Prog Med Phys.  2020 Dec;31(4):145-152. 10.14316/pmp.2020.31.4.145.

High-Dose-Rate Electron-Beam Dosimetry Using an Advanced Markus Chamber with Improved IonRecombination Corrections

• Jeong DH
• Lee M
• Lim H
• Kang SK
• Jang KW

• KMID: 2510475
• DOI: http://doi.org/10.14316/pmp.2020.31.4.145

Abstract

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 k _s 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.

Keyword

Ion recombination correction factor; Dose-per-pulse; Advanced Markus chamber; Boag model; Electron beam dosimetry

Figure

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  Fig. 1 Experimental setup (a) for reference dosimetry using an Advanced Markus® chamber and (b) for percentage depth dose measurements using a radiochromic film in water. LINAC, linear accelerator.

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  Fig. 2 Ratio of the ion-collection efficiency (f1/f2) and the correction factor (1/f1) calculated as a function of u1 using Boag’s model.

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  Fig. 3 Comparison of the ion-recombination correction factors recalculated in this work with data published by Laitano et al. [7] for 5 polarizing voltage ratios. See text for the explanation of the difference between graphs (a) and (b).

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  Fig. 4 The ion-recombination correction factors at V1/V2=400/200 and V1/V2=300/100 calculated in this work using Boag’s model. The correction factors calculated by the conventional methods are included for comparison.

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  Fig. 5 Measured percentage depth dose (PDD) curve used to determine the beam-quality index R50, including the reference depth zref and the practical range Rp.

Reference

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