Consequences of air around an ionization chamber: Are existing solid phantoms suitable for reference dosimetry on an MR‐linac?

Hackett, S.; Asselen, Bram van; Wolthaus, J.; Kok, J G M; Woodings, S.; Lagendijk, J.J.W.; Raaymakers, B W

doi:10.1118/1.4952727

Cited by 58 publications

(104 citation statements)

References 23 publications

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“…The effect was smaller for thinner air gaps, but still potentially significant (> 0.5%) for air gaps as small as 0.05 mm. These results are consistent with the effects measured in previous studies …”

Section: Discussionsupporting

confidence: 94%

“…In a 1.5 T magnetic field, the gyroradii of the electrons entering the chamber and contributing to the dose was mostly on the order of 2 to 4 mm (Fig. ), which is 10 times larger than the air gap thicknesses implicated in previous observations . The total contribution from electrons with gyroradii of the same order as the suspected air gap thicknesses (≤ 0.5 mm) was 0.2% or less.…”

Section: Discussionmentioning

confidence: 68%

“…The air gap effect observed by Hackett et al was a reduction in chamber signal associated with submillimeter air gap thicknesses, so the energy contributions of most interest are those of electrons with a gyroradius on that scale. A reduction in chamber signal implies a loss of energy reaching the chamber which is why we are studying the electrons as they first enter rather than as they leave.…”

Section: Resultsmentioning

confidence: 97%

“…However, readings from ionization chambers can be perturbed when placed inside conventional solid water phantoms in a magnetic field . This effect manifests as an increase in the orientation dependence of the chamber when rotated about its long axis, with variations for the PTW 30013 Farmer chamber of more than 1% observed by Hackett et al, for an air gap likely to be on the order of 0.1 mm thick, and up to 3.8% observed by Agnew et al for a 0.3 mm asymmetric air gap. We previously reported similar variations .…”

Section: Introductionmentioning

confidence: 99%

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Monte Carlo study of the chamber-phantom air gap effect in a magnetic field

O'Brien

Sawakuchi

2017

Med. Phys.

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Purpose The aim of this study was to examine the effect of sub-millimeter air gaps that may exist between an ionization chamber and solid phantoms when measurements are performed in a magnetic field. Methods Geant4 Monte Carlo simulations were performed using a model of a PTW 30013 Farmer chamber in a water phantom. Symmetrical and asymmetrical air gaps of various thicknesses were modeled surrounding the chamber, and the dose to the air cavity of the chamber was scored in each case. Magnetic fields were modeled parallel to the long axis of the chamber with strengths of 0, 0.35 T, 1.0 T and 1.5 T. To examine the phenomenon in more detail, the gyroradii of the electrons responsible for the energy deposited in the chamber were scored as they entered the chamber and the total energy deposited was split into three components: energy originating from inside the chamber, in the immediate vacinity of the chamber, or outside the chamber. Results Differences in the chamber dose of 1.6% were observed for asymmetric air gaps just 0.2 mm thick. No effect greater than 0.5% was observed for the symmetrical air gaps investigated in this work (1.4 mm thick or less) for this chamber/magnetic field configuration. The mean gyroradius of contributing electrons as they first enter the chamber was 4 mm. The presence of the air gap reduced the energy contributions from electrons released in the immediate vicinity of the chamber, and this loss was not completely compensated for when a magnetic field is present. Conclusions The gyroradius of most electrons was too large to be responsible for the air gap effect via the electron return effect; instead, the effect is attributed to the loss of energy contributions from electrons originating inside the air gap volume, which is not completely compensated for by more distant electrons owing to their reduced range in the magnetic field. When the chamber is parallel with the magnetic field, symmetric air gaps have a smaller effect (<0.5%) compared to asymmetric air-gaps (up to 1.6%) in the chamber response.

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Section: Discussionsupporting

confidence: 94%

Section: Discussionmentioning

confidence: 68%

Section: Resultsmentioning

confidence: 97%

Section: Introductionmentioning

confidence: 99%

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Monte Carlo study of the chamber-phantom air gap effect in a magnetic field

O'Brien

Sawakuchi

2017

Med. Phys.

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“…In the case of MR-linacs, they are particularly attractive because of the limited space available inside the MRI bore. However, Hackett et al [17] recently showed that ionization chambers exhibit a significantly increased rotational dependence in the presence of a B-field when used in plastic phantoms. This effect was attributed to the presence of sub-millimeter air gaps surrounding the chamber inside the phantom insert cavity.…”

Section: Air Gapsmentioning

confidence: 99%

Dosimetry in the presence of strong magnetic fields

O'Brien

Schupp

Pencea

et al. 2017

J. Phys.: Conf. Ser.

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Abstract. Magnetic resonance imaging-guided radiotherapy (MRIgRT) is an emerging technology that requires the use of radiation fields in the presence of magnetic (B) fields. In the presence of B-fields the Lorentz force influences the trajectories of the secondary electrons, which in turn affects both the dose distribution in water and the dose-response of ionization chambers and several other detectors. Thus, dosimetry in the presence of a B-field requires understanding both the B-field effects on the dose distribution and the response of detectors. In this paper we present measured data to show effects of the B-field on the dose distributions, response of ionization chambers, and presence of air-gaps surrounding the sensitive volume of the detector.

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Characteristics of the Exradin W1 scintillator in the magnetic field

Yoon

Kim

Choi

et al. 2019

J Applied Clin Med Phys

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To investigate the angular dependency of the W1 scintillator with and without a magnetic field, the beam incidence angles to the detector varied from 0° to 360° at intervals of 30° when the detector was pointed in both the craniocaudal and right‐to‐left directions. The beam incidence angles also varied from 0° to 360° at intervals of 45° when the W1 scintillator was in the anterior‐to‐posterior direction. To investigate the field size dependency of the W1 scintillator with and without a magnetic field, the doses by an identical beam‐on time were measured at various square field sizes and the measured doses were normalized to the dose at the field of 10.5 cm × 10.5 cm (FS10.5). With and without a magnetic field, the deviations of the doses to the dose at the beam incident angle of 0° were always less than 1% regardless of the dosimeter positioning relative to the magnetic field direction. When the field sizes were equal to or less than FS10.5, the differences in the output factors with and without a magnetic field were less than 0.7%. However, those were larger than 1% at fields larger than FS10.5, and up to 3.1%. The W1 scintillator showed no angular dependency to the magnetic field. Differences larger than 1% in the output factors with and without a magnetic field were observed at field sizes larger than 10.5 cm × 10.5 cm.

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Consequences of air around an ionization chamber: Are existing solid phantoms suitable for reference dosimetry on an MR‐linac?

Cited by 58 publications

References 23 publications

Monte Carlo study of the chamber-phantom air gap effect in a magnetic field

Monte Carlo study of the chamber-phantom air gap effect in a magnetic field

Dosimetry in the presence of strong magnetic fields

Characteristics of the Exradin W1 scintillator in the magnetic field

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