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Muir B, Davis S, Dhanesar S, Hillman Y, Iakovenko V, Kim GGY, Alves VGL, Lei Y, Lowenstein J, Renaud J, Sarfehnia A, Siebers J, Tantôt L. AAPM WGTG51 Report 385: Addendum to the AAPM's TG-51 protocol for clinical reference dosimetry of high-energy electron beams. Med Phys 2024. [PMID: 38980220 DOI: 10.1002/mp.17277] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2023] [Revised: 03/29/2024] [Accepted: 06/14/2024] [Indexed: 07/10/2024] Open
Abstract
An Addendum to the AAPM's TG-51 protocol for the determination of absorbed dose to water is presented for electron beams with energies between 4 MeV and 22 MeV (1.70 cm ≤ R 50 ≤ 8.70 cm $1.70\nobreakspace {\rm cm} \le R_{\text{50}} \le 8.70\nobreakspace {\rm cm}$ ). This updated formalism allows simplified calibration procedures, including the use of calibrated cylindrical ionization chambers in all electron beams without the use of a gradient correction. Newk Q $k_{Q}$ data are provided for electron beams based on Monte Carlo simulations. Implementation guidance is provided. Components of the uncertainty budget in determining absorbed dose to water at the reference depth are discussed. Specifications for a reference-class chamber in electron beams include chamber stability, settling, ion recombination behavior, and polarity dependence. Progress in electron beam reference dosimetry is reviewed. Although this report introduces some major changes (e.g., gradient corrections are implicitly included in the electron beam quality conversion factors), they serve to simplify the calibration procedure. Results for absorbed dose per linac monitor unit are expected to be up to approximately 2 % higher using this Addendum compared to using the original TG-51 protocol.
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Affiliation(s)
- Bryan Muir
- Metrology Research Centre, National Research Council of Canada, Ottawa, Ontario, Canada
| | - Stephen Davis
- Department of Radiation Oncology, Miami Cancer Institute, Miami, Florida, USA
| | - Sandeep Dhanesar
- Department of Radiation Oncology, Houston Methodist Hospital, Houston, Texa, USA
| | - Yair Hillman
- Department of Radiation Oncology, Sharett Institute of Oncology, Hadassah Medical Center, Hebrew University of Jerusalem, Jerusalem, Israel
| | | | - Grace Gwe-Ya Kim
- Department of Radiation Medicine and Applied Sciences, UC San Diego School of Medicine, San Diego, California, USA
| | | | - Yu Lei
- Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, Nebraska, USA
| | - Jessica Lowenstein
- Department of Radiation Physics, UT M.D. Anderson Cancer Center, Houston, Texa, USA
| | - James Renaud
- Metrology Research Centre, National Research Council of Canada, Ottawa, Ontario, Canada
| | - Arman Sarfehnia
- Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada
- Department of Medical Physics, Odette Cancer Centre, Sunnybrook Health Sciences Centre, Toronto, Ontario, Canada
| | - Jeffrey Siebers
- Department of Radiation Oncology, University of Virginia, Charlottesville, Virginia, USA
| | - Laurent Tantôt
- Département de radio-oncologie, CIUSSS de l'Est-de-l'Île-de-Montréal - Hôpital Maisonneuve-Rosemont, Montreal, Quebec, Canada
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Evaluation of the polarity effect of Roos parallel plate ionization chamber in build-up region. POLISH JOURNAL OF MEDICAL PHYSICS AND ENGINEERING 2022. [DOI: 10.2478/pjmpe-2022-0015] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Abstract
Purpose: Despite widespread studying of the polarity effect of Roos parallel plate ion chamber in electron beams as mentioned in several protocols, no investigations have up till now studied this effect in photon beams in the build-up region. It is important to examine its polarity effect in the build-up region for photon beams, so this is the first work that focuses in to evaluate the polarity effect of the Roos chamber in the surface and build-up region and comparing its effect with other chambers.
Methods: In this study, the Roos chamber was irradiated by a Theratron 780E 60Co beam to a known polarity effect. The Polarity effects of 5×5 up to 35×35 cm2 field sizes at positive and negative polarizing voltages were measured in the build-up region from surface to 0.7 cm in a solid water phantom.
Results: The polarity ratios (PRs) were obtained at 1.020 ± 0.00 and 1.015 ± 0.00 for field sizes 5 × 5 up to 35 × 35 cm2, respectively. For the same fields, the percentage of polarity effects (%PEs) was obtained at 1.99% ± 0.00% and 1.47% ± 0.02%, respectively. The results found that the %PEs decrease with increased field sizes and depths. Moreover, the %PEs exhibited a decrease with an increased percentage surface dose (%SD). The uncertainty of %PE was estimated as 0.01% for all measurements in this study.
Conclusions: As a result, the average %PE of the Roos chamber described here is equal to 0.756% ± 0.013% for all depths and field sizes for the 60Co γ-ray beam. It has introduced a less percentage of polarity effect than other chambers.
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Mirandola A, Maestri D, Magro G, Mastella E, Molinelli S, Rossi E, Russo S, Vai A, Ciocca M. Determination of ion recombination and polarity effects for the PTW Advanced Markus ionization chamber in synchrotron based scanned proton and carbon ion beams. Phys Med 2022; 96:149-156. [DOI: 10.1016/j.ejmp.2022.03.007] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/23/2022] [Accepted: 03/09/2022] [Indexed: 10/18/2022] Open
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Garty G, Harken A, Brenner D. Traceable dosimetry for MeV ion beams. JOURNAL OF INSTRUMENTATION : AN IOP AND SISSA JOURNAL 2022; 17:T02002. [PMID: 35497570 PMCID: PMC9052763 DOI: 10.1088/1748-0221/17/02/t02002] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Standard dosimetry protocols exist for highly penetrating photon and particle beams used in the clinic and in research. However, these protocols cannot be directly applied to shallow penetration MeV-range ion beams. The Radiological Research Accelerator Facility has been using such beams for almost 50 years to irradiate cell monolayers, using self-developed dosimetry, based on tissue equivalent ionization chambers. To better align with the internationally accepted standards, we describe implementation of a commercial, NIST-traceable, air-filled ionization chamber for measurement of absorbed dose to water from low energy ions, using radiation quality correction factors calculated using TRS-398 recommendations. The reported dose does not depend on the ionization density in the range of 10-150 keV/μm.
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Affiliation(s)
- G. Garty
- Radiological Research Accelerator Facility, Columbia University, 136 S. Broadway, Irvington, NY 10533, U.S.A
| | - A.D. Harken
- Radiological Research Accelerator Facility, Columbia University, 136 S. Broadway, Irvington, NY 10533, U.S.A
| | - D.J. Brenner
- Radiological Research Accelerator Facility, Columbia University, 136 S. Broadway, Irvington, NY 10533, U.S.A
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Baghani HR, Robatjazi M, Andreoli S. Comparing the dosimeter-specific corrections for influence quantities of some parallel-plate ionization chambers in conventional electron beam dosimetry. Appl Radiat Isot 2021; 179:110031. [PMID: 34801928 DOI: 10.1016/j.apradiso.2021.110031] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2021] [Revised: 11/06/2021] [Accepted: 11/14/2021] [Indexed: 11/28/2022]
Abstract
The performance characteristics of some widely employed parallel-plate ionization chambers in dosimetry of conventional high energy electron beams were evaluated and compared in the present study following the recommendations of the IAEA TRS-398 reference dosimetry protocol. Three different types of PTW-made parallel-plate ionization chambers including Roos (TM34001), Markus (TM23343), and Advanced Markus (TM34045) were employed, and correction factors for polarity (kpol), recombination (ks), and quality conversion factor ( [Formula: see text] ) were determined at different nominal electron energies of 4, 6, 9, 12, 16, and 20 MeV produced by a Varian Trilogy clinical Linac. All measurements were performed inside a MP3-M automatic water phantom in the reference condition of 100 cm SSD (source to surface distance), reference measurement depth (zref), and 10 × 10 cm2 field size at the phantom surface. The maximum and minimum range of kpol deviations from unity were respectively found for Markus and Roos ionization chambers. The maximum ks values also belonged to the Markus ionization chamber, while the minimum ks values were observed for the Advanced Markus chamber. The measured ks values through recommendations of the TRS-398 dosimetry protocol were in good accordance with those obtained by Jaffe-plot analysis for all considered ionization chambers. The type of employed ionization chamber can minimally affect the measured electron beam quality index (R50), while it can have a more considerable impact on [Formula: see text] value, especially in the case of the Markus chamber. From the results, it can be concluded that the Roos and Advanced Markus ionization chambers have a superior performance in the case of electron beam dosimetry, although all considered ionization chambers fulfilled the criteria requested by relevant reference dosimetry protocols.
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Affiliation(s)
| | - Mostafa Robatjazi
- Medical Physics and Radiological Sciences Department, Sabzevar University of Medical Sciences, Sabzevar, Iran; Non-Communicable Diseases Research Center, Sabzevar University of Medical Sciences, Sabzevar, Iran
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Fares M, Messai A, Messaoudi M, Negara K, Debili MY, Begaa S. Initial and volume recombination losses in gamma versatile ionization chamber VGIC for gamma ray dosimetry. Appl Radiat Isot 2021; 175:109794. [PMID: 34111648 DOI: 10.1016/j.apradiso.2021.109794] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/31/2020] [Revised: 05/21/2021] [Accepted: 05/25/2021] [Indexed: 10/21/2022]
Abstract
A versatile gamma ionization chamber (VGIC) has been designed, developed and characterized, in order to study experimentally its characteristics according to the geometry of the electrodes, the volume and pressure of the filler gas for the design of a gamma sealed chamber. The tests were conducted under the IEC (International Electro-technical Commission). The results obtained in various nuclear tests of the characterization and calibration of the ionization chamber gamma VGIC developed at our laboratory were presented in this paper. Three irradiators were operated, irradiator intensive gamma (60Co: 1.25 MeV), medium intensive gamma (137Cs: 0.662 MeV) and low-intensity gamma (60Co). Saturation curves and linearity were identified and the operating range and the sensitivity of the chamber have been deducted. The current, voltage (I,V) characteristics of the chamber filled, with argon gas at 0.4 M pa pressure, for gamma ray irradiator sources were studied, the chamber was irradiated with gamma rays using different gamma sources. The plateau region is reached above 200 V and the detector operating voltage is found to be 600V. It is observed that in the plateau region the slope is constant with an increase in the exposure rate. The (1/I, 1/V) and (I, l/V2) characteristic curves reveal the presence of the initial and volume recombination losses. The volume recombination losses are found to be smaller than the initial recombination losses.
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Affiliation(s)
- Mohamed Fares
- Laboratory of Nuclear Detectors, Department Detection and Measurement, Nuclear Research Centre of Birine, P.o.Box 180, Ain Oussera, 17200, Djelfa, Algeria.
| | - Adnane Messai
- Laboratory of Nuclear Detectors, Department Detection and Measurement, Nuclear Research Centre of Birine, P.o.Box 180, Ain Oussera, 17200, Djelfa, Algeria.
| | - Mohammed Messaoudi
- Reactor Chemistry Department, Nuclear Research Centre of Birine, P.o. Box 180, Ain Oussera, 17200, Djelfa, Algeria.
| | - Kassida Negara
- Laboratory of Nuclear Detectors, Department Detection and Measurement, Nuclear Research Centre of Birine, P.o.Box 180, Ain Oussera, 17200, Djelfa, Algeria.
| | - Mohamed Yacine Debili
- LM2S, Physics Department, Faculty of Science, University of Badji Mokhtar - Annaba, P.O. Box. BP 12, Annaba, 23000, Algeria.
| | - Samir Begaa
- Reactor Chemistry Department, Nuclear Research Centre of Birine, P.o. Box 180, Ain Oussera, 17200, Djelfa, Algeria.
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Huang CY, Yang B, Lam WW, Tang KK, Li TC, Law WK, Cheung KY, Yu SK. Effects on skin dose from unwanted air gaps under bolus in an MR-guided linear accelerator (MR-linac) system. Phys Med Biol 2021; 66:065021. [PMID: 33607641 DOI: 10.1088/1361-6560/abe837] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
Bolus is commonly used in MV photon radiotherapy to increase superficial dose and improve dose uniformity for treating shallow lesions. However, irregular patient body contours can cause unwanted air gaps between a bolus and patient skin. The resulting dosimetric errors could be exacerbated in MR-Linac treatments, as secondary electrons generated by photons are affected by the magnetic field. This study aimed to quantify the dosimetric effect of unwanted gaps between bolus and skin surface in an MR-Linac. A parallel-plate ionization chamber and EBT3 films were utilized to evaluate the surface dose under bolus with various gantry angles, field sizes, and different air gaps. The results of surface dose measurements were then compared to Monaco 5.40 Treatment Planning System (TPS) calculations. The suitability of using a parallel-plate chamber in MR-Linac measurement was validated by benchmarking the percentage depth dose and output factors with the microDiamond detector and air-filled ionization chamber measurements in water. A non-symmetric response of the parallel-plate chamber to oblique beams in the magnetic field was characterized. Unwanted air gaps significantly reduced the skin dose. For a frontal beam, skin dose was halved when there was a 5 mm gap, a much larger difference than in a conventional linac. Skin dose manifested a non-symmetric pattern in terms of gantry angle and gap size. The TPS overestimated skin dose in general, but shared the same trend with measurement when there was no air gap, or the gap size was larger than 5 mm. However, the calculated and measured results had a large discrepancy when the bolus-skin gap was below 5 mm. When treating superficial lesions, unwanted air gaps under the bolus will compromise the dosimetric goals. Our results highlight the importance of avoiding air gaps between bolus and skin when treating superficial lesions using an MR-Linac system.
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Kranzer R, Poppinga D, Weidner J, Schüller A, Hackel T, Looe HK, Poppe B. Ion collection efficiency of ionization chambers in ultra-high dose-per-pulse electron beams. Med Phys 2021; 48:819-830. [PMID: 33251606 DOI: 10.1002/mp.14620] [Citation(s) in RCA: 30] [Impact Index Per Article: 10.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/19/2020] [Revised: 09/30/2020] [Accepted: 11/18/2020] [Indexed: 02/03/2023] Open
Abstract
PURPOSE The ion collection efficiency of vented ionization chambers has been investigated in an ultra-high dose-per-pulse (DPP) electron beam. The role of the chamber design and the electric field strength in the sensitive air volume have been evaluated. METHODS An advanced Markus chamber and three specially designed parallel plate air-filled ionization chambers (EWC: End Window Chamber) with varying electrode distance of 0.5, 1, and 2 mm have been investigated. Their ion collection efficiencies were determined experimentally using two methods: extrapolation of Jaffé plots and comparison against a DPP-independent reference detector. The latter was achieved by calibrating a current transformer against alanine dosimeters. All measurements were performed in a 24 MeV electron beam with DPP values between 0.01 and 3 Gy. Additionally, the numerical approach introduced by Gotz et al. was implemented taking into account space charge effects at these ultra-high DPPs. The method has been extended to obtain time-resolved and position-dependent electric field distortions within the air cavity. RESULTS The ion collection efficiency of the investigated ionization chambers drops significantly in the ultra-high DPP range. The extent of this drop is dependent on the electrode distance, the applied chamber voltage and thus the field strength in the sensitive air volume. For the Advanced Markus chamber, a good agreement between the experimental, numerical and the results of Petersson et al. could be shown. Using the three EWCs with different electrode spacing, an improvement of the ion collection efficiency and a reduction of the polarity effect with decreasing electrode distance could be demonstrated. Furthermore, the results revealed that the determination of the ion collection efficiency from the Jaffé plots and therefore also from two-voltage method typically underestimate the ion collection efficiency in the region of high dose-per-pulse (3 to 130 mGy) and overestimate the ion collection efficiency at ultra-high dose-per-pulse (>1 Gy per pulse). CONCLUSIONS In this work, the ion collection efficiency determined with different methods and ionization chambers have been compared and discussed. As expected, an increase of the electric field in the ionization chamber, either by applying a higher bias voltage or a reduction of the electrode distance, improves the ion collection efficiency and also reduces the polarity effect. For the Advanced Markus chamber, the experimental results obtained by comparison against a reference agree well with the numerical solution. Based on these results, it seems possible to keep the recombination loss less than or equal to 5% up to a dose-per-pulse of 3 Gy with an appropriately designed ionization chamber, which corresponds to the level accepted in conventional radiotherapy dosimetry protocols.
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Affiliation(s)
- Rafael Kranzer
- PTW-Freiburg, Freiburg, 79115, Germany.,University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, 26121, Germany
| | | | | | - Andreas Schüller
- Physikalisch-Technische Bundesanstalt, Braunschweig, 38116, Germany
| | - Thomas Hackel
- Physikalisch-Technische Bundesanstalt, Braunschweig, 38116, Germany
| | - Hui Khee Looe
- University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, 26121, Germany
| | - Björn Poppe
- University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University, Oldenburg, 26121, Germany
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Schönfeld AB, Schönfeld AA, Looe HK, Poppe B, de Wilde RL. Experimental determination of the recombination correction factor k S for SNC 125c, SNC 350p and SNC 600c ionization chambers in pulsed photon beams. Z Med Phys 2020; 30:300-304. [PMID: 32278506 DOI: 10.1016/j.zemedi.2020.03.001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2019] [Revised: 03/06/2020] [Accepted: 03/06/2020] [Indexed: 10/24/2022]
Abstract
Accurate ionization chamber measurements of the absorbed dose to water require the correction of incomplete collection of charges created within the chamber volume. According to current dosimetry protocols such as the TRS-398 or the DIN 6800-2, incomplete charge collection is accounted for by the correction factor ks, which can be determined numerically or experimentally. The method proposed by Burns & McEwen (Phys. Med. Biol., 1998) was used in this study to determine the coefficients γ and δ used for the calculation of the correction factor ks of three ionization chambers, the SNC 125c, the SNC 600c and the SNC 350p (all Sun Nuclear Corp., Melbourne, Florida) for an absorbed dose to water range of 0.2mGy to 1.6mGy per pulse in pulsed photon beams. The shift of the effective point of measurement from the reference point Δz and the correction factor kr were determined for the SNC 350p according to the draft DIN 6800-2:2019-07.
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Affiliation(s)
- Ann-Britt Schönfeld
- University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany.
| | | | - Hui Khee Looe
- University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany
| | - Björn Poppe
- University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital, Carl von Ossietzky University Oldenburg, Germany
| | - Rudy Leon de Wilde
- University Clinic for Gynecology, Pius Hospital, Medical Campus of the Carl von Ossietzky University Oldenburg, Germany
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Martin-Martin G, Aguilar PB, Barbés B, Guibelalde E. Assessment of ion recombination correction and polarity effects for specific ionization chambers in flattening-filter-free photon beams. Phys Med 2019; 67:176-184. [PMID: 31734555 DOI: 10.1016/j.ejmp.2019.07.018] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 04/11/2019] [Revised: 07/23/2019] [Accepted: 07/24/2019] [Indexed: 11/17/2022] Open
Abstract
PURPOSE To investigate ion recombination correction and polarity effects in four ion chamber models in flattening-filter-free (FFF) beams to (1) evaluate their suitability for reference dosimetry; (2) assess the accuracy of the two-voltage technique (TVA) against the Bruggmoser formalism; and (3) examine the influence of the accelerator type on the recombination correction. METHODS Jaffé plots were created for a variety of microchambers, small-volume and Farmer-type chambers to obtain kS, the recombination correction factor, using two different types of accelerators. These values were plotted against dose-per-pulse and Jaffé plots for opposite polarities were created to determine which chambers meet the AAPM TG-51 addendum recombination and polarity specifications. RESULTS Nearly all small-volume chambers exhibited reference-class behavior with respect to ion recombination and polarity effects. The microchambers exhibited anomalous recombination and polarity effects, precluding their use for reference dosimetry in FFF beams. For the reference-class chambers, agreement between TVA-determined kS values and Jaffé and Bruggmoser formalisms-determined kS values was within 0.1%. No significant differences were found between the kS values obtained with the two different accelerators used in this work. CONCLUSIONS This study stresses the need to characterize ion recombination correction and polarity effects for small-volume chambers and microchambers on an individual chamber basis and with the more rigorous criteria of the AAPM TG-51 addendum. Furthermore, the study demonstrated the suitability of the TVA method for chambers that exhibit reference-class behavior in FFF beams. Finally, this work has shown that the recombination correction does not depend on the type of accelerator but on its dose-per-pulse.
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Affiliation(s)
- Guadalupe Martin-Martin
- Medical Physics and Radiation Protection Service, Hospital Universitario de Fuenlabrada, C/ Camino del Molino 2, 28492 Fuenlabrada, Madrid, Spain.
| | - Pedro Borja Aguilar
- Clínica Universidad de Navarra, Department of Radiation Physics, Avenida Pío XII, 31080 Pamplona, Navarra, Spain
| | - Benigno Barbés
- Clínica Universidad de Navarra, Department of Radiation Physics, Avenida Pío XII, 31080 Pamplona, Navarra, Spain
| | - Eduardo Guibelalde
- Medical Physics Group, Department of Radiology, University Complutense of Madrid, 28040 Madrid, Spain
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Petersson K, Jaccard M, Germond JF, Buchillier T, Bochud F, Bourhis J, Vozenin MC, Bailat C. High dose-per-pulse electron beam dosimetry - A model to correct for the ion recombination in the Advanced Markus ionization chamber. Med Phys 2017; 44:1157-1167. [PMID: 28094853 DOI: 10.1002/mp.12111] [Citation(s) in RCA: 117] [Impact Index Per Article: 16.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/23/2016] [Revised: 01/11/2017] [Accepted: 01/11/2017] [Indexed: 11/12/2022] Open
Abstract
PURPOSE The purpose of this work was to establish an empirical model of the ion recombination in the Advanced Markus ionization chamber for measurements in high dose rate/dose-per-pulse electron beams. In addition, we compared the observed ion recombination to calculations using the standard Boag two-voltage-analysis method, the more general theoretical Boag models, and the semiempirical general equation presented by Burns and McEwen. METHODS Two independent methods were used to investigate the ion recombination: (a) Varying the grid tension of the linear accelerator (linac) gun (controls the linac output) and measuring the relative effect the grid tension has on the chamber response at different source-to-surface distances (SSD). (b) Performing simultaneous dose measurements and comparing the dose-response, in beams with varying dose rate/dose-per-pulse, with the chamber together with dose rate/dose-per-pulse independent Gafchromic™ EBT3 film. Three individual Advanced Markus chambers were used for the measurements with both methods. All measurements were performed in electron beams with varying mean dose rate, dose rate within pulse, and dose-per-pulse (10-2 ≤ mean dose rate ≤ 103 Gy/s, 102 ≤ mean dose rate within pulse ≤ 107 Gy/s, 10-4 ≤ dose-per-pulse ≤ 101 Gy), which was achieved by independently varying the linac gun grid tension, and the SSD. RESULTS The results demonstrate how the ion collection efficiency of the chamber decreased as the dose-per-pulse increased, and that the ion recombination was dependent on the dose-per-pulse rather than the dose rate, a behavior predicted by Boag theory. The general theoretical Boag models agreed well with the data over the entire investigated dose-per-pulse range, but only for a low polarizing chamber voltage (50 V). However, the two-voltage-analysis method and the Burns & McEwen equation only agreed with the data at low dose-per-pulse values (≤ 10-2 and ≤ 10-1 Gy, respectively). An empirical model of the ion recombination in the chamber was found by fitting a logistic function to the data. CONCLUSIONS The ion collection efficiency of the Advanced Markus ionization chamber decreases for measurements in electron beams with increasingly higher dose-per-pulse. However, this chamber is still functional for dose measurements in beams with dose-per-pulse values up toward and above 10 Gy, if the ion recombination is taken into account. Our results show that existing models give a less-than-accurate description of the observed ion recombination. This motivates the use of the presented empirical model for measurements with the Advanced Markus chamber in high dose-per-pulse electron beams, as it enables accurate absorbed dose measurements (uncertainty estimation: 2.8-4.0%, k = 1). The model depends on the dose-per-pulse in the beam, and it is also influenced by the polarizing chamber voltage, with increasing ion recombination with a lowering of the voltage.
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Affiliation(s)
- Kristoffer Petersson
- CHUV, Institut de Radiophysique, Rue du Grand-Pré 1, CH-1007, Lausanne, Switzerland
| | - Maud Jaccard
- CHUV, Institut de Radiophysique, Rue du Grand-Pré 1, CH-1007, Lausanne, Switzerland
| | | | - Thierry Buchillier
- CHUV, Institut de Radiophysique, Rue du Grand-Pré 1, CH-1007, Lausanne, Switzerland
| | - François Bochud
- CHUV, Institut de Radiophysique, Rue du Grand-Pré 1, CH-1007, Lausanne, Switzerland
| | - Jean Bourhis
- CHUV, Service de Radio-Oncologie, Rue du Bugnon 46, CH - 1011, Lausanne, Switzerland
| | | | - Claude Bailat
- CHUV, Institut de Radiophysique, Rue du Grand-Pré 1, CH-1007, Lausanne, Switzerland
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Bailey M, Shipley DR, Manning JW. Roos and NACP-02 ion chamber perturbations and water-air stopping-power ratios for clinical electron beams for energies from 4 to 22 MeV. Phys Med Biol 2015; 60:1087-105. [PMID: 25586026 DOI: 10.1088/0031-9155/60/3/1087] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Empirical fits are developed for depth-compensated wall- and cavity-replacement perturbations in the PTW Roos 34001 and IBA / Scanditronix NACP-02 parallel-plate ionisation chambers, for electron beam qualities from 4 to 22 MeV for depths up to approximately 1.1 × R₅₀,D. These are based on calculations using the Monte Carlo radiation transport code EGSnrc and its user codes with a full simulation of the linac treatment head modelled using BEAMnrc. These fits are used with calculated restricted stopping-power ratios between air and water to match measured depth-dose distributions in water from an Elekta Synergy clinical linear accelerator at the UK National Physical Laboratory. Results compare well with those from recent publications and from the IPEM 2003 electron beam radiotherapy Code of Practice.
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Affiliation(s)
- M Bailey
- Radiation Dosimetry, National Physical Laboratory, Hampton Road, Teddington, Middlesex, TW11 0LW, UK
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13
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Muir BR, McEwen MR, Rogers DWO. Determination of relative ion chamber calibration coefficients from depth-ionization measurements in clinical electron beams. Phys Med Biol 2014; 59:5953-69. [PMID: 25211012 DOI: 10.1088/0031-9155/59/19/5953] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Abstract
A method is presented to obtain ion chamber calibration coefficients relative to secondary standard reference chambers in electron beams using depth-ionization measurements. Results are obtained as a function of depth and average electron energy at depth in 4, 8, 12 and 18 MeV electron beams from the NRC Elekta Precise linac. The PTW Roos, Scanditronix NACP-02, PTW Advanced Markus and NE 2571 ion chambers are investigated. The challenges and limitations of the method are discussed. The proposed method produces useful data at shallow depths. At depths past the reference depth, small shifts in positioning or drifts in the incident beam energy affect the results, thereby providing a built-in test of incident electron energy drifts and/or chamber set-up. Polarity corrections for ion chambers as a function of average electron energy at depth agree with literature data. The proposed method produces results consistent with those obtained using the conventional calibration procedure while gaining much more information about the behavior of the ion chamber with similar data acquisition time. Measurement uncertainties in calibration coefficients obtained with this method are estimated to be less than 0.5%. These results open up the possibility of using depth-ionization measurements to yield chamber ratios which may be suitable for primary standards-level dissemination.
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Affiliation(s)
- B R Muir
- Measurement Science and Standards, National Research Council of Canada, Ottawa, ON, K1A 0R6, Canada
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Di Venanzio C, Marinelli M, Milani E, Prestopino G, Verona C, Verona-Rinati G, Falco MD, Bagalà P, Santoni R, Pimpinella M. Characterization of a synthetic single crystal diamond Schottky diode for radiotherapy electron beam dosimetry. Med Phys 2013; 40:021712. [DOI: 10.1118/1.4774360] [Citation(s) in RCA: 45] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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Zink K, Wulff J. Beam quality corrections for parallel-plate ion chambers in electron reference dosimetry. Phys Med Biol 2012; 57:1831-54. [DOI: 10.1088/0031-9155/57/7/1831] [Citation(s) in RCA: 30] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Muir BR, McEwen MR, Rogers DWO. Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams. Med Phys 2012; 39:1618-31. [DOI: 10.1118/1.3687864] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022] Open
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17
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Bass G, Thomas R, Pearce J. The calibration of parallel-plate electron ionization chambers at NPL for use with the IPEM 2003 code of practice: summary data. Phys Med Biol 2009; 54:N115-24. [DOI: 10.1088/0031-9155/54/8/n01] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
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Bruggmoser G, Saum R, Schmachtenberg A, Schmid F, Schüle E. Determination of the recombination correction factorkSfor some specific plane-parallel and cylindrical ionization chambers in pulsed photon and electron beams. Phys Med Biol 2006; 52:N35-50. [PMID: 17202615 DOI: 10.1088/0031-9155/52/2/n01] [Citation(s) in RCA: 50] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
It has been shown from an evaluation of the inverse reading of the dosemeter (1/M) against the inverse of the polarizing voltage (1/V), obtained with a number of commercially available ionization chambers, using dose per pulse values between 0.16 and 5 mGy, that a linear relationship between the recombination correction factor kS and dose per pulse (DPP) can be found. At dose per pulse values above 1 mGy the method of a general equation with coefficients dependent on the chamber type gives more accurate results than the Boag method. This method was already proposed by Burns and McEwen (1998, Phys. Med. Biol. 43 2033) and avoids comprehensive and time-consuming measurements of Jaffé plots which are a prerequisite for the application of the multi-voltage analysis (MVA) or the two-voltage analysis (TVA). We evaluated and verified the response of ionization chambers on the recombination effect in pulsed accelerator beams for both photons and electrons. Our main conclusions are: (1) The correction factor k(S) depends only on the DPP and the chamber type. There is no influence of radiation type and energy. (2) For all the chambers investigated there is a linear relationship between kS and DPP up to 5 mGy/pulse, and for two chambers we could show linearity up to 40 mGy/pulse. (3) A general formalism, such as that of Boag, characterizes chambers exclusively by the distance of the electrodes and gives a trend for the correction factor, and therefore (4) a general formalism has to reflect the influence of the chamber construction on the recombination by the introduction of chamber-type dependent coefficients.
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Affiliation(s)
- G Bruggmoser
- Univ.-Klinikum Freiburg, Klinik f Strahlenheilkunde, FB Med. Physik, Freiburg, Germany.
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Kron T, McNiven A, Witruk B, Kenny M, Battista J. An experimental study of recombination and polarity effect in a set of customized plane parallel ionization chambers. AUSTRALASIAN PHYSICS & ENGINEERING SCIENCES IN MEDICINE 2006; 29:291-9. [PMID: 17260582 DOI: 10.1007/bf03178393] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/01/2022]
Abstract
Plane parallel ionization chambers are an important tool for dosimetry and absolute calibration of electron beams used for radiotherapy. Most dosimetric protocols require corrections for recombination and polarity effects, which are to be determined experimentally as they depend on chamber design and radiation quality. Both effects were investigated in electron beams from a linear accelerator (Varian 21CD) for a set of four tissue equivalent plane parallel ionization chambers customized for the present research by Standard Imaging (Madison WI). All four chambers share the same design and air cavity dimensions, differing only in the diameter of their collecting electrode and the corresponding width of the guard ring. The diameters of the collecting electrodes were 2 mm, 4 mm, 10 mm and 20 mm. Measurements were taken using electron beams of nominal energy 6 to 20 MeV in a 10 cm x 10 cm field size with a SSD of 100 cm at various depths in a Solid Water slab phantom. No significant variation of recombination effect was found with radiation quality, depth of measurement or chamber design. However, the polarity effect exceeded 5% for the chambers with small collecting electrode for an effective electron energy below 4 MeV at the point of measurement. The magnitude of the effect increased with decreasing electron energy in the phantom. The polarity correction factor calculated following AAPM protocol TG51 ranged from approximately 1.00 for the 20.0 mm chamber to less than 0.95 for the 2 mm chamber at 4.1 cm depth in a electron beam of nominally 12 MeV. By inverting the chamber it could be shown that the polarity effect did not depend on the polarity of the electrode first traversed by the electron beam. Similarly, the introduction of an air gap between the overlying phantom layer and the chambers demonstrated that the angular distribution of the electrons at the point of measurement had a lesser effect on the polarity correction than the electron energy itself. The magnitude of the absolute difference between charge collected at positive and negative polarity was found to correlate with the area of the collecting electrode which is consistent with the explanation that differences in thickness of the collecting electrodes and the number of electrons stopped in them contribute significantly to the polarity effect. Overall, the polarity effects found in the present study would have a negligible effect on electron beam calibration at a measurement depth recommended by most calibration protocols. However, the present work tested the corrections under extreme conditions thereby aiming at greater understanding of the mechanism underlying the correction factors for these chambers. This may lead to better chamber design for absolute dosimetry and electron beam characterization with less reliance on empirical corrections.
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Affiliation(s)
- T Kron
- Department of Physical Sciences, Peter MacCallum Cancer Centre, Melbourne, Australia.
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