ESTRO 2020 Abstract book

S742 ESTRO 2020

due to the size of most dosimeters, eg. sufficient spatial resolutions, precise dose distribution, proper absorption, and energy dependency of the detector. In this context, the idea of this research work is aimed at developing a micro-optically driven end-fiber technology in the engineering of X-ray dosimeters and hence address the recent bottlenecks of the commercial dosimeter employed in the small field dosimetry. Material and Methods A fiber optic detector based on inorganic scintillating materials was developed with the size of about 200-micron diameter and performance test measured under small field irradiation of high energy photon at 6MV & 15 MV delivered by an Elekta Linear Accelerator (LINAC). Two commercial dosimeters namely, PTW PinPoint 3D and microDiamond were used to compare PDD and Beam profile measurements under Elekta Linear Acceleration (LINAC) radiations. The modified subtraction method has been considered to quantify and eliminate the negligible cerenkov background from the X ray-excited Photoluminescence (XEP) optical signal. Measurements were performed in a water tank under IAEA Technical reports series recommendations (IAEA TRS 381 and TRS 483). Results Small field of the size of 3*3 cm 2 to 0.5*0.5 cm 2 were measured, characterize and compare with commercial detectors for different dosimetry parameters. Good agreement was obtained among all measurements with the reference detectors and reliability of the measurements was shown for the smallest field sizes. The FWHM was measured 5.5 mm for the smallest available square size beam of 5 x 5 mm 2 , where the discrepancy of 0.5 mm is due to the scattering effects inside the water and convolution effect between field and sensor geometries. Percentage Depth Dose (PDD) factor dependence variation with water depth exhibits nearly the same behavior for all tested detectors whereas small size beam profiling comparison in the in-field measurement shows only 0% - 1.2% variation with the reference detector. The fibered detector allows to perform dose measurements at very high accuracy and reliability from low (50 cGy/min) to high dose rates (800 cGy/min) and found to be independent of the delivered dose rate variation. The calibration of the detector easily achieved owing to the linear behavior of the detected optical signal with dose and the repeatability test under the respective dose of 100 cGy and 20 cGy. Conclusion The developed fiber optic sensor is easy to access and can be used to accurately measure the dose delivered using small field irradiation during the small volume of tumor treatment. The author’s measurement shows that despite using a non-water equivalent sensor, the detector can be a powerful candidate for beam characterization and quality assurance in, e.g., EBRT, radiosurgery, IMBT, and brachytherapy. PO-1317 Experimental determination of kBMQ up to a magnetic flux density of 1.5 T S. Pojtinger 1,2 , R. Kapsch 1 , D. Thorwarth 2 1 Physikalisch-Technische Bundesanstalt, Dosimetry for radiation therapy and diagnostic radiology, Braunschweig, Germany ; 2 University Hospital Tübingen, Biomedical Physics, Tübingen, Germany Purpose or Objective Hybrid devices consisting of a linear accelerator and a magnetic resonance tomograph (MR-linacs) are currently available in several clinical institutions. For these new devices – as in conventional radiation therapy – the traceable determination of the water absorbed dose must be guaranteed under reference conditions. However, dose measurements using ionization chambers are influenced by magnetic fields. To consider these effects, many authors propose the use of a magnetic field correction factor

PO-1315 Development and Evaluation of Flexible Dosimeter for Surface Dose Measurement in Radiotherapy J. Kim 1 , M. Han 2 , Y.M. Moon 3 , S.K. Park 3 1 Dongnam Institute of Radiological and Medical Sciences, Department of Radiation Oncology, Busan, Korea Republic of ; 2 College of Medicine- Inje University, Department of Radiation Oncology, Busan, Korea Republic of ; 3 Dongnam Institute of Radiological and Medical Sciences, Departments of Radiation Oncology, Busan, Korea Republic of Purpose or Objective Radiotherapy has the possibility of adverse reactions due to overlapping dose at skin surface binding parts. Exposure of the skin to 2Gy or higher dose scan cause adverse reactions such as erythema, desquamation,and necrosis. To prevent them, the skin dose is predicted through a treatment planning system. but its accuracy is only 50±25%. Thus,we need dosimeters that can accurately measure the skin dose from computed tomography images and calculated treatment plans in clinical practice. The existing skin dosimeters such as glass rod dosimeter and optically stimulated luminescent dosimeter check the point dose. However, analog-type integrated solid state dosimeters are difficult to analyze the dose of body surface and have low positioning accuracy because the attachment site is decided with naked eye. Meanwhile,to overcome morphological limitations,materials based on flexible polymer-based thin film technology have been actively researched. In this paper, we wanted to make a flexible detector using PbI2 and evaluate its performance. Material and Methods As a basic study on a patch-type skin dosimeter, this study produced a PbI2 dosimeter based on silicon rubber for the human skin using the particle in binder(PIB) method. To evaluate the performance of this dosimeter, reproducibility and linearity were evaluated by 1, 10, 100, 1000 and 10000 bending counts at 6 MV and 15 MV. The reproducibility was examined by irradiating a 1 Gy dose at the dose rate of 4 Gy/min 10 times repeatedly. The linearity was tested by irradiating 0.01 to 10Gy doses at the doserate of 4Gy/min. Results Consequently,when the relative standard deviation(R-SD) as the evaluation criterion for reproducibility was set to1.5%or lower,theS-RD of the 1000 bending count was 1.74% at 6MV and the S-RD of the 10000 bending count at 15MV was2.2%. Thus,these results were higher than the criterion. When the coefficient of determination(R-Sq) as the evaluation criterion for linearity was set at 0.9900 or higher,the R-Sq of the 10000 bending count was 0.9730 at 6MV and 0.9812 at 15MV. Thus, these results were lower than the criterion. When the bending count increased to more than 10000,the performance showed fine variations. Conclusion This study introduces a new type of flexible functional material and the problem of performance variations can be improved through further research using various electrodes. PO-1316 Real-time fibered X-ray dosimeter for small field dosimetry in high energy radiation oncology S.B.C. Debnath 1 , D. Tonneau 1 , C. Fauquet 1 , A. Goncalves 2 , A. Tallet 3 , J. Darreon 3 1 Aix Marseille Univeristy- CNRS, CINaM UMR 7325, Marseille, France ; 2 Institut Paoli-Calmettes, UMR 7258- Inserm UMR 1068- CRCM, Marseille, France ; 3 Institut Paoli-Calmettes, Radiotherapy, Marseille, France Purpose or Objective Small field irradiation to treat small volume tumors in the field of radiation dosimetry is one of the major challenges