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Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors

PURPOSE: Reference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiati...

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Autores principales: Woodings, S. J., van Asselen, B., van Soest, T. L., de Prez, L. A., Lagendijk, J. J. W., Raaymakers, B. W., Wolthaus, J. W. H.
Formato: Online Artículo Texto
Lenguaje:English
Publicado: John Wiley and Sons Inc. 2019
Materias:
Acceso en línea:https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6852601/
https://www.ncbi.nlm.nih.gov/pubmed/31131902
http://dx.doi.org/10.1002/mp.13623
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author Woodings, S. J.
van Asselen, B.
van Soest, T. L.
de Prez, L. A.
Lagendijk, J. J. W.
Raaymakers, B. W.
Wolthaus, J. W. H.
author_facet Woodings, S. J.
van Asselen, B.
van Soest, T. L.
de Prez, L. A.
Lagendijk, J. J. W.
Raaymakers, B. W.
Wolthaus, J. W. H.
author_sort Woodings, S. J.
collection PubMed
description PURPOSE: Reference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65‐G detectors are waterproof Farmer‐type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber‐to‐chamber variation and determine whether generic chamber type‐specific magnetic field correction factors can be applied for each of the PTW30013 and FC65‐G type ion chambers when they are oriented anti‐parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field. METHODS: The experiment was conducted with 12 PTW30013 and 13 FC65‐G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross‐calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI‐linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR‐compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter‐clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber‐type determined. RESULTS: The magnetic field correction factors within both PTW30013 and FC65‐G chamber‐types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65‐G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%. CONCLUSION: The consistency of the results for the PTW30013 and FC65‐G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65‐G chamber. Values for each chamber‐type (with careful attention to beam energy, magnetic field strength and beam‐field‐chamber orientations) can be applied from the literature.
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spelling pubmed-68526012019-11-21 Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors Woodings, S. J. van Asselen, B. van Soest, T. L. de Prez, L. A. Lagendijk, J. J. W. Raaymakers, B. W. Wolthaus, J. W. H. Med Phys COMPUTATIONAL AND EXPERIMENTAL DOSIMETRY PURPOSE: Reference dosimetry in a strong magnetic field is made more complex due to (a) the change in dose deposition and (b) the change in sensitivity of the detector. Potentially it is also influenced by thin air layers, interfaces between media, relative orientations of field, chamber and radiation, and minor variations in ion chamber stem or electrode construction. The PTW30013 and IBA FC65‐G detectors are waterproof Farmer‐type ion chambers that are suitable for reference dosimetry. The magnetic field correction factors have previously been determined for these chamber types. The aim of this study was to assess the chamber‐to‐chamber variation and determine whether generic chamber type‐specific magnetic field correction factors can be applied for each of the PTW30013 and FC65‐G type ion chambers when they are oriented anti‐parallel (ǁ) to, or perpendicular (⊥) to, the magnetic field. METHODS: The experiment was conducted with 12 PTW30013 and 13 FC65‐G chambers. The magnetic field correction factors were measured using a practical method. In this study each chamber was cross‐calibrated against the local standard chamber twice; with and without magnetic field. Measurements with 1.5 T magnetic field were performed with the 7 MV FFF beam of the MRI‐linac. Measurements without magnetic field (0 T) were performed with the 6 MV conventional beam of an Elekta Agility linac. A prototype MR‐compatible PTW MP1 phantom was used along with a prototype holder that facilitated measurements with the chamber aligned 90° counter‐clockwise (⊥) and 180° (ǁ) to the direction of the magnetic field. A monitor chamber was also mounted on the holder and all measurements were normalized so that the effect of variations in the output of each linac was minimized. Measurements with the local standard chamber were repeated during the experiment to quantify the experimental uncertainty. Recombination was measured in the 6 MV beam. Beam quality correction factors were applied. Differences in recombination and beam quality between beams are constant within each chamber type. By comparing the results for the two cross calibrations the magnetic field correction factors can be determined for each chamber, and the variation within the chamber‐type determined. RESULTS: The magnetic field correction factors within both PTW30013 and FC65‐G chamber‐types were found to be very consistent, with observed standard deviations for the PTW30013 of 0.19% (ǁ) and 0.13% (⊥), and for the FC65‐G of 0.15% (ǁ) and 0.17% (⊥). These variations are comparable with the standard uncertainty (k = 1) of 0.24%. CONCLUSION: The consistency of the results for the PTW30013 and FC65‐G chambers implies that it is not necessary to derive a new factor for every new PTW30013 or FC65‐G chamber. Values for each chamber‐type (with careful attention to beam energy, magnetic field strength and beam‐field‐chamber orientations) can be applied from the literature. John Wiley and Sons Inc. 2019-06-17 2019-08 /pmc/articles/PMC6852601/ /pubmed/31131902 http://dx.doi.org/10.1002/mp.13623 Text en © 2019 The Authors. Medical Physics published by Wiley Periodicals, Inc. on behalf of American Association of Physicists in Medicine This is an open access article under the terms of the http://creativecommons.org/licenses/by/4.0/ License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
spellingShingle COMPUTATIONAL AND EXPERIMENTAL DOSIMETRY
Woodings, S. J.
van Asselen, B.
van Soest, T. L.
de Prez, L. A.
Lagendijk, J. J. W.
Raaymakers, B. W.
Wolthaus, J. W. H.
Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title_full Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title_fullStr Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title_full_unstemmed Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title_short Technical Note: Consistency of PTW30013 and FC65‐G ion chamber magnetic field correction factors
title_sort technical note: consistency of ptw30013 and fc65‐g ion chamber magnetic field correction factors
topic COMPUTATIONAL AND EXPERIMENTAL DOSIMETRY
url https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6852601/
https://www.ncbi.nlm.nih.gov/pubmed/31131902
http://dx.doi.org/10.1002/mp.13623
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