Reference dosimetry in magnetic fields: formalism and ionization chamber correction factors

O'Brien, Daniel; Roberts, David A.; Ibbott, Geoffrey S.; Sawakuchi, Gabriel O.

doi:10.1118/1.4959785

Cited by 153 publications

(287 citation statements)

References 34 publications

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“…Beyond the build-up region, the reduced electron range has shifted the depth dose curve closer to the depth kerma curve. This shift results in a 0.5% lower dose at these depths for the same photon fluence [12]. More significantly, the compression of the build-up region enhances the dose at dmax by 1.9%.…”

Section: Methodsmentioning

confidence: 79%

“…As a result, the use of the PDD as a beam quality specifier must be re-examined in the presence of a B-field in terms of its relationship to the stopping power ratios. In contrast, alternative beam quality specifiers, such as the 10 20 , that depend on dose ratios in the transient equilibrium region are relatively unaffected by the B-field [12].…”

Section: Methodsmentioning

confidence: 95%

“…Note that in closed bore systems, such as the Elekta MR-linac, the cryostat acts as an additional source of contaminant electrons which also have the effect of shifting dmax closer to the surface [12]. These electrons are swept away when a transverse B-field is present and do not significantly influence the surface dose.…”

Section: Pdd and Dose Profilesmentioning

confidence: 99%

“…In that case, they found that the chamber response did not require correction for B-fields up to 1 T regardless of the orientation of the chamber. O'Brien et al [12] proposed a general formalism for reference dosimetry in the presence of a B-field, and they also calculated correction factors for a range of Farmer chambers for the transverse 1.5-T Elekta MR-linac system, where the beam is always perpendicular to the magnetic field. In this configuration, they showed that the correction for chambers that are aligned parallel to the B-field (1.5 T) (Figure 3a) is less than 1%.…”

Section: Ionization Chamber Responsementioning

confidence: 99%

“…As a result, there is great interest in developing new external-beam radiotherapy machines with integrated MRI [3][4][5][6][7]. However, since MRI requires magnetic (B) fields, the Lorentz force influences the trajectories of the secondary electrons, which in turn affects both the dose distribution in water [1,8,9] and the dose-response of ionization chambers [6,[10][11][12] and several other detectors [13,14]. 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.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Methodsmentioning

confidence: 79%

Section: Methodsmentioning

confidence: 95%

Section: Pdd and Dose Profilesmentioning

confidence: 99%

Section: Ionization Chamber Responsementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Dosimetry in the presence of strong magnetic fields

O'Brien

Schupp

Pencea

et al. 2017

J. Phys.: Conf. Ser.

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Measurement validation of treatment planning for a MR‐Linac

Chen

Paulson

Ahunbay

et al. 2019

J Applied Clin Med Phys

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Purpose The magnetic field can cause a nonnegligible dosimetric effect in an MR‐Linac system. This effect should be accurately accounted for by the beam models in treatment planning systems (TPS). The purpose of the study was to verify the beam model and the entire treatment planning and delivery process for a 1.5 T MR‐Linac based on comprehensive dosimetric measurements and end‐to‐end tests. Material and methods Dosimetry measurements and end‐to‐end tests were performed on a preclinical MR‐Linac (Elekta AB) using a multitude of detectors and were compared to the corresponding beam model calculations from the TPS for the MR‐Linac. Measurement devices included ion chambers (IC), diamond detector, radiochromic film, and MR‐compatible ion chamber array and diode array. The dose in inhomogeneous phantom was also verified. The end‐to‐end tests include the generation, delivery, and comparison of 3D and IMRT plan with measurement. Results For the depth dose measurements with Farmer IC, micro IC and diamond detector, the absolute difference between most measurement points and beam model calculation beyond the buildup region were <1%, at most 2% for a few measurement points. For the beam profile measurements, the absolute differences were no more than 1% outside the penumbra region and no more than 2.5% inside the penumbra region. Results of end‐to‐end tests demonstrated that three 3D static plans with single 5 × 10 cm2 fields (at gantry angle 0°, 90° and 270°) and two IMRT plans successfully passed gamma analysis with clinical criteria. The dose difference in the inhomogeneous phantom between the calculation and measurement was within 1.0%. Conclusions Both relative and absolute dosimetry measurements agreed well with the TPS calculation, indicating that the beam model for MR‐Linac properly accounts for the magnetic field effect. The end‐to‐end tests verified the entire treatment planning process.

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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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Reference dosimetry in magnetic fields: formalism and ionization chamber correction factors

Cited by 153 publications

References 34 publications

Dosimetry in the presence of strong magnetic fields

Dosimetry in the presence of strong magnetic fields

Measurement validation of treatment planning for a MR‐Linac

Characteristics of the Exradin W1 scintillator in the magnetic field

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