Charged particle transport in magnetic fields in EGSnrc

Malkov, Victor; Rogers, D. W. O.

doi:10.1118/1.4954318

Cited by 60 publications

(112 citation statements)

References 46 publications

(48 reference statements)

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“…No effect greater than 0.5% was seen for symmetrical air gaps with thicknesses of 1.4 mm or less for any of the magnetic field strengths studied. Malkov and Rogers calculated a 0.9% effect for a 0.5 mm symmetrical air gap and a 1.6% effect for a 1.0 mm air gap. Note that Malkov and Rogers performed their simulations with the 1.5 T magnetic field perpendicular to the chamber, the beam was modeled in EGSnrc as a parallel beam using an energy spectrum from a 6 MV Elekta SL25 beam, the chamber used was an NE2571 Farmer chamber and was positioned at the center of a 4.3 cm diameter delrin cylinder representing a measurement in a build‐up cap.…”

Section: Discussionmentioning

confidence: 99%

“…Malkov and Rogers calculated a 0.9% effect for a 0.5 mm symmetrical air gap and a 1.6% effect for a 1.0 mm air gap. Note that Malkov and Rogers performed their simulations with the 1.5 T magnetic field perpendicular to the chamber, the beam was modeled in EGSnrc as a parallel beam using an energy spectrum from a 6 MV Elekta SL25 beam, the chamber used was an NE2571 Farmer chamber and was positioned at the center of a 4.3 cm diameter delrin cylinder representing a measurement in a build‐up cap. Also, their values were based on a comparison of the ratios of the chamber response with and without a magnetic field for each air gap, rather than just the ratio of chamber response with and without the air gap.…”

Section: Discussionmentioning

confidence: 99%

“…Because this effect disappears when the chamber is used in a water phantom, it has been attributed to the presence of small pockets of air trapped between the chamber and the plastic phantom material. Malkov and Rogers used EGSnrc (modified to account for the magnetic field) to simulate the effect of a 0.5 mm and 1 mm uniform air gap surrounding a modeled NE2571 chamber and found that such gaps can change the response by up to 1% depending on the magnetic field strength. Although magnetic fields are known to enhance or suppress the dose at tissue‐air interfaces owing to the electron return effect, it remains unclear how this could result in such a large effect with the submillimeter air gaps found in these phantoms.…”

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: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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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“…The appropriateness of IC‐based dosimetry in conventional solid phantoms in the presence of a B ‐field has been investigated by several groups, and, to date, have identified the effect of the clearance air gap present between the chamber and phantom as a source of hindering perturbation . Most recently, O’Brien and Sawakuchi studied this effect by simulating a PTW 30013 IC in water with surrounding symmetric and asymmetric air gaps of various thicknesses in the presence of B‐ fields in the range of (0.35–1.5) T using MC.…”

Section: Discussionmentioning

confidence: 99%

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

et al. 2020

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Purpose In this work, the fabrication, operation, and evaluation of a probe‐format graphite calorimeter — herein referred to as Aerrow — as an absolute clinical dosimeter of high‐energy photon beams while in the presence of a B = 1.5 T magnetic field is described. Comparable to a cylindrical ionization chamber (IC) in terms of utility and usability, Aerrow has been developed for the purpose of accurately measuring absorbed dose to water in the clinic with a minimum disruption to the existing clinical workflow. To our knowledge, this is the first reported application of graphite calorimetry to magnetic resonance imaging (MRI)‐guided radiotherapy. Methods Based on a previously numerically optimized and experimentally validated design, an Aerrow prototype capable of isothermal operation was constructed in‐house. Graphite‐to‐water dose conversions as well as magnetic field perturbation factors were calculated using Monte Carlo, while heat transfer and mass impurity corrections and uncertainties were assessed analytically. Reference dose measurements were performed in the absence and presence of a B = 1.5 T magnetic field using Aerrow in the 7 MV FFF photon beam of an Elekta MRI‐linac and were directly compared to the results obtained using two calibrated reference‐class IC types. The feasibility of performing solid phantom‐based dosimetry with Aerrow and the possible influence of clearance gaps is also investigated by performing reference‐type dosimetry measurements for multiple rotational positions of the detector and comparing the results to those obtained in water. Results In the absence of the B‐field, as well as in the parallel orientation while in the presence of the B‐field, the absorbed dose to water measured using Aerrow was found to agree within combined uncertainties with those derived from TG‐51 using calibrated reference‐class ICs. Statistically significant differences on the order of (2–4)%, however, were observed when measuring absorbed dose to water using the ICs in the perpendicular orientation in the presence of the B‐field. Aerrow had a peak‐to‐peak response of about 0.5% when rotated within the solid phantom regardless of whether the B‐field was present or not. Conclusions This work describes the successful use of Aerrow as a straightforward means of measuring absolute dose to water for large high‐energy photon fields in the presence of a 1.5 T B‐field to a greater accuracy than currently achievable with ICs. The detector‐phantom air gap does not appear to significantly influence the response of Aerrow in absolute terms, nor does it contribute to its rotational dependence. This work suggests that the accurate use of solid phantoms for absolute point dose measurement is possible with Aerrow.

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“…For this, we utilized the Monte Carlo package EGSnrc (PRESTA-II) with the recently published enhanced algorithm for electron transport in electro-magnetic fields [5]. An Elekta 6MV FFF linear accelerator beam model served as input for a 30x30x20 cm 3 water phantom in which the chamber was placed in 10cm depth.…”

Section: Methodsmentioning

confidence: 99%

Optimal orientation for ionization chambers in MRgRT reference dosimetry

Pojtinger¹,

Dohm²,

Thorwarth³

2017

Current Directions in Biomedical Engineering

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Abstract:The interest in hybrid systems combining magnetic resonance imaging and medical linear accelerator (MR-Linac) is rapidly increasing due to the clinical availability of different systems. Reference dosimetry is a critical issue for integrating these devices into clinical practice. However, the response of ionization chambers changes according to the distinct orientation of the chamber with respect to the magnetic field. In this study, we have carried out Monte Carlo simulations to identify an optimal orientation for thimble type chambers in MRgRT reference dosimetry. Our findings suggest that an orientation where the chamber axis is parallel to the magnetic field axis should be preferred.

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Charged particle transport in magnetic fields in EGSnrc

Cited by 60 publications

References 46 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

Absolute dosimetry of a 1.5 T MR‐guided accelerator‐based high‐energy photon beam in water and solid phantoms using Aerrow

Optimal orientation for ionization chambers in MRgRT reference dosimetry

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