Monte Carlo calculated beam quality correction factors for photon reference dosimetry
Damian Czarnecki,
Germany
PD-0810
Abstract
Monte Carlo calculated beam quality correction factors for photon reference dosimetry
Authors: Damian Czarnecki1, Mohamad Alissa2,3, Andreas A. Schoenfeld4, Klemens Zink2,5
1 University of Applied Sciences Mittelhessen, Institute of Medical Physics and Radiation Protection, Giessen, Germany; 2University of Applied Sciences Mittelhessen, Institute of Medical Physics and Radiation Protection, Giessen, Germany; 3University Medical Center Giessen-Marburg, Department of Radiotherapy and Radiooncology, Gießen, Germany; 4Sun Nuclear Corp., research, Melbourne, USA; 5University Medical Center Giessen-Marburg, Marburg Ion Beam Therapy Center, Marburg, Germany
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Purpose or Objective
Ionization
chambers used in radiotherapy require a beam quality correction factor kQ 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 kQ. For the widely used ionization chambers
SNC 600c and SNC 125c from Sun Nuclear Corporation (Melbourne, FL) there are no kQ values available. In this study, kQ values for this ionization chambers were
calculated according to international and national dosimetry protocols: TG-51,
TRS-398 and DIN 6800-2.
Material 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 kQ
as a function of photon beam quality %dd(10)x
and TPR20,10 for the Farmer type ionization chamber SNC 600c and the
scanning ionization chamber SNC 125c. The Figure presents the beam quality correction
factor kQ calculated according to the international
dosimetry protocols TG-51 and TRS-398 as well as the Germany dosimetry protocol
DIN 6800-2. kQ 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 kQ values as a function of photon beam
quality specifier %dd(10)x (left panels: (a), (c)) and TPR20,10
(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 (1σ). 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 kQ 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 kQ values are not available for a specific
ionization chamber.