Abstracts - BMTMedPhys 2017 – Dresden, September 10–13 • DOI 10.1515/bmt-2017-5099 Biomed. Eng.-Biomed. Tech. 2017; 62(s1): S517–S520 • © by Walter de Gruyter • Berlin • Boston S517 V 173 Dose measurement in the steep dose gradients around brachytherapy sources Frank W. Hensley, Ruprecht-Karls-Universität, Heidelberg, Deutschland, [email protected] Michael Andrassy, Eckert & Ziegler BEBIG GmbH, Berlin, Deutschland, [email protected] Ndimofor Chofor, Medizinische Strahlenphysik, Pius-Hospital Oldenburg und Carl-von-Ossietzky-Universität Oldenburg, Oldenburg, Deutschland, [email protected] Dietrich Harder, Georg-August-Universität, Göttingen, Deutschland, [email protected] Günther Hartmann, Deutsches Krebsforschungszentrum (DKFZ), Heidelberg, Deutschland, [email protected] Theodor Kaulich, Medizinische Physik, Universitätsklinik für Radioonkologie, Tübingen, Deutschland, [email protected] Michael Kollefrath, Medizinische Physik, Klinik für Strahlenheilkunde, Freiburg i. Br., Deutschland, [email protected] Michael Niekamp, Elekta GmbH, Hamburg, Deutschland, [email protected] Björn Poppe, Medizinische Strahlenphysik, Pius-Hospital Oldenburg und Carl-von-Ossietzky-Universität Oldenburg, Oldenburg, Deutschland, [email protected] Thorsten Schneider, Physikalisch-Technische Bundesanstalt, Braunschweig, Deutschland, [email protected] Andreas Schönfeld, Medizinische Strahlenphysik, Pius-Hospital Oldenburg und Carl-von-Ossietzky-Universität Oldenburg, Oldenburg, Deutschland, [email protected] Edmund Schüle, Physikalisch-Technische Werkstätten Dr. Pychlau GmbH, Freiburg i. Br., Deutschland, [email protected] Hans-Joachim Selbach, Physikalisch-Technische Bundesanstalt, Braunschweig, Deutschland, hans- [email protected] Frank-Andre Siebert, Christian-Albrechts-Universität, Kiel, Deutschland, [email protected] Ulrich Quast, Universität Duisburg-Essen, Witten, Deutschland, [email protected] Renate Walter, Klinikum Augsburg, Augsburg, Deutschland, [email protected] Golam Abu Zakaria, Klinikum Oberberg, Gummersbach, Deutschland, golamabu.zakaria@klinikum- oberberg.de Dose measurements at a few cm distance from brachytherapy sources are difficult due to the complexity of positioning the source and detector with the required precision of around 0.1 mm, and due to the uncertainties in detector response caused by energy-dependence, unclear effective point of measurement and volume averaging of detector signal across the extended sensitive volume. Verification of new calculation algorithms in brachytherapy incorporating tissue inhomogeneities, the calibration and relative dosimetry of kV-irradiators used at short source-tissue-distances in intraoperative radiotherapy and also clinical situations when applying these techniques require verification by measurement for which the present codes of practice for dosimetry in radiotherapy are not applicable. A formalism under preparation by the DIN 6803-3 working group describes the determination of absorbed dose to water from measurement by multiplication of calibration and correction factors defined in analogy to the well-known codes of practice: , where is the factor correcting for the radiation quality at the point of measurement and corrects for the volume averaging effect. Values of are given in the work by Chofor et al. (Z.Med.Phys. 26 (2015) 238). Otherwise, small solid detectors can be cross-calibrated to a small ionization chamber at a point in water at around 3-5 cm distance from the source with the chamber’s effective point of measurement and determined as proposed by Schönfeld et al. (to be published). DIN 6803-3 will describe these approaches to dose measurement in the source vicinity with acceptable uncertainty, preferentially for detectors with small volume averaging. Abstracts - BMTMedPhys 2017 – Dresden, September 10–13 • DOI 10.1515/bmt-2017-5099 Biomed. Eng.-Biomed. Tech. 2017; 62(s1): S517–S520 • © by Walter de Gruyter • Berlin • Boston S518 V 175 MR-µ-imaging based 3-dimensional-polymer gel dosimetry in comparison to 2D-film and 1D-diamond dosimetry of mm-sized photon pencil beams Andreas Berg, High-field MR-CE, Medical University of Vienna; Center for Medical Physics and Biomedical Engineering, Wien, Österreich, [email protected] Gerd Heilemann, Department of Radiation Oncology, Division Medical Radiation Physics, Medical University of Vienna, Wien, Österreich, [email protected] Dietmar Georg, Department of Radiation Oncology, Division Medical Radiation Physics, Medical University of Vienna, Wien, Österreich, [email protected] In small-field dosimetry of mm-sized beams the performance characteristics: spatial resolution and dose discrimination with reference to detector-noise represent the most important criteria for detectors. 3D-Magnetic- Resonance-based Polymer-gel-Dosimetry (MRPD) avoids the distortion of the radiation field due to water/tissue equivalence of the whole detector and allows for the 3-dimensional measurement of the dose distribution within one single multi-slice imaging experiment. We investigated the performance characteristics of several dosimeters known for their high spatial resolution capabilities for the following challenging setting: a 6MV photon-field below (depth=12mm) 4 differently sized lead collimators (thickness=1cm) with diameters of d4=1,92mm down to d1=0,46mm. We compared the results on lateral dose-profiles of different 3D-MR-µ-imaging based measurement protocols: a short, medium and long lasting protocol at different nominal spatial resolution (pixel- size:156-468µm) to two different film (EBT3) scanning protocols (pixel-size: 35-169µm) and a µ-diamond detector (PTW60019) with profiles at high- and low-resolution direction. Results: a short 3D-multi-slice parameter multi-echo MR-measurement protocol lasting about 20 min is capable of detecting all of the very small radiation-fields down to d1=460µm at relative modulation strength of about 5% of the incident dose. The dose noise is dependent on the MR-protocol used and might be reduced with long lasting (h) protocols down to below 1,5%. 3.) We consider here film dosimetry at pixel-size of 169µm as the best reference standard with regard to spatial resolution in 2dimensions. The high-resolution film scanning protocol at 720dpi suffers from noise and is hardly able to differentiate the low-level dose-modulations below d1=500µm collimation. 4.) The diamond detector stands out for excellent signal-to-noise-ratio and reports the same FWHM- diameter values as film and gel for the high-resolution scanning protocol within errors. However it does not confirm the non-single-Gaussian-like profiles with slight dose-enhancement at the edges for the 1,92mm and 1,54mm collimations, which film and gel appear to indicate. Abstracts - BMTMedPhys 2017 – Dresden, September 10–13 • DOI 10.1515/bmt-2017-5099 Biomed. Eng.-Biomed. Tech. 2017; 62(s1): S517–S520 • © by Walter de Gruyter • Berlin • Boston S519 V 176 The influence of magnetic fields on the lateral dose response function of various photon dosimetry detectors Björn Delfs, Universitätsklinik für med. Strahlenphysik, Universität Oldenburg, Oldenburg, Deutschland, [email protected] Dietrich Harder, Medical Physics and Biophysics, Georg August University, Göttingen, Deutschland, dietrich- [email protected] Björn Poppe, Universitätsklinik für med. Strahlenphysik, Universität Oldenburg, Oldenburg, Deutschland, [email protected] Hui Khee Looe, Universitätsklinik für med. Strahlenphysik, Universität Oldenburg, Oldenburg, Deutschland, [email protected] With the introduction of MR-Linac systems for image guided radiotherapy, new challenges in clinical dosimetry emerge due to the Lorentz force which modifies the trajectories of the secondary electrons compared to the case without magnetic field. The aim of this study is to determine the 1D lateral dose response functions, K(x-ξ), of clinically established photon dosimetry detectors under the influence of magnetic fields. Function K(x-ξ) is the convolution kernel transforming the true dose profile D(ξ) into the disturbed signal profile M(x) measured with a detector. For the air-filled PTW Semiflex 3D 31021 ionization chamber, the PTW microDiamond 60019, the PTW Si diode 60017 and the scintillation detector Exradin W1, kernels K(x-ξ) were determined by Monte-Carlo simulation using 0.25 mm wide 60Co and 6 MV slit beams. The detectors were modelled following manufacturers’ specifications and placed at 5 cm water depth. Magnetic fields of 0, 0.5, 1 and 1.5 T, oriented perpendicular to the beam axis, were applied. Distortions of K(x-ξ) were found to depend on the magnetic field, the density of the detector’s structural components, and the orientation of the detector relative to the magnetic field. Kernel K(x-ξ) of the air-filled ionization chamber with its low-density air cavity shows the strongest asymmetrical deformation by action of the magnetic field. The semiconductor detectors with materials of larger density than water also show asymmetrical, but smaller distortions of their K(x-ξ) functions. Kernel K(x-ξ) of the nearly water equivalent W1 was found to be almost independent from the magnetic field. Functions K(x-ξ) of various detectors in magnetic fields can be used to derive correction factors, and the absence of magnetic field influences on kernel K(x-ξ) of a water-equivalent detector