photocuring. We cured the material in a mold with a 395 nm LED for a few minutes. We made 4 cylindrical pellets of 4.75 mm diameter and 2.78 mm thickness (same size than alanine dosimeters used in this work). Pellets of the new material and alanine were irradiated in a 60 Co gamma source with a dose rate of about 8 Gy min -1 . They were given doses of 5, 10, 20, 30, 50, 75 and 100 Gy. The EPR signal for both dosimeters was obtained by a Bruker EMX-micro spectrometer by inserting the pellets into the resonator in a quartz tube. Absorbance and fluorescence signals of the pellets of our material were measured with a Shimazdu UV-2700 spectrophotometer and an Ocean Optics QE6500 spectrometer respectively. Fluorescence was excited with a diode laser. Results A clear EPR signal was obtained for our material, and this signal increased with dose. The peak-to-peak amplitude of the EPR spectra are shown in the figures. Although alanine and PEGDA have similar characteristics in terms of its water equivalence (similar effective atomic number, mass density and electronic density), their EPR signal is very different. recorded afterwards. Material and Methods