Abstract
Although several studies provide data for reference dosimetry, the SNC600c and SNC125c ionization chambers (Sun Nuclear Corporation, Melbourne, FL) are in clinical use worldwide for which no beam quality correction factors kQ are available. The goal of this study was to calculate beam quality correction factors kQ for these ionization chambers according to dosimetry protocols TG-51, TRS 398 and DIN 6800-2. Monte Carlo simulations using EGSnrc have been performed to calculate the absorbed dose to water and the dose to air within the active volume of ionization chamber models. Both spectra and simulations of beam transport through linear accelerator head models were used as radiation sources for the Monte Carlo calculations. 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 25 MV. The fitting curves through the calculated values showed a root mean square deviation between 0.0010 and 0.0017. The investigated ionization chamber models (SNC600c, SNC125c) are not included in above mentioned dosimetry protocols, but are in clinical use worldwide. This study covered this knowledge gap and compared the calculated results with published kQ values for similar ionization chambers. Agreements with published data were observed in the 95% confidence interval, confirming the use of data for similar ionization chambers, when there are no kQ values available for a given ionization chamber.
Highlights
Ionization chamber measurements of the absorbed dose to water in high energy photon beams are described in national and international dosimetry protocols
Data sets obtained by Monte Carlo simulations and measurements at primary standards laboratories have been used to derive consensus data for beam quality correction factors kQ according to the international dosimetry protocol TRS 398 of the IAEA (International Atomic Energy Agency) [4]
The Monte Carlo calculated data are presented in comparison with calori metric measurements of two SNC600c ionization chambers that have been taken from NRC Report PIRS 3327 [22]
Summary
Ionization chamber measurements of the absorbed dose to water in high energy photon beams are described in national and international dosimetry protocols. The determination of kQ values with high accuracy and the investigation of its influencing quantities are essential to reduce the uncertainties of dose measurements. Numerous research groups have published experimental and Monte Carlo based correction factors kQ which may be used for an update of national and international dosimetry protocols [1,2,3]. Data sets obtained by Monte Carlo simulations and measurements at primary standards laboratories have been used to derive consensus data for beam quality correction factors kQ according to the international dosimetry protocol TRS 398 of the IAEA (International Atomic Energy Agency) [4]. KQ values are only valid in the respective reference conditions, which vary between different dosimetry protocols
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