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KMID : 1038720200310040145
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Progress in Medical Physics
2020 Volume.31 No. 4 p.145 ~ p.152
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High-Dose-Rate Electron-Beam Dosimetry Using an Advanced Markus Chamber with Improved IonRecombination Corrections
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Jeong Dong-Hyeok
Lee Man-Woo
Lim Heui-Jin
Kang Sang-Koo
Jang Kyoung-Won
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Abstract
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Purpose: In ionization-chamber dosimetry for high-dose-rate electron beams一above 20 mGy/pulse一the ion-recombination correction methods recommended by the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) are not appropriate, because they overestimate the correction factor. In this study, we suggest a practical ion-recombination correction method, based on Boag’s improved model, and apply it to reference dosimetry for electron beams of about 100 mGy/pulse generated from an electron linear accelerator (LINAC).
Methods: This study employed a theoretical model of the ion-collection efficiency developed by Boag and physical parameters used by Laitano et al. We recalculated the ion-recombination correction factors using two-voltage analysis and obtained an empirical fitting formula to represent the results. Next, we compared the calculated correction factors with published results for the same calculation conditions. Additionally, we performed dosimetry for electron beams from a 6 MeV electron LINAC using an Advanced Markus® ionization chamber to determine the reference dose in water at the source-to-surface distance (SSD)=100 cm, using the correction factors obtained in this study.
Results: The values of the correction factors obtained in this work are in good agreement with the published data. The measured dose-per-pulse for electron beams at the depth of maximum dose for SSD=100 cm was 115 mGy/pulse, with a standard uncertainty of 2.4%. In contrast, the ks values determined using the IAEA and AAPM methods are, respectively, 8.9% and 8.2% higher than our results.
Conclusions: The new method based on Boag’s improved model provides a practical method of determining the ion-recombination correction factors for high dose-per-pulse radiation beams up to about 120 mGy/pulse. This method can be applied to electron beams with even higher doseper-pulse, subject to independent verification.
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KEYWORD
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Ion recombination correction factor, Dose-per-pulse, Advanced Markus chamber, Boag model, Electron beam dosimetry
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