ESTRO 2020 Abstract Book

S699 ESTRO 2020

chamber’s stem. Additionally, the positioning of the chambers with respect to the beam’s axis and magnetic field; and the magnetic field strength also play an essential role. Nevertheless, all chambers show increase or decrease of dose response in magnetic field by not more than 5% up to 1.5 T. The correction factors derived from Monte Carlo simulations with detailed chamber models considering their effective sensitive volumes show agreement better than 1% to measurements. Conclusion A comprehensive study on the dose response of compact chambers in magnetic field has been carried out by measurements and Monte Carlo simulations. By comparing the results of chambers with different designs, the underlying mechanisms of the magnetic field dependent dose response of these compact chambers have been revealed. PO-1326 Evaluation of Percentage Depth Dose Measurement of High-energy Electron Beams using new TLD Sheet K. Sasaki 1 , Y. Shiota 2 , M. Miura 2 1 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan ; 2 Iwata City Hospital, Medical Physics, Iwata, Japan Purpose or Objective Although there are various devices for measuring high- energy radiation, Radiographic films, Radiochromic films, and parallel plate ionization chambers are generally used to obtain surface or build-up region doses with high accuracy. In this study, we measured the depth dose of high energy electron beams using a new TLD sheet dosimeter with an effective atomic number close to the water and high measurement flexibility and investigated the characteristics of electron beam measurement. Material and Methods Percentage depth dose (PDD) was measured using 4 MeV, 6 MeV, 9 MeV, and 12 MeV electron beams from Novalis TX (BrainLAB / Varian Medical systems) LINAC. Measurements were taken with a NACP-02 parallel plate ionization chamber (IBA), gafchromic films (Ashland) and thermoluminescence dosimeter sheets (TLD sheet, Toyo Medic Co., Ltd). The chemical formula of TLD sheet is LiB3O5, and its thickness is 150 micrometers, which is about half of the gafchromic film. Measurements using TLD sheets were performed in two ways. The first method was a parallel incidence method in which a sheet was sandwiched between plastic slab phantoms (WE211, Kyoto Kagaku) and a radiation beam was incident in parallel on the sheet. The second method was a stack method in which the radiation beam was incident vertically on the stacked TLD sheets. Results Depth scaling was performed using the density of the slab phantom in the parallel incidence method, and that was performed by the measured data of the TLD sheet in the stack method. The relationship between the measured digital value and the dose was linear, and the difference due to energy was very small. PDDs measured with ionization chambers or TLD sheets showed consistent data agreement at deeper depths than the maximum dose depth, but TLD sheets tended to be lower in the build-up region (Figure 1)

Conclusion It has been proven, that it is possible to measure the output factors directly in water with radiochromic film without any protection. This practice reveals itself very useful when measuring small field’s beams.

PO-1325 Measurement and MC simulation of magnetic field correction factors of compact ionization chambers I. Büsing 1 , B. Delfs 1 , T. Tekin 1 , A. Schönfeld 1 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital - Carl von Ossietzky University, Oldenburg, Germany Purpose or Objective Farmer-type ionization chambers are commonly used for reference dosimetry at MR linacs. The alteration of their dose response in magnetic field and the underlying mechanisms have been frequently studied and reported. However, compact ionization chambers are also used for measurements in situations where high spatial resolution is required, such as small fields and lateral profiles. Therefore, it is of the same importance to study their magnetic field dependent dose response, for which reports in the literature are still rare. Furthermore, the correct modelling of the effective sensitive volume of an ionization chamber in Monte Carlo simulation has been shown to be of high importance. Since this might play a more essential role in smaller chambers due to their more compact design, this aspect was also investigated in this work. Material and Methods Six compact ionization chambers from two manufacturers (PTW 31021, PTW 31022, PTW 31023, PTW 31016, IBA CC01-G, IBA CC003-S) were investigated. Measurements were performed using a 4 cm x 4 cm field of a 6 MV photon beam to evaluate the magnetic field correction factors k B,Q of these chambers. An electromagnet was positioned in the beam, with which the magnetic field can be varied up to 1.4 T. The chambers were positioned with their reference points axially and radially in 5 cm water depth. For selected chambers, measurements were repeated with smaller field sizes of 2 cm x 2 cm and 0.5 cm x 0.5 cm to assess the possible field size dependence of the correction factors. Monte Carlo simulations were performed using the EGSnrc code. For fully-guarded chambers, the effective sensitive volumes were evaluated using a high-resolution proton microbeam measurements that were then compared to computation results obtained from finite element analysis. The role of effective sensitive volume was studied. Results The modification of dose responses of the investigated ionization chambers shows strong dependence on the chamber’s design, that is, the shape of sensitive volume including the presence of non-active air volume; and the components surrounding the air volume such as the