pmp Progress in Medical Physics Progress in Medical Physics 2508-4445 2508-4453 Korean Society of Medical Physics 10.14316/pmp.2018.29.1.16 pmp-29-16 Original Article Use of Cylindrical Chambers as Substitutes for Parallel-Plate Chambers in Low-Energy Electron Dosimetry ChunMinsoo ∗ AnHyun Joon ∗ KangSeong-Hee † ChoJin Dong ‡ ParkJong Min ∗ † ‡ § ΙΙ KimJung-in ∗ ‡ § Department of Radiation Oncology, Seoul National University Hospital, Seoul, Korea Department of Radiation Oncology, Seoul National University Bundang Hospital, Seongnam, Korea Biomedical Research Institute, Seoul National University Hospital, Seoul, Korea Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, Korea Center for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea Corresponding author Jung-in Kim (madangin@gmail.com) Tel: 82-2-2072-3573 Fax: 82-2-765-3317 03 2018 20 03 2018 29 1 16 22 28022018 28032018 29032018 Copyright © 2018 Korean Society of Medical Physics 2018 This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Abstract

Current dosimetry protocols recommend the use of parallel-plate chambers in electron dosimetry because the electron fluence perturbation can be effectively minimized. However, substitutable methods to calibrate and measure the electron output and energy with the widely used cylindrical chamber should be developed in case a parallel-plate chamber is unavailable. In this study, we measured the correction factors and absolute dose-to-water of electrons with energies of 4, 6, 9, 12, 16, and 20 MeV using Farmer-type and Roos chambers by varying the dose rates according to the AAPM TG-51 protocol. The ion recombination factor and absolute dose were found to be varied across the chamber types, energy, and dose rate, and these phenomena were remarkable at a low energy (4 MeV), which was in good agreement with literature. While the ion recombination factor showed a difference across chamber types of less than 0.4%, the absolute dose differences between them were largest at 4 MeV at approximately 1.5%. We therefore found that the absolute dose with respect to the dose rate was strongly influenced by ion-collection efficiency. Although more rigorous validation with other types of chambers and protocols should be performed, the outcome of the study shows the feasibility of replacing the parallel-plate chamber with the cylindrical chamber in electron dosimetry.

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Used ion chambers (a) Farmer-type chamber, and (b) Roos chamber.

Differences in (a) ion recombination factors (Pion), and (b) dose across two different chamber types.

The impact of dose rate on dosimetric parameters with an order of ion recombination factor (Pion), collected charge, and the absolute dose. Measurements with (a) Farmer-type chamber, and (b) Roos chamber.

Numerical values for ion recombination factor (Pion), and dose according to the chamber-type, energy, and dose rate.

Dose rate Ion recombination factor (Pion) Dose (cGy)
4 MeV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV 4 MeV 6 MeV 9 MeV 12 MeV 16 MeV 20 MeV
Farmer chamber
100 MU/min 1.006 1.012 1.009 1.009 1.008 1.010 100.355 100.080 99.676 99.735 99.661 99.072
300 MU/min 1.008 1.012 1.010 1.010 1.008 1.011 100.748 100.156 99.776 99.910 99.635 99.277
600 MU/min 1.007 1.011 1.010 1.010 1.008 1.010 100.799 100.154 99.875 100.032 99.709 99.174
1,000 MU/min 1.009 1.011 1.011 1.010 1.010 1.011 101.219 100.302 100.176 100.104 100.078 99.325
Roos chamber
100 MU/min 1.008 1.011 1.009 1.010 1.008 1.010 101.852 99.620 100.012 100.459 100.039 98.962
300 MU/min 1.009 1.010 1.009 1.011 1.009 1.011 102.060 99.579 100.012 100.539 100.196 99.283
600 MU/min 1.011 1.012 1.011 1.011 1.012 1.013 102.434 100.064 100.450 100.697 100.703 99.565
1,000 MU/min 1.009 1.012 1.010 1.009 1.011 1.012 102.222 100.064 100.327 100.496 100.623 99.441