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Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane‐parallel ionization chamber.

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Title: Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane‐parallel ionization chamber.
Authors: Gebauer, Benjamin1,2 (AUTHOR), Baumann, Kilian‐Simon3,4,5 (AUTHOR), Fuchs, Hermann6,7 (AUTHOR), Georg, Dietmar6,7 (AUTHOR), Oborn, Brad M.2,8,9 (AUTHOR), Looe, Hui‐Khee10 (AUTHOR), Lühr, Armin11 (AUTHOR) armin.luehr@tu-dortmund.de
Source: Medical Physics. Mar2024, Vol. 51 Issue 3, p2293-2305. 13p.
Subjects: Ionization chambers, Magnetic field measurements, Proton beams, Correction factors, Proton magnetic resonance, Protons, Air pressure
Abstract: Background: The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)‐dependent change in the dosage of ionization chambers in magnetic resonance imaging‐integrated proton therapy (MRiPT) is considered by the correction factor kB⃗,M,Q$k_{\vec{B},M,Q}$, which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. Purpose: In this study, kB⃗,M,Q$k_{\vec{B},M,Q}$ was both measured and simulated with high accuracy for a plane‐parallel ionization chamber at different clinical relevant proton energies and MF strengths. Material and methods: The dose‐response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10 ×$\times$ 10 cm 2$^2$ quasi mono‐energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm 2$^2$ in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL‐calculated 0.32, 0.5, and 1 T MF maps of the electromagnet. kB⃗,M,Q$k_{\vec{B},M,Q}$ was calculated for the measurements for all energies and MF strengths based on the equation: kB⃗,M,Q=MQMQB⃗$k_{\vec{B},M,Q}=\frac{M_\mathrm{Q}}{M_\mathrm{Q}^{\vec{B}}}$, where MQB⃗$M_\mathrm{Q}^{\vec{B}}$ and MQ$M_\mathrm{Q}$ were the temperature and air‐pressure corrected detector readings with and without the MF, respectively. MC‐based correction factors were determined as kB⃗,M,Q=DdetDdetB⃗$k_{\vec{B},M,Q}=\frac{D_\mathrm{det}}{D_\mathrm{det}^{\vec{B}}}$, where DdetB⃗$D_\mathrm{det}^{\vec{B}}$ and Ddet$D_\mathrm{det}$ were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose‐to‐water (DW$D_{W}$) and the shift in depth of the Bragg peak. Results: The detector showed a reduced dose‐response for all measured energies and MF strengths, resulting in experimentally determined kB⃗,M,Q$k_{\vec{B},M,Q}$ values larger than unity. For all energies and MF strengths examined, kB⃗,M,Q$k_{\vec{B},M,Q}$ ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non‐linear with a maximum at 1 T and 252.7 MeV. The MC simulated kB⃗,M,Q$k_{\vec{B},M,Q}$ values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact on DW$D_{W}$ was smaller 0.2 %. Conclusion: For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement of kB⃗,M,Q$k_{\vec{B},M,Q}$ was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculating kB⃗,M,Q$k_{\vec{B},M,Q}$ for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT. [ABSTRACT FROM AUTHOR]
Copyright of Medical Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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  Data: Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane‐parallel ionization chamber.
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  Data: <searchLink fieldCode="AR" term="%22Gebauer%2C+Benjamin%22">Gebauer, Benjamin</searchLink><relatesTo>1,2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Baumann%2C+Kilian‐Simon%22">Baumann, Kilian‐Simon</searchLink><relatesTo>3,4,5</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Fuchs%2C+Hermann%22">Fuchs, Hermann</searchLink><relatesTo>6,7</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Georg%2C+Dietmar%22">Georg, Dietmar</searchLink><relatesTo>6,7</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Oborn%2C+Brad+M%2E%22">Oborn, Brad M.</searchLink><relatesTo>2,8,9</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Looe%2C+Hui‐Khee%22">Looe, Hui‐Khee</searchLink><relatesTo>10</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Lühr%2C+Armin%22">Lühr, Armin</searchLink><relatesTo>11</relatesTo> (AUTHOR)<i> armin.luehr@tu-dortmund.de</i>
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  Data: <searchLink fieldCode="JN" term="%22Medical+Physics%22">Medical Physics</searchLink>. Mar2024, Vol. 51 Issue 3, p2293-2305. 13p.
– Name: Subject
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  Data: <searchLink fieldCode="DE" term="%22Ionization+chambers%22">Ionization chambers</searchLink><br /><searchLink fieldCode="DE" term="%22Magnetic+field+measurements%22">Magnetic field measurements</searchLink><br /><searchLink fieldCode="DE" term="%22Proton+beams%22">Proton beams</searchLink><br /><searchLink fieldCode="DE" term="%22Correction+factors%22">Correction factors</searchLink><br /><searchLink fieldCode="DE" term="%22Proton+magnetic+resonance%22">Proton magnetic resonance</searchLink><br /><searchLink fieldCode="DE" term="%22Protons%22">Protons</searchLink><br /><searchLink fieldCode="DE" term="%22Air+pressure%22">Air pressure</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: Background: The combination of magnetic resonance imaging and proton therapy offers the potential to improve cancer treatment. The magnetic field (MF)‐dependent change in the dosage of ionization chambers in magnetic resonance imaging‐integrated proton therapy (MRiPT) is considered by the correction factor kB⃗,M,Q$k_{\vec{B},M,Q}$, which needs to be determined experimentally or computed via Monte Carlo (MC) simulations. Purpose: In this study, kB⃗,M,Q$k_{\vec{B},M,Q}$ was both measured and simulated with high accuracy for a plane‐parallel ionization chamber at different clinical relevant proton energies and MF strengths. Material and methods: The dose‐response of the Advanced Markus chamber (TM34045, PTW, Freiburg, Germany) irradiated with homogeneous 10 ×$\times$ 10 cm 2$^2$ quasi mono‐energetic fields, using 103.3, 128.4, 153.1, 223.1, and 252.7 MeV proton beams was measured in a water phantom placed in the MF of an electromagnet with MF strengths of 0.32, 0.5, and 1 T. The detector was positioned at a depth of 2 g/cm 2$^2$ in water, with chamber electrodes parallel to the MF lines and perpendicular to the proton beam incidence direction. The measurements were compared with TOPAS MC simulations utilizing COMSOL‐calculated 0.32, 0.5, and 1 T MF maps of the electromagnet. kB⃗,M,Q$k_{\vec{B},M,Q}$ was calculated for the measurements for all energies and MF strengths based on the equation: kB⃗,M,Q=MQMQB⃗$k_{\vec{B},M,Q}=\frac{M_\mathrm{Q}}{M_\mathrm{Q}^{\vec{B}}}$, where MQB⃗$M_\mathrm{Q}^{\vec{B}}$ and MQ$M_\mathrm{Q}$ were the temperature and air‐pressure corrected detector readings with and without the MF, respectively. MC‐based correction factors were determined as kB⃗,M,Q=DdetDdetB⃗$k_{\vec{B},M,Q}=\frac{D_\mathrm{det}}{D_\mathrm{det}^{\vec{B}}}$, where DdetB⃗$D_\mathrm{det}^{\vec{B}}$ and Ddet$D_\mathrm{det}$ were the doses deposited in the air cavity of the ionization chamber model with and without the MF, respectively. Furthermore, MF effects on the chamber dosimetry are studied using MC simulations, examining the impact on the absorbed dose‐to‐water (DW$D_{W}$) and the shift in depth of the Bragg peak. Results: The detector showed a reduced dose‐response for all measured energies and MF strengths, resulting in experimentally determined kB⃗,M,Q$k_{\vec{B},M,Q}$ values larger than unity. For all energies and MF strengths examined, kB⃗,M,Q$k_{\vec{B},M,Q}$ ranged between 1.0065 and 1.0205. The dependence on the energy and the MF strength was found to be non‐linear with a maximum at 1 T and 252.7 MeV. The MC simulated kB⃗,M,Q$k_{\vec{B},M,Q}$ values agreed with the experimentally determined correction factors within their standard deviations with a maximum difference of 0.6%. The MC calculated impact on DW$D_{W}$ was smaller 0.2 %. Conclusion: For the first time, measurements and simulations were compared for proton dosimetry within MFs using an Advanced Markus chamber. Good agreement of kB⃗,M,Q$k_{\vec{B},M,Q}$ was found between experimentally determined and MC calculated values. The performed benchmarking of the MC code allows for calculating kB⃗,M,Q$k_{\vec{B},M,Q}$ for various ionization chamber models, MF strengths and proton energies to generate the data needed for a proton dosimetry protocol within MFs and is, therefore, a step towards MRiPT. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Medical Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1002/mp.16797
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      – Code: eng
        Text: English
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        PageCount: 13
        StartPage: 2293
    Subjects:
      – SubjectFull: Ionization chambers
        Type: general
      – SubjectFull: Magnetic field measurements
        Type: general
      – SubjectFull: Proton beams
        Type: general
      – SubjectFull: Correction factors
        Type: general
      – SubjectFull: Proton magnetic resonance
        Type: general
      – SubjectFull: Protons
        Type: general
      – SubjectFull: Air pressure
        Type: general
    Titles:
      – TitleFull: Proton dosimetry in a magnetic field: Measurement and calculation of magnetic field correction factors for a plane‐parallel ionization chamber.
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              M: 03
              Text: Mar2024
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              Y: 2024
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