ESTRO 2020 Abstract book

S17 ESTRO 2020

Organic materials are an attractive technology for advanced dosimetry applications as they combine the electronic advantages of semiconducting substrates with the dosimetric, chemical and mechanical benefits of organic compounds such as plastics. We assessed the potential use of organic photodetectors for radiotherapy dose verification when coupled with a plastic scintillator. Characterising their response to radiation under different clinically relevant conditions when pre-irradiated up to 40kGy of gamma radiation for investigation of radiation damage effects in medical radiation applications. Material and Methods The Organic Photodetectors (OPDs) characterised were provided by ISORG (Grenoble, France) with an area of 1x1cm^2 and 500nm active area thickness. The OPDs were tested prior to and following irradiation up to a total dose of 40kGy by a cobalt-60 gamma source (dose rate=1.2kGy/h). The OPDs response was tested by exposure to ionising radiation by indirect interaction (converted into optical photons with a peak emission of 420nm plastic scintillator (BC400, Saint-Gobain)). Energy dependence was tested by photon irradiation from kV to MV energy using medical linear accelerators. Dose linearity measurements were performed for 6MV photons and 6MeV electrons at calibration conditions. In order to characterise the detector’s response under different irradiation dose rates, response was recorded by varying the source to surface distance from 90-300cm. Percentage depth dose and output factor measurements were obtained in reference conditions and compared to an IC (CC13).

Results A linear relationship was obtained between pulse dose and collection efficiencies, the latter increasing as the pulse dose decreased in all cases. These fittings got worse as the pulse repetition frequency decreased, probably as a result of an increasing uncertainty in measurements as the pulse frequency was reduced. Pulse frequency did not reveal a so clear relationship with collection efficiency, although in a general way, efficiency increased as pulse frequency decreased (Figures 1 and 2).

Results The samples after pre-irradiation, show that the response to irradiation by 6MV photons is consistent and reproducible when used with a scintillator. Response of samples equipped with 0.5mm plastic scintillator were observed to be linear with both photon and electron beams from 25-500Gy at 600MU/min. The sensitivity of the organic photodetectors to 6MV photons was determined to be (190±0.28)pC/cGy and (170±0.11)pC/cGy for the non- irradiated and pre-irradiated samples biased at 2V, respectively. We observed no dose rate response variation despite the low mobility of organic semiconductors (Fig.1). The energy dependence was measured for a pre-irradiated organic device for orthovoltage energies from an x-ray tube and compared to the response from pulsed 6MV photons, Fig.2. The gradient from 30-100keV is due to the absorption of the plastic scintillator as proved by comparison with absorption data from NIST. After 100keV the energy dependence plateaus proving the advantage of using organic detectors for dosimetry.

Conclusion Liquid-filled ionization chambers collection efficiencies increase the lower the pulse dose and pulse frequency are. These parameters vary during an VMAT delivery so they will influence the verification of the patient treatments. It can be significant in SRS and SBRT where unflattened beams are employed and tolerance levels are strict. Efficiencies corrections make the SRS measurements more consistent with those of the TPS. However, uncertainties in the way the pulse dose is calculated with the developed software are not compatible with clinical treatments. Therefore, a new more accurate correction method would be necessary. PH-0049 Organic semiconductors photodiodes for ionising radiation dosimetry J. Posar 1 , J. Davis 1 , P. Sellin 2 , M. Griffith 3 , O. Dhez 4 , M.L.F. Lerch 1 , A.B. Rosenfeld 1 , M. Petasecca 1 1 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia ; 2 University of Surrey, Department of Physics, Surrey, United Kingdom ; 3 University of Newcastle, School of Mathematical and Physical Sciences, Newcastle, Australia ; 4 ISORG, Business Development, Grenoble, France

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