검색
검색 팝업 닫기

Ex) Article Title, Author, Keywords

Article

Split Viewer

Original Article

Progress in Medical Physics 2018; 29(1): 16-22

Published online March 31, 2018 https://doi.org/10.14316/pmp.2018.29.1.16

Copyright © Korean Society of Medical Physics.

Use of Cylindrical Chambers as Substitutes for Parallel-Plate Chambers in Low-Energy Electron Dosimetry

Minsoo Chun*, Hyun Joon An*, Seong-Hee Kang, Jin Dong Cho, Jong Min Park*,‡,§,ΙΙ, Jung-in Kim*,‡,§

*Department of Radiation Oncology, Seoul National University Hospital, Seoul, Department of Radiation Oncology, Seoul National University Bundang Hospital, Seongnam, Biomedical Research Institute, Seoul National University Hospital, §Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, ΙΙCenter for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea

Correspondence to:Jung-in Kim(madangin@gmail.com)

Tel: 82-2-2072-3573
Fax: 82-2-765-3317

Received: February 28, 2018; Revised: March 28, 2018; Accepted: March 29, 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/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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.

KeywordsElectron dosimetry, AAPM TG-51, Parallel plate chamber, Farmer chamber

The various international dosimetry protocols have recommended the use of parallel plate ionization chamber in electron beam calibration, and especially in case of low energy electron (R50<4 g/cm2 or <10 MeV).14) This is mainly because the replacement correction factor was not well defined in cylindrical chamber, although several previous studies have reported about these issues but still controversial.57) In electron dosimetry with parallel plate chamber, the replacement correction factor can be taken as unity due to the design of the parallel plate chamber consisting of thin-foiled entrance window and the air-filled cavity, which could effectively minimize the electron fluence perturbations.8,9)

However, there should be another substitute method to measure electron beam output and energy in case of unavailability of parallel plate chamber, or for the convenience of experimental set-up. This interchangeable approach could be done with widely-used cylindrical chamber, such as Farmer-type or thimble chamber, their cross calibration of course should be pre-verified. A previous study has verified the temporal use of cylindrical chamber in the measurement of 6 MeV electron beam with International Atomic Energy Agency (IAEA), Technical Reports Series No. 398 (TRS-398) protocols, but those with American Association of Physicists in Medicine (AAPM) Task Group (TG)-51 protocol was not reported yet.10)

In this study, we measured the electron beams of six energies (4, 6, 9, 12, 16, and 20 MeV) according to AAPM TG-51 protocols with Farmer-type (TN 30013, PTW-Freiburg, Freiburg, Germany) and Roos chamber (TN 34001, PTW-Freiburg, Freiburg, Germany). By varying the dose rate, the impact of dose rate on electron dosimetry was rigorously investigated to validate its clinical appropriateness.

1. Experimental setup

Electron beams of six energies (4, 6, 9, 12, 16, and 20 MeV) were measured with a linear accelerator (Trilogy, Varian Medical Systems, Palo Alto, CA). Small one dimensional water phantom (WP1D Phantom, IBA Dosimetry, Schwarzenbruck, Germany) of 42×36×36 cm3 was setup, and source-to-surface distance (SSD) was set to 100 cm. The 10×10 cone was used in accordance with an initial beam modeling.

Two types of ion chambers, 0.6 cc Farmer chamber and 0.35 cc Roos chamber were used in this study as shown in Fig. 1. Their corresponding absorbed dose to water calibration factor (ND,w60Co) were provided by the secondary standards dosimetry laboratories (SSDL) within an year. An UNIDOS-E electrometer (PTW-Freiburg, Freiburg, Germany) was used to read collected charge for each measurement.

2. Electron beam calibration and measurement

All measurements in this study were performed according to AAPM TG-51 protocols as following Eq. (1)

DWQ=MkQND,w60Co

Where, M and kQ denote fully corrected reading, and chamber-specific beam quality correction factor, respectively. The fully corrected reading M was acquired by multiplication of Mraw with ion recombination factors (Pion), polarization correction factor (Ppol), electrometer correction factor (Pelec), and corrections for standard environmental conditions (PTP). All collection factors were obtained at reference depth Pion=0.6 R50−0.1 (cm) with respect to each energy regardless of chamber type.

The Pion were measured by varying dose rate with 100, 300, 600, and 1000 MU/min according to the Eq. (2), where VH be the normal operating voltage and VL be the bias reduced by the factor 2, and Mraw* be the chamber reading for each bias. Ppol was measured with reference dose-rate (1,000 MU/min) where the reference dosimetry was being performed.1)

Pion(VH)=1-VH/VLMrawH/MrawL-VH/VL

Beam quality conversion factor (kQ) was provided by Eq. (3), and acquired by chamber-specific manner.

kQ=PgrQkR50

where, PgrQ and kR50 indicates gradient correction factor for cylindrical chamber and beam quality-dependent absolute dose calibration factor specified by R50, respectively. The gradient correction factor for parallel-plate chamber is not necessary, and for cylindrical chamber, PgrQ was presented by a function of the radius of the chamber cavity, rcav, as following Eq. (4).

PgrQ=Mraw(dref+0.5rcav)Mraw(dref)

kR50 was presented by the product of photon-electron conversion factor (kecal) and electron beam quality conversion factor (k′R50). kecal was chamber-specific, and 0.896 for Farmber chamber, and 0.901 for Roos chamber in this study. k′R50 was also provided according to the chamber-type and beam quality (R50) as shown in Eq. (5) and (6).

kR50(cyl)=0.9905+0.0710e(-R50/3.67)kR50(pp)=1.2239-0.145(R50)0.214

After all calibration and conversion factors were obtained, the measured dose was normalized by reference percent depth dose (PDD) at dref to present the absolute dose at dmax. The absolute differences in Pion and absolute dose were calculated, and their relationships were observed.

1. Ion recombination factor

Ion recombination factor according to the six electron energies and chamber types were provided in Table 1, and also the Pion differences between chambers were presented in Fig. 2(a) by 100 folds numerical value. The largest magnitude of Pion differences across two chambers were at 4, and 16 MeV showing less than 0.004. Pion difference (×100) according to the energy in certain dose rate were −0.043, −0.035, −0.23, and −0.026, for 100, 300, 600, and 1,000 MU/min respectively. The closest differences in Pion across two chambers was acquired where the reference dosimetry was being performed (1,000 MU/min).

2. Absolute dose to water

The absolute dose with respect to each dose rate and energies were presented in Table 1, and dose difference across two chambers were provided in Fig. 2(b). The largest dose differences across two chambers were definitely observed at 4 MeV showing −1.497, 0.577, −0.336, −0.724, −0.681, and −0.229 for 4, 6, 9, 12, 16, and 20 MeV. Absolute dose difference according to the energy in certain dose rate were −0.394, −0.361, −0.454, and −0.328, for 100, 300, 600, and 1,000 MU/min respectively. Although absolute dose difference in 4 MeV was relatively higher than those of energies equal or greater than 6 MeV, those with 1,000 MU/min were less than 1% while larger than 1.3% in other dose rate.

By closely examining Eq (1). and other correction factors, the absolute dose to water could be determined by multiplication of the raw reading, Pion, Ppol, PTP, Pelec, kQ, and ND,w60Co. Among them, the dose-rate dependent variables were raw reading and Pion, while other factors cannot influence the absolute dose to water by varying dose rate. Fig. 3 showed the impact of dose rate on the measurement of Pion, raw reading, and absolute dose across chambers. It can be showed that an almost linear patterns on absolute dose with respect to dose rate were mainly influenced by the collected charge not by Pion for both chambers.

This study investigated the electron reference dosimetry with Farmer-type and Roos chamber, and the impact of dose rate on dosimetric parameters. Including AAPM TG-51 and IAEA TRS-398 protocols, various studies have been made to verify the appropriateness of chamber types in electron dosimetry. Although other literatures have still argued about them, it is noticeable that they commonly recommended using parallel plate chamber in low electron energies rather than cylindrical chamber.5,6,8,11) All acceptance with recommendations has been made, but substitutable methods should be prepared in case of unavailability of parallel plate chamber, such as with widely-used cylindrical chamber.

The variations on ion recombination factor could reflect different extent of incomplete ion collection, and this discrepancy was dominant in 4 MeV as shown in Fig. 2(a). However, Pion difference on 4 MeV with 1,000 MU/min was relatively small, thus the dose difference could be minimized less than 1% by the selection of dose rate on calibration circumstances (1,000 MU/min). The dose difference across chambers were higher especially in 4 MeV as shown in Fig. 2(b). The dose difference between two chambers were −1.32, 0.42, −0.22, −0.58, −0.54, and −0.06 cGy on averages, and −1.50, 0.58, −0.34, −0.72, −0.68, and −0.22 on maximum magnitude. The relatively large dose difference on 4 MeV across chamber types was shown, and this is mainly because of the appreciable perturbation in cylindrical chambers. Also necessities to extrapolate the beam quality factors in the energy range of R50 less than 2 cm (<6 MeV) could boost the dose difference relatively high. Because the other correction factors were acquired with fixed dose rate, the dose difference according to dose rate were only influenced by Pion and ion collection efficiency with respect to the different dose rate. These verification results could suggest that the reference electron dosimetry in 4 MeV even with Farmer chamber can be reached less than 1% difference with Roos chamber when the user measured the beam with dose-rate where the reference dosimetry were being performed.

Ion collection efficiency was strongly influenced by the dose rate of the pulsed beam, which can be determined by both dose per pulse and pulse repetition frequency.12) Lang et al.12) investigated the impacts of dose per pulse in ion collection efficiency, and reported that the ion collection efficiency could be decreased by 6% at the maximum dose rate. Takei et al.13) reported that influences of pulse repetition frequency in ion collection, and they claimed that there were decreases in the collected charge within 1% for electron dosimetry. By summarizing the previous studies, the ion collection efficiency can be influenced by the dose rate of the pulsed beam. As shown in Fig. 3, the graphs on middle row could present the linear pattern of collected charge by increasing the dose rate, which is consistent with the previous literatures. However, ion recombination factor (Pion) had no statistical founding with dose rate regardless of energy and chamber type. It can be interpreted that the different types of chamber could influence the Pion less than only 0.4%.

Although we concluded that the usage of Farmer chamber in electron dosimetry could provide output difference less than 1% with dose-rate for reference dosimetry, this does not mean Farmer chamber could replace the parallel-plate chamber directly. For more accurate output measurement with Farmer chamber, the cross calibration between two chambers should be made in each clinic one by one. This can be regarded as our limitation of our study, and measurements with other combinations of parallel plate chambers should be preceded to generalize the use of cross factor such as Markus chamber, Advanced-Markus, and others.

Also, there exists the necessity of more rigorous and wide experiments with IAEA TRS-398. While other feasibility studies of using cylindrical chamber in low energy electron dosimetry has been made infrequently, we believe that the combination and comparison between AAPM TG-51 and IAEA TRS-398 protocols with Monte Carlo based simulations could effectively support the use of cylindrical chamber in electron beam dosimetry and thereby boost the convenience of the radiation quality assurance routine.10) Nonetheless, our study results showed a possibility to use cylindrical chamber in low energy dosimetry with a medium dose rate.

We observed the close relationship between ion collection efficiency and absolute dose regardless of chamber type suggesting the proper selection of the dose rate could influence on electron dosimetry. The study results could suggest that the cylindrical chamber can be used in electron dosimetry with dose rate for reference dosimetry as a substitute of parallel plate chamber.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2017M2A2A7A02020641 and 2017M2A2A7A02020643).

All relevant data are within the paper and its Supporting Information files.

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

Dose rateIon recombination factor (Pion)Dose (cGy)


4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV
Farmer chamber
 100 MU/min1.0061.0121.0091.0091.0081.010100.355100.08099.67699.73599.66199.072
 300 MU/min1.0081.0121.0101.0101.0081.011100.748100.15699.77699.91099.63599.277
 600 MU/min1.0071.0111.0101.0101.0081.010100.799100.15499.875100.03299.70999.174
 1,000 MU/min1.0091.0111.0111.0101.0101.011101.219100.302100.176100.104100.07899.325
Roos chamber
 100 MU/min1.0081.0111.0091.0101.0081.010101.85299.620100.012100.459100.03998.962
 300 MU/min1.0091.0101.0091.0111.0091.011102.06099.579100.012100.539100.19699.283
 600 MU/min1.0111.0121.0111.0111.0121.013102.434100.064100.450100.697100.70399.565
 1,000 MU/min1.0091.0121.0101.0091.0111.012102.222100.064100.327100.496100.62399.441
  1. Almond PR, Biggs PJ, Coursey BM, Hanson WF, Huq MS, and Nath R, et al. AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med Phys 1999;26:1847-70.
    Pubmed CrossRef
  2. Andreo P, Burns D, Hohlfeld K, Huq MS, Kanai T, and Laitano F, et al. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. IAEA TRS 2000;398.
  3. Party IW, Thwaites D, DuSautoy A, Jordan T, McEwen M, and Nisbet A, et al. The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration. Phys Med Biol 2003;48:2929.
    Pubmed CrossRef
  4. Gerbi BJ, Antolak JA, Deibel FC, Followill DS, Herman MG, and Higgins PD, et al. Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25. Med Phys 2009;36:3239-79.
  5. Laitano RF, Guerra AS, Pimpinella M, Caporali C, and Petrucci A. Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse. Phys Med Biol 2006;51:6419-36.
    Pubmed CrossRef
  6. Piermattei A, Canne SD, Azario L, Russo A, Fidanzio A, and Micelit R, et al. The saturation loss for plane parallel ionization chambers at high dose per pulse values. Phys Med Biol 2000;45:1869-83.
    Pubmed CrossRef
  7. Burns DT, and McEwen MR. Ion recombination corrections for the NACP parallel-plate chamber in a pulsed electron beam. Phys Med Biol 1998;43:2033-45.
    Pubmed CrossRef
  8. Martisikova M, Ackermann B, and Jakel O. Analysis of uncertainties in Gafchromic EBT film dosimetry of photon beams. Phys Med Biol 2008;53:7013-27.
    Pubmed CrossRef
  9. Kim SH, Huh H, Choi SH, Choi J, Kim HJ, Lim C, and Shin DO. The Study on the Use of a Cylindrical Ionization Chamber for the Calibration of a 6 MeV Electron Beam. Korean J Med Phys 2009;20:317-323.
    Pubmed CrossRef
  10. Sathiyan S, and Ravikumar M. Absolute dose determination in high-energy electron beams: Comparison of IAEA dosimetry protocols. J Med Phys India 2008;33:108.
    CrossRef
  11. Lang S, Hrbacek J, and Leong A, et al. Ion-recombination correction for different ionization chambers in high dose rate flatteningfilterfree photon beams. Phys Med Biol 2012;57:2819-27.
    Pubmed KoreaMed CrossRef
  12. Takei Hideyuki, et al. General ion recombination effect in a liquid ionization chamber in high-dose-rate pulsed photon and electron beams. J Radiat Res 2018.
    Pubmed CrossRef

Article

Original Article

Progress in Medical Physics 2018; 29(1): 16-22

Published online March 31, 2018 https://doi.org/10.14316/pmp.2018.29.1.16

Copyright © Korean Society of Medical Physics.

Use of Cylindrical Chambers as Substitutes for Parallel-Plate Chambers in Low-Energy Electron Dosimetry

Minsoo Chun*, Hyun Joon An*, Seong-Hee Kang, Jin Dong Cho, Jong Min Park*,‡,§,ΙΙ, Jung-in Kim*,‡,§

*Department of Radiation Oncology, Seoul National University Hospital, Seoul, Department of Radiation Oncology, Seoul National University Bundang Hospital, Seongnam, Biomedical Research Institute, Seoul National University Hospital, §Institute of Radiation Medicine, Seoul National University Medical Research Center, Seoul, ΙΙCenter for Convergence Research on Robotics, Advanced Institutes of Convergence Technology, Suwon, Korea

Correspondence to:Jung-in Kim(madangin@gmail.com)

Tel: 82-2-2072-3573
Fax: 82-2-765-3317

Received: February 28, 2018; Revised: March 28, 2018; Accepted: March 29, 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/4.0) 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.

Keywords: Electron dosimetry, AAPM TG-51, Parallel plate chamber, Farmer chamber

Introduction

The various international dosimetry protocols have recommended the use of parallel plate ionization chamber in electron beam calibration, and especially in case of low energy electron (R50<4 g/cm2 or <10 MeV).14) This is mainly because the replacement correction factor was not well defined in cylindrical chamber, although several previous studies have reported about these issues but still controversial.57) In electron dosimetry with parallel plate chamber, the replacement correction factor can be taken as unity due to the design of the parallel plate chamber consisting of thin-foiled entrance window and the air-filled cavity, which could effectively minimize the electron fluence perturbations.8,9)

However, there should be another substitute method to measure electron beam output and energy in case of unavailability of parallel plate chamber, or for the convenience of experimental set-up. This interchangeable approach could be done with widely-used cylindrical chamber, such as Farmer-type or thimble chamber, their cross calibration of course should be pre-verified. A previous study has verified the temporal use of cylindrical chamber in the measurement of 6 MeV electron beam with International Atomic Energy Agency (IAEA), Technical Reports Series No. 398 (TRS-398) protocols, but those with American Association of Physicists in Medicine (AAPM) Task Group (TG)-51 protocol was not reported yet.10)

In this study, we measured the electron beams of six energies (4, 6, 9, 12, 16, and 20 MeV) according to AAPM TG-51 protocols with Farmer-type (TN 30013, PTW-Freiburg, Freiburg, Germany) and Roos chamber (TN 34001, PTW-Freiburg, Freiburg, Germany). By varying the dose rate, the impact of dose rate on electron dosimetry was rigorously investigated to validate its clinical appropriateness.

Materials and Methods

1. Experimental setup

Electron beams of six energies (4, 6, 9, 12, 16, and 20 MeV) were measured with a linear accelerator (Trilogy, Varian Medical Systems, Palo Alto, CA). Small one dimensional water phantom (WP1D Phantom, IBA Dosimetry, Schwarzenbruck, Germany) of 42×36×36 cm3 was setup, and source-to-surface distance (SSD) was set to 100 cm. The 10×10 cone was used in accordance with an initial beam modeling.

Two types of ion chambers, 0.6 cc Farmer chamber and 0.35 cc Roos chamber were used in this study as shown in Fig. 1. Their corresponding absorbed dose to water calibration factor (ND,w60Co) were provided by the secondary standards dosimetry laboratories (SSDL) within an year. An UNIDOS-E electrometer (PTW-Freiburg, Freiburg, Germany) was used to read collected charge for each measurement.

2. Electron beam calibration and measurement

All measurements in this study were performed according to AAPM TG-51 protocols as following Eq. (1)

DWQ=MkQND,w60Co

Where, M and kQ denote fully corrected reading, and chamber-specific beam quality correction factor, respectively. The fully corrected reading M was acquired by multiplication of Mraw with ion recombination factors (Pion), polarization correction factor (Ppol), electrometer correction factor (Pelec), and corrections for standard environmental conditions (PTP). All collection factors were obtained at reference depth Pion=0.6 R50−0.1 (cm) with respect to each energy regardless of chamber type.

The Pion were measured by varying dose rate with 100, 300, 600, and 1000 MU/min according to the Eq. (2), where VH be the normal operating voltage and VL be the bias reduced by the factor 2, and Mraw* be the chamber reading for each bias. Ppol was measured with reference dose-rate (1,000 MU/min) where the reference dosimetry was being performed.1)

Pion(VH)=1-VH/VLMrawH/MrawL-VH/VL

Beam quality conversion factor (kQ) was provided by Eq. (3), and acquired by chamber-specific manner.

kQ=PgrQkR50

where, PgrQ and kR50 indicates gradient correction factor for cylindrical chamber and beam quality-dependent absolute dose calibration factor specified by R50, respectively. The gradient correction factor for parallel-plate chamber is not necessary, and for cylindrical chamber, PgrQ was presented by a function of the radius of the chamber cavity, rcav, as following Eq. (4).

PgrQ=Mraw(dref+0.5rcav)Mraw(dref)

kR50 was presented by the product of photon-electron conversion factor (kecal) and electron beam quality conversion factor (k′R50). kecal was chamber-specific, and 0.896 for Farmber chamber, and 0.901 for Roos chamber in this study. k′R50 was also provided according to the chamber-type and beam quality (R50) as shown in Eq. (5) and (6).

kR50(cyl)=0.9905+0.0710e(-R50/3.67)kR50(pp)=1.2239-0.145(R50)0.214

After all calibration and conversion factors were obtained, the measured dose was normalized by reference percent depth dose (PDD) at dref to present the absolute dose at dmax. The absolute differences in Pion and absolute dose were calculated, and their relationships were observed.

Results

1. Ion recombination factor

Ion recombination factor according to the six electron energies and chamber types were provided in Table 1, and also the Pion differences between chambers were presented in Fig. 2(a) by 100 folds numerical value. The largest magnitude of Pion differences across two chambers were at 4, and 16 MeV showing less than 0.004. Pion difference (×100) according to the energy in certain dose rate were −0.043, −0.035, −0.23, and −0.026, for 100, 300, 600, and 1,000 MU/min respectively. The closest differences in Pion across two chambers was acquired where the reference dosimetry was being performed (1,000 MU/min).

2. Absolute dose to water

The absolute dose with respect to each dose rate and energies were presented in Table 1, and dose difference across two chambers were provided in Fig. 2(b). The largest dose differences across two chambers were definitely observed at 4 MeV showing −1.497, 0.577, −0.336, −0.724, −0.681, and −0.229 for 4, 6, 9, 12, 16, and 20 MeV. Absolute dose difference according to the energy in certain dose rate were −0.394, −0.361, −0.454, and −0.328, for 100, 300, 600, and 1,000 MU/min respectively. Although absolute dose difference in 4 MeV was relatively higher than those of energies equal or greater than 6 MeV, those with 1,000 MU/min were less than 1% while larger than 1.3% in other dose rate.

By closely examining Eq (1). and other correction factors, the absolute dose to water could be determined by multiplication of the raw reading, Pion, Ppol, PTP, Pelec, kQ, and ND,w60Co. Among them, the dose-rate dependent variables were raw reading and Pion, while other factors cannot influence the absolute dose to water by varying dose rate. Fig. 3 showed the impact of dose rate on the measurement of Pion, raw reading, and absolute dose across chambers. It can be showed that an almost linear patterns on absolute dose with respect to dose rate were mainly influenced by the collected charge not by Pion for both chambers.

Discussion

This study investigated the electron reference dosimetry with Farmer-type and Roos chamber, and the impact of dose rate on dosimetric parameters. Including AAPM TG-51 and IAEA TRS-398 protocols, various studies have been made to verify the appropriateness of chamber types in electron dosimetry. Although other literatures have still argued about them, it is noticeable that they commonly recommended using parallel plate chamber in low electron energies rather than cylindrical chamber.5,6,8,11) All acceptance with recommendations has been made, but substitutable methods should be prepared in case of unavailability of parallel plate chamber, such as with widely-used cylindrical chamber.

The variations on ion recombination factor could reflect different extent of incomplete ion collection, and this discrepancy was dominant in 4 MeV as shown in Fig. 2(a). However, Pion difference on 4 MeV with 1,000 MU/min was relatively small, thus the dose difference could be minimized less than 1% by the selection of dose rate on calibration circumstances (1,000 MU/min). The dose difference across chambers were higher especially in 4 MeV as shown in Fig. 2(b). The dose difference between two chambers were −1.32, 0.42, −0.22, −0.58, −0.54, and −0.06 cGy on averages, and −1.50, 0.58, −0.34, −0.72, −0.68, and −0.22 on maximum magnitude. The relatively large dose difference on 4 MeV across chamber types was shown, and this is mainly because of the appreciable perturbation in cylindrical chambers. Also necessities to extrapolate the beam quality factors in the energy range of R50 less than 2 cm (<6 MeV) could boost the dose difference relatively high. Because the other correction factors were acquired with fixed dose rate, the dose difference according to dose rate were only influenced by Pion and ion collection efficiency with respect to the different dose rate. These verification results could suggest that the reference electron dosimetry in 4 MeV even with Farmer chamber can be reached less than 1% difference with Roos chamber when the user measured the beam with dose-rate where the reference dosimetry were being performed.

Ion collection efficiency was strongly influenced by the dose rate of the pulsed beam, which can be determined by both dose per pulse and pulse repetition frequency.12) Lang et al.12) investigated the impacts of dose per pulse in ion collection efficiency, and reported that the ion collection efficiency could be decreased by 6% at the maximum dose rate. Takei et al.13) reported that influences of pulse repetition frequency in ion collection, and they claimed that there were decreases in the collected charge within 1% for electron dosimetry. By summarizing the previous studies, the ion collection efficiency can be influenced by the dose rate of the pulsed beam. As shown in Fig. 3, the graphs on middle row could present the linear pattern of collected charge by increasing the dose rate, which is consistent with the previous literatures. However, ion recombination factor (Pion) had no statistical founding with dose rate regardless of energy and chamber type. It can be interpreted that the different types of chamber could influence the Pion less than only 0.4%.

Although we concluded that the usage of Farmer chamber in electron dosimetry could provide output difference less than 1% with dose-rate for reference dosimetry, this does not mean Farmer chamber could replace the parallel-plate chamber directly. For more accurate output measurement with Farmer chamber, the cross calibration between two chambers should be made in each clinic one by one. This can be regarded as our limitation of our study, and measurements with other combinations of parallel plate chambers should be preceded to generalize the use of cross factor such as Markus chamber, Advanced-Markus, and others.

Also, there exists the necessity of more rigorous and wide experiments with IAEA TRS-398. While other feasibility studies of using cylindrical chamber in low energy electron dosimetry has been made infrequently, we believe that the combination and comparison between AAPM TG-51 and IAEA TRS-398 protocols with Monte Carlo based simulations could effectively support the use of cylindrical chamber in electron beam dosimetry and thereby boost the convenience of the radiation quality assurance routine.10) Nonetheless, our study results showed a possibility to use cylindrical chamber in low energy dosimetry with a medium dose rate.

Conclusion

We observed the close relationship between ion collection efficiency and absolute dose regardless of chamber type suggesting the proper selection of the dose rate could influence on electron dosimetry. The study results could suggest that the cylindrical chamber can be used in electron dosimetry with dose rate for reference dosimetry as a substitute of parallel plate chamber.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No.2017M2A2A7A02020641 and 2017M2A2A7A02020643).

Conflicts of Interest

The authors have nothing to disclose.

Availability of Data and Materials

All relevant data are within the paper and its Supporting Information files.

Tables

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

Dose rateIon recombination factor (Pion)Dose (cGy)


4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV
Farmer chamber
 100 MU/min1.0061.0121.0091.0091.0081.010100.355100.08099.67699.73599.66199.072
 300 MU/min1.0081.0121.0101.0101.0081.011100.748100.15699.77699.91099.63599.277
 600 MU/min1.0071.0111.0101.0101.0081.010100.799100.15499.875100.03299.70999.174
 1,000 MU/min1.0091.0111.0111.0101.0101.011101.219100.302100.176100.104100.07899.325
Roos chamber
 100 MU/min1.0081.0111.0091.0101.0081.010101.85299.620100.012100.459100.03998.962
 300 MU/min1.0091.0101.0091.0111.0091.011102.06099.579100.012100.539100.19699.283
 600 MU/min1.0111.0121.0111.0111.0121.013102.434100.064100.450100.697100.70399.565
 1,000 MU/min1.0091.0121.0101.0091.0111.012102.222100.064100.327100.496100.62399.441

Fig 1.

Figure 1.Used ion chambers (a) Farmer-type chamber, and (b) Roos chamber.
Progress in Medical Physics 2018; 29: 16-22https://doi.org/10.14316/pmp.2018.29.1.16

Fig 2.

Figure 2.Differences in (a) ion recombination factors (Pion), and (b) dose across two different chamber types.
Progress in Medical Physics 2018; 29: 16-22https://doi.org/10.14316/pmp.2018.29.1.16

Fig 3.

Figure 3.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.
Progress in Medical Physics 2018; 29: 16-22https://doi.org/10.14316/pmp.2018.29.1.16

Table 1 Numerical values for ion recombination factor (Pion), and dose according to the chamber-type, energy, and dose rate.

Dose rateIon recombination factor (Pion)Dose (cGy)


4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV4 MeV6 MeV9 MeV12 MeV16 MeV20 MeV
Farmer chamber
 100 MU/min1.0061.0121.0091.0091.0081.010100.355100.08099.67699.73599.66199.072
 300 MU/min1.0081.0121.0101.0101.0081.011100.748100.15699.77699.91099.63599.277
 600 MU/min1.0071.0111.0101.0101.0081.010100.799100.15499.875100.03299.70999.174
 1,000 MU/min1.0091.0111.0111.0101.0101.011101.219100.302100.176100.104100.07899.325
Roos chamber
 100 MU/min1.0081.0111.0091.0101.0081.010101.85299.620100.012100.459100.03998.962
 300 MU/min1.0091.0101.0091.0111.0091.011102.06099.579100.012100.539100.19699.283
 600 MU/min1.0111.0121.0111.0111.0121.013102.434100.064100.450100.697100.70399.565
 1,000 MU/min1.0091.0121.0101.0091.0111.012102.222100.064100.327100.496100.62399.441

References

  1. Almond PR, Biggs PJ, Coursey BM, Hanson WF, Huq MS, and Nath R, et al. AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med Phys 1999;26:1847-70.
    Pubmed CrossRef
  2. Andreo P, Burns D, Hohlfeld K, Huq MS, Kanai T, and Laitano F, et al. Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water. IAEA TRS 2000;398.
  3. Party IW, Thwaites D, DuSautoy A, Jordan T, McEwen M, and Nisbet A, et al. The IPEM code of practice for electron dosimetry for radiotherapy beams of initial energy from 4 to 25 MeV based on an absorbed dose to water calibration. Phys Med Biol 2003;48:2929.
    Pubmed CrossRef
  4. Gerbi BJ, Antolak JA, Deibel FC, Followill DS, Herman MG, and Higgins PD, et al. Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25. Med Phys 2009;36:3239-79.
  5. Laitano RF, Guerra AS, Pimpinella M, Caporali C, and Petrucci A. Charge collection efficiency in ionization chambers exposed to electron beams with high dose per pulse. Phys Med Biol 2006;51:6419-36.
    Pubmed CrossRef
  6. Piermattei A, Canne SD, Azario L, Russo A, Fidanzio A, and Micelit R, et al. The saturation loss for plane parallel ionization chambers at high dose per pulse values. Phys Med Biol 2000;45:1869-83.
    Pubmed CrossRef
  7. Burns DT, and McEwen MR. Ion recombination corrections for the NACP parallel-plate chamber in a pulsed electron beam. Phys Med Biol 1998;43:2033-45.
    Pubmed CrossRef
  8. Martisikova M, Ackermann B, and Jakel O. Analysis of uncertainties in Gafchromic EBT film dosimetry of photon beams. Phys Med Biol 2008;53:7013-27.
    Pubmed CrossRef
  9. Kim SH, Huh H, Choi SH, Choi J, Kim HJ, Lim C, and Shin DO. The Study on the Use of a Cylindrical Ionization Chamber for the Calibration of a 6 MeV Electron Beam. Korean J Med Phys 2009;20:317-323.
    Pubmed CrossRef
  10. Sathiyan S, and Ravikumar M. Absolute dose determination in high-energy electron beams: Comparison of IAEA dosimetry protocols. J Med Phys India 2008;33:108.
    CrossRef
  11. Lang S, Hrbacek J, and Leong A, et al. Ion-recombination correction for different ionization chambers in high dose rate flatteningfilterfree photon beams. Phys Med Biol 2012;57:2819-27.
    Pubmed KoreaMed CrossRef
  12. Takei Hideyuki, et al. General ion recombination effect in a liquid ionization chamber in high-dose-rate pulsed photon and electron beams. J Radiat Res 2018.
    Pubmed CrossRef
Korean Society of Medical Physics

Vol.35 No.2
June 2024

pISSN 2508-4445
eISSN 2508-4453
Formerly ISSN 1226-5829

Frequency: Quarterly

Current Issue   |   Archives

Stats or Metrics

Share this article on :

  • line