ESTRO 36 Abstract Book

S403 ESTRO 36 2017 _______________________________________________________________________________________________

dose and dose-rate gradients, high dose rates and softening of photon energy spectrum with depth. A single crystal synthetic diamond detector PTW 60019 (marketed as microDiamond) (PTW, Freiburg, Germany) has a small active volume and was designed for such measurements in high energy photon, electron and proton beams. It can be read out directly with standard electrometers used at radiotherapy departments, unlike thermoluminescent detectors, which are currently the most used dosimeters in BT but have to be pre- and post-processed with dedicated equipment. Hence the purpose of this study was to evaluate the suitability of a microDiamond for the determination of absorbed dose to water in an HDR 192 Ir beam quality. The use of three microDiamond samples also allowed for assessment of their individual reproducibility. Material and Methods In-phantom measurements were performed using the microSelectron HDR 192 Ir BT treatment unit. Oncentra treatment planning system (TPS) was used to create irradiation plans for a cubical PMMA phantom with a microDiamond positioned at one of the three source-to- detector distances (SDDs) (1.5, 2.5 and 5.5 cm). The source was stepped by 0.5 cm over the total length of 6 cm to yield absorbed dose of 2 Gy at the reference point of the detector. A phantom correction factor was applied to account for the difference between the experimental phantom and the spherical water phantom used for absorbed dose calculations made with the TPS. The same measurements were repeated for all three detectors (mD1, mD2, mD3). Results Experimentally determined absorbed dose to water deviated from that calculated with the TPS from -1 to +2 % and agreed to within experimental uncertainties for all the detectors and SDDs (Figure 1). The mD2 overestimated absorbed dose to water by up to 2% compared with the estimates by the other two detectors. A decrease in the difference with increasing SDD suggests that it might be related to differences in the position of the active volume inside the detector which is of higher importance closer to the source where dose gradients are steeper. The combined relative uncertainty in experimentally determined absorbed dose to water did not exceed 2% () for all the detectors and SDDs. A variation in raw readings was within 2% over the investigated range. Conclusion Preliminary results indicate that the dosimetric properties of a microDiamond obviate the need for multiple correction factors and facilitate dosimetry of HDR 192 Ir BT sources. This, together with the convenience of use, shows high potential of a microDiamond for quality assurance of HDR BT treatment units at clinical sites. It must be noted, nevertheless, that individual characterization of a microDiamond is required to achieve high accuracy. PO-0770 The distortions of the dose response functions of dosimeters in the presence of a magnetic field H.K. Looe 1 , B. Delfs 1 , D. Harder 2 , B. Poppe 1 1 Carl von Ossietky University, University Clinic for Medical Radiation Physics, Oldenburg, Germany 2 Georg August University, Prof em.- Medical Physics and Biophysics, Göttingen, Germany Purpose or Objective The new developments of MRgRT have opened new possibilities for high precision image-guided radiotherapy. However, the secondary electrons liberated within the medium by the primary photon beam are subjected to the Lorentz force. Therefore, the trajectories of the secondary electrons in non-water media, such as an air- filled cavity or a high-density semiconductor, will differ significantly from that in water. In this work we demonstrate, using simple geometries, that the shape of the lateral dose response functions of clinical detectors

will depend on the electron density of the detector material, the beam quality and the magnetic field. The dosimetric implications are discussed and correction

strategies are proposed. Material and Methods

Based on the convolution model (Looe et al 2015), the one- dimensional lateral dose response function, K(x-ξ), acting as the convolution kernel transforming the true dose profile D(ξ) into the measured signal profile M(x), was derived by Monte-Carlo simulation for a simple cylindrical detector placed at 5 cm depth in water using 60 Co and 6 MV slit beams. The cylinder with 1.13 mm radius and 2 mm height was filled with water of normal density (1 g/cm 3 ), low density (0.0012 g/cm 3 ) and enhanced density (3 g/cm 3 ), where the latter two represent the density of an air-filled ionization chamber and a semiconductor detector respectively. Simulations were performed using the EGSnrc package, and 0.5, 1.0 and 1.5 T magnetic fields were applied. Results Fig. 1 shows the derived kernels K(x-ξ) without and with magnetic field for the three detector densities and two beam qualities. The shape of K(x-ξ) without magnetic field has been discussed in Looe et al 2015 in terms of the electron density of the detector material. The effect of the magnetic field on the secondary electrons’ trajectories in a non-water equivalent medium is manifested as a distortion of K(x-ξ). It is worth mentioning that function K(x-ξ) for water with normal density (middle panels) does not vary in the presence of a magnetic field, and the shape of this function merely represents the geometrical volume-averaging effect.

Fig. 1. Area normalized K(x-ξ) for the cylindrical detector voxels of 'low”, 'normal”, and 'enhanced” density without and with, 0.5, 1.0 and 1.5 T magnetic field. Conclusion It has been shown for the first time that the lateral dose response functions K(x-ξ) of non-water equivalent detectors will be distorted by a magnetic field, showing asymmetrical detector response, even if the detector’s construction is symmetrical. The distortions are attributed to the differences in charged particle trajectories within the detectors having electron density other than of normal water. The effect of a magnetic field on a detector’s response can be characterized by the area-normalized convolution kernel K(x-ξ, y-η). As previously proposed (Looe et al 2015), corrections based on the convolution model can be applied to account for the detector’s volume effect in the presence of magnetic field:

PO-0771 The dose response functions of an air-filled ionization chamber in the presence of a magnetic field B. Delfs 1 , D. Harder 2 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical