ESTRO 36 Abstract Book

S402 ESTRO 36 2017 _______________________________________________________________________________________________

screen has been replaced by a water equivalent build-up material [1]; a dual detector combining a standard EPID and an array dosimeter [2]; and an EPID comprising a plastic scintillator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). recorded afterwards. Material and Methods

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.

Conclusion We have obtained an EPR signal for our solid polymer dosimeter. The EPR signal increases linearly with dose for the medical dose range, but it saturates for higher doses. Although it is not comparable to the EPR dosimetry using alanine, this signal could be a source of improved understanding of the underlying dosimetric characteristics of this material and it may be a supporting feature to the optical signals from the dosimeter. We further foresee interesting applications in particle therapy beams since the signal production in solid-state dosimeters are generally dependent on the ionization density. PO-0769 A microDiamond for determination of absorbed dose around high-dose-rate 192Ir brachytherapy sources V. Kaveckyte 1 , A. Malusek 1 , H. Benmaklouf 2 , G. Alm Carlsson 1 , A. Carlsson Tedgren 2 1 Linköping University, Radiation Physics IMH, Linköping, Sweden 2 Karolinska University Hospital, Radiation physics, Stockholm, Sweden Purpose or Objective Experimental dosimetry of high-dose-rate (HDR) 192 Ir brachytherapy (BT) sources is complicated due to steep