ESTRO 2022 - Abstract Book

S730

Abstract book

ESTRO 2022

Ionization chambers used in radiotherapy require a beam quality correction factor k Q for dosimetry in clinical high energy photon fields. However, there are still ionization chamber in clinical use worldwide with no published beam quality correction factors k Q . For the widely used ionization chambers SNC 600c and SNC 125c from Sun Nuclear Corporation (Melbourne, FL) there are no k Q values available. In this study, k Q values for this ionization chambers were calculated according to international and national dosimetry protocols: TG-51, TRS-398 and DIN 6800-2. Materials and Methods All Monte Carlo simulations presented in this work have been performed using EGSnrc. Absorbed dose to water was calculated in a small cylindrical water voxel with a radius of 0.5 cm and a height of 0.2 cm. To calculate the dose in the sensitive volume of the investigated ionization chambers, a detailed Monte Carlo based model of the chambers was created according to technical drawings provided by the manufacturer. Tabulated spectra as well as simulations of beam transport through linear accelerator head models were used as high energy photon radiation sources for the Monte Carlo calculations. Results Figure 1 shows the beam quality correction factor k Q as a function of photon beam quality % dd (10) x and TPR 20,10 for the Farmer type ionization chamber SNC 600c and the scanning ionization chamber SNC 125c. The Figure presents the beam quality correction factor k Q calculated according to the international dosimetry protocols TG-51 and TRS-398 as well as the Germany dosimetry protocol DIN 6800-2. k Q values as a function of the respective beam quality specifier Q were fitted against recommended equations for photon beam dosimetry in the range of 4 MV to 18 MV. The fitting curves through the calculated values showed a root mean square deviation between 0.0010 and 0.0017.

Figure 1: Monte Carlo calculated k Q values as a function of photon beam quality specifier %dd(10) x (left panels: (a), (c)) and TPR 20,10 (right panels: (b), (d)) for the SNC 600c and the SNC 125 ionization chamber according to the investigated dosimetry protocols. Error bars indicate the statistical uncertainties (1 σ ). Fit curves to the data are shown with a 95% confidence interval. Conclusion The investigated ionization chamber models are not included in above-mentioned dosimetry protocols. This study addressed this knowledge gap by providing data for this ionization chamber for reference dosimetry. In addition, a comparison of the calculated values with published k Q data for similar ionization chambers shows agreement with published data within the 95% confidence interval. These results confirm the use of data for similar ionization chambers when k Q values are not available for a specific ionization chamber.

PD-0811 Lateral dose response of an ionization chamber in an external magnetite field

M. Alissa 1,2 , K. Zink 1,3 , A. A. Schoenfeld 4 , D. Czarneck 1

1 Institute for Medical Physics and Radiation Protection, University of Applied Sciences Mittelhessen, Giessen, Germany; 2 Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Giessen, Germany; 3 Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany; 4 Sun Nuclear Corporation , Research, Melbourne, USA Purpose or Objective Integrating magnetic resonance tomography (MRI) with medical linear accelerators allows monitoring the tumor during radiotherapy treatment. In this study, the effect of an external magnetic field on the spatial response within an ionization chamber was investigated using Monte Carlo simulations. Materials and Methods The SNC 125c ionization chamber (Sun Nuclear Corp., Melbourne, USA) was modeled in detail with the C++ class library of the EGSnrc Monte Carlo code system and placed in a water phantom at a depth of 10 cm. To investigate the spatial response of the ion chamber a photon pencil beam with dimensions Δ f = 0,1 mm x 0,1 mm was scanned over the chamber in x- and y-direction and the average dose deposited in the active volume of the chamber was scored. Additionally, a magnetic field of 1.5 T in x- and ±y-direction was applied in separate simulations (see Fig 1). A MR-linac photon spectrum was used for these simulations.