Technical Note: Ion chamber angular dependence in a magnetic field

Reynolds, M; Rathee, S; Fallone, B. G.

doi:10.1002/mp.12405

Cited by 19 publications

(21 citation statements)

References 25 publications

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“…The statistical uncertainty on all of these difference is about 0.14%. These results are in agreement with the findings of Reynolds et al., in which a 3° misalignment in any direction about the the 90° and 270° orientations produced a maximum variation in dose of roughly 1% and an average variation of about 0.5%. Furthermore, although slightly larger

Δ k_{Q}^{mag} false(italicθ false)

values are observed for 0° and 180° in Table , the advantage of nearly completely eliminating the effect of the sensitive volume on k

_{Q}^{italicmag}

, as compared to the 90° and 270° simulations, and the absence of a difference between the k

_{Q}^{italicmag}

values at 0 and 180°, as described in Section 3.A, makes the C‐I and C‐III orientations the ideal orientations for ion chamber dosimetry in magnetic fields.…”

Section: Resultssupporting

confidence: 93%

“…The statistical uncertainty on all of these difference is about 0.14%. These results are in agreement with the findings of Reynolds et al, 17 in which a 3°misalignment in Table III, the advantage of nearly completely eliminating the effect of the sensitive volume on k mag Q , as compared to the 90°and 270°simulations, and the absence of a difference between the k mag Q values at 0 and 180°, as described in Section 3.A, makes the C-I and C-III orientations the ideal orientations for ion chamber dosimetry in magnetic fields. Additionally, as is seen in the Dk mag Q ðh; DVÞ results, Table III, in these orientations the lack of variation in k mag Q demonstrates that the details of the sensitive volume are negligible (as compared to several percent change due to sensitive volume changes in the C-II and C-IV orientations).…”

Section: B Sensitive Volume Effectssupporting

confidence: 94%

“…The aim of this work is to evaluate, using the EGSnrc Monte Carlo simulation code, both k B and k mag Q for a variety of chambers (cylindrical and parallel-plate), magnetic field values, and beam qualities. The variation of the ion chamber's reading as a function of orientation in the magnetic field has been shown previously, 15,17,18 and, here, k mag Q is evaluated as a function of angle with respect to the magnetic field in order to determine an optimal orientation for magnetic field reference dosimetry. k B and k mag Q are also determined for a few select parallel-plate chambers to evaluate the possible use of these chambers in magnetic fields.…”

Section: Introductionmentioning

confidence: 92%

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Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

2018

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Orienting the chamber parallel to the magnetic field when the field is perpendicular to the photon beam will minimize the effect of the magnetic field on chamber response, and eliminate the problem of the unknown sensitive volume. Values of k and kQmag can bring ion chamber dosimetry in magnetic fields in-line with the TG-51 protocol. PP chamber are sensitive to the magnetic field and variation in chamber response due to small angular changes makes them unlikely candidates for clinical reference dosimetry in magnetic fields. The stability in TPR1020, as a function of magnetic fields and beam qualities, makes it the best beam quality specifier in magnetic fields.

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Δ k_{Q}^{mag} false(italicθ false)

values are observed for 0° and 180° in Table , the advantage of nearly completely eliminating the effect of the sensitive volume on k

_{Q}^{italicmag}

, as compared to the 90° and 270° simulations, and the absence of a difference between the k

_{Q}^{italicmag}

values at 0 and 180°, as described in Section 3.A, makes the C‐I and C‐III orientations the ideal orientations for ion chamber dosimetry in magnetic fields.…”

Section: Resultssupporting

confidence: 93%

Section: B Sensitive Volume Effectssupporting

confidence: 94%

Section: Introductionmentioning

confidence: 92%

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Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

2018

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“…In addition to these challenges, all quality assurance equipment must be MR compatible, and the impact of the magnetic field on the response of the QA equipment including ion chambers must be well characterized. [14][15][16][17][18] The characterization of a research version of the Monaco treatment planning system (TPS) 19 and the commissioning of the MRI scanner using research MR sequences that are not widely available have been previously reported. 20 Despite these initial efforts, there is little published guidance on commissioning the Elekta Unity in its current FDA-approved state.…”

mentioning

confidence: 99%

Commissioning of a 1.5T Elekta Unity MR‐linac: A single institution experience

Snyder

Aubin

Yaddanapudi

et al. 2020

J Applied Clin Med Phys

101

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MR image-guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR-linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR-to-MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator-triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end-to-end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350mm-diameter spherical volume, the peak-to-peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator-triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS-calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 ± 1.1% (3%/ 2 mm) and end-to-end testing of adapted plans showed agreement within 0.4% between ion-chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end-to-end test using an IROC MRgRT phantom.

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“…Using a δ u of 0.02 or smaller has become common for Monte Carlo simulations which use similar theories 19,20,40,77 , but this produces a potentially severe increase in computational time. The solution for a dose calculation in the presence of a magnetic field is expected to converge as δ u decreases, and determining the largest possible δ u that does not cause variations in the solution is required to improve efficiency.…”

Section: Lorentz Force In a Condensed History Calculationmentioning

confidence: 99%

Charged Particle Transport in Magnetic Fields in the EGSnrc Monte Carlo Code System

Malkov¹

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The advent of magnetic resonance guided radiation therapy provides a promising technology for dealing with tumour motion and anatomical variations during treatment.These machines possess a variety of beam energies, geometrical configurations, and different magnetic field strengths. Although photon beams do not directly experience the influence of the magnetic field, electrons set in motion will curve and impact dose distributions. Clinical reference dosimetry protocols rely on correction factors which account for the change in detector response for different beam qualities in the absence of a magnetic field. The effect of the magnetic field poses challenges for dosimetry, as ion chambers and solid state detectors respond disproportionately to the actual change in the dose to the media in the presence of the magnetic field. This necessitates an adaptation of current dosimetry protocols through calculation of high precision magnetic field and beam quality correction factors which account for detector response variation.In this work, charged particle transport in magnetic fields is implemented in EGSnrc and is shown to pass the Fano cavity test at the 0.1 % level. Further good agreement with experimental ion chamber measurements is shown, and important effects such as air gaps and the unknown sensitive volume of the chamber are determined to cause several percent variation in the calculated ion chamber dose. Ion chamber magnetic field correction factors are then evaluated for over thirty cylindrical ionization chamber and a select number of parallel-plate chambers. Magnetic field correction factors for the majority of cylindrical chambers are within 1 % of unity, while parallel-plate chambers require correction factors on the order of several percent and, unlike cylindrical chambers, no optimal orientation is available to reduce the effect of the magnetic field. The %dd(10) x beam-quality specifier is shown to have a strong dependence of the magnetic field strength, and the TPR 20 10 is determined to be the optimal beam-quality specifier in magnetic fields. Collectively, this work contributes to the EGSnrc gold standard Monte Carlo code and to the evolving field of clinical reference dosimetry in magnetic fields.ii To my supervisor and mentor, Dr. David W. O. Rogers, I wish to extend my sincere gratitude for giving me the opportunity to work with, and learn from, him. From the first time that I met Dave, he has provided me with exceptional guidance and support. Dave's door has always been open when I needed help, and his insights and calm manner have always helped point the way forward. I have been truly lucky in my choice of supervisor and it is one of the best decisions I have ever made. I would also like to thank all of my friends, office mates, and colleagues at Carleton University. Martin Martinov has been an instrumental help throughout my research, and provided many helpful suggestions and constantly applied his excellent programming knowledge to aide everyone in the group. Also, I am very appreciative to all...

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Technical Note: Ion chamber angular dependence in a magnetic field

Abstract: Within a magnetic field-oriented orthogonal to the radiation beam, the ionization chamber dose-response fluctuates greatly as a function of polar and azimuthal angle, where a parallel field yields a more homogeneous dose-response.

Cited by 19 publications

References 25 publications

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality

Commissioning of a 1.5T Elekta Unity MR‐linac: A single institution experience

Charged Particle Transport in Magnetic Fields in the EGSnrc Monte Carlo Code System

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