Breast dosimetry in transverse and longitudinal field MRI‐Linac radiotherapy systems

Mahdavi, Seied Rabi; Esmaeeli, A. D.; Pouladian, Majid; Monfared, Ali Shabestani; Sardari, Dariush; Bagheri, Saeid

doi:10.1118/1.4906193

Cited by 12 publications

(20 citation statements)

References 32 publications

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“…Current and developing machines make use of linac or 60 Co beam sources and have a range of magnetic fields strengths which are oriented either parallel or perpendicular to the incoming photon beam. The effect of the magnetic field is felt by the secondary electrons and has been shown to have a notable impact on patient dose distributions, particularly in regions of large density changes . Furthermore, the charge collected per MU in ion chambers has been shown to vary by several percent when making measurements in a magnetic field .…”

Section: Introductionmentioning

confidence: 99%

“…The effect of the magnetic field is felt by the secondary electrons and has been shown to have a notable impact on patient dose distributions, particularly in regions of large density changes. [8][9][10] Furthermore, the charge collected per MU in ion chambers has been shown to vary by several percent when making measurements in a magnetic field. 11,12,13 The magnitude of the influence on the charge reading depends heavily on the ion chamber, beam quality, and on magnetic field magnitude and orientation with respect to the chamber and the photon beam.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

2018

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“…28 Future work involves developing the phantom recipe further to include dosimetric gels in order to perform end-to-end testing and validate dose distributions derived from emerging technologies such as the MRI-Linac. 17,33,34…”

Section: Discussionmentioning

confidence: 99%

Technical Note: Multipurpose CT, ultrasound, and MRI breast phantom for use in radiotherapy and minimally invasive interventions

et al. 2016

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“…Current and developing machines are configured with magnetic fields ranging between 0.35 T and 1.5 T, with linac or Cobalt‐60 photon beams that are either parallel or perpendicular to the magnetic field. The magnetic field's effect on the trajectory of secondary electrons has been shown to affect dose distributions, particularly near interfaces between materials with very different densities . Furthermore, it has been found that the charge collected per MU in the ion chamber changes by plus or minus several percent in the presence of magnetic fields .…”

Section: Introductionmentioning

confidence: 99%

Sensitive volume effects on Monte Carlo calculated ion chamber response in magnetic fields

Malkov

Rogers

2017

Medical Physics

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Purpose: The development of magnetic resonance-guided radiation therapy (MRgRT) necessitates accurate Monte Carlo (MC) models of ion chambers for computing ion chamber corrections to compensate for the presence of the magnetic field. This study evaluates the sensitivity of the ion chamber dose response in a magnetic field on the collection volume used in the MC simulation. Methods: The EGSnrc system's egs_chamber application is used with a recently developed and validated magnetic field transport code. The calculated dose to the sensitive volume of the chamber per unit incident photon fluence, normalized to that at 0 T, is evaluated as a function of magnetic field for the PTW 30013, PTW 31006, PTW 31010, Exradin A12S, and Exradin A1SL chambers. The sensitive region is varied by excluding the volume corresponding to either 0, 0.5, or 1 mm of distance away from the stem. The photon field, magnetic field, and ion chamber are all oriented perpendicular to each other as in the majority of published experimental works. Results: The calculations for a Co-60 source demonstrate that variations from the 0 mm simulations are on the order of several percent with a maximum deviation, occurring at 0.5 T, of 1.75 AE 0.03% and 3.39 AE 0.06% for the 0.5 mm or 1 mm simulations, respectively, for a 0.057 cm 3 A1SL chamber. Larger volume chambers showed smaller, but still non-negligible, variations. Simulations of the A1SL chamber with a 7 MV photon source, corresponding to the Elekta MR-linac machine, demonstrate that the effect is slightly reduced but still persists with a maximum deviation of 1.97 AE 0.08% for the 1 mm reduction. Conclusions: Usually, the geometric sensitive volume of the ion chamber is used in MC calculation as a substitute for the potentially unknown, smaller, true collection volume (governed by the complex electric field distribution inside the chamber). The calculations in this study demonstrate that even a small variation in simulated volume can lead to fairly large variations in the MC calculated ion chamber response in a magnetic field. This is an important effect that must be addressed to ensure proper calibration of MRgRT machines using MC ion chamber correction factors. This effect may play a role, even where there is no magnetic field, in small-field dosimetry when volume averaging effect are important.

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Breast dosimetry in transverse and longitudinal field MRI‐Linac radiotherapy systems

Cited by 12 publications

References 32 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

Technical Note: Multipurpose CT, ultrasound, and MRI breast phantom for use in radiotherapy and minimally invasive interventions

Sensitive volume effects on Monte Carlo calculated ion chamber response in magnetic fields

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