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

O'Brien, Daniel; Sawakuchi, Gabriel O.

doi:10.1002/mp.12290

Cited by 50 publications

(68 citation statements)

References 12 publications

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“…Here the variation in k

_{Q}^{italicmag}

due to changes in the sensitive volume are looked at, and a resolution of the issue is sought through finding an optimal orientation with respect to the magnetic field. Air gaps surrounding the chamber have been shown to cause several percent change in the chamber response . These changes are notable in plastic phantoms, and the issue is negligible when using a water phantom for ion chamber measurements.…”

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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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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“…Here the variation in k

_{Q}^{italicmag}

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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“…Although a systematic difference appears to exist between the two phantoms, the combined uncertainty in this comparison is approximately 0.4% ( k = 1), thus no statistical significance can be ascribed. Elucidation of this expected small effect will require the MC simulation of air gaps surrounding Aerrow in various configurations (i.e., gap thickness and symmetry, B ‐field magnitude, and detector orientation with respect to the radiation and B ‐fields), in‐line with the chamber‐focused study by O'Brien and Sawakuchi …”

Section: Discussionmentioning

confidence: 91%

“…34 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. 7,[16][17][18]65,66 Most recently, O'Brien and Sawakuchi 18 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. They concluded that the effect was not attributable to the electron return effect, but rather to the loss of electron contributions originating within the air gap, which is not entirely compensated for by more distant electrons.…”

Section: Discussionmentioning

confidence: 99%

“…Elucidation of this expected small effect will require the MC simulation of air gaps surrounding Aerrow in various configurations (i.e., gap thickness and symmetry, B-field magnitude, and detector orientation with respect to the radiation and B-fields), in-line with the chamber-focused study by O'Brien and Sawakuchi. 18…”

Section: Discussionmentioning

confidence: 99%

“…7,[12][13][14][15] Aside from complicating clinical reference dosimetry measurements, there exists an outstanding issue surrounding the use of ICs in solid phantoms: The interaction between the magnetic field and secondary charged particles in the clearance gaps surrounding the chamber that can result in an unpredictable rotational dependence (e.g., rotating the detector about its major axis) on the order of several percent. [16][17][18][19] Clinically, this is of considerable importance, as the use of ICs in solid phantoms is ubiquitous in practice for both machine and patient-specific quality assurance, where time efficiency is as critical as measurement precision.…”

Section: Introductionmentioning

confidence: 99%

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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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MOSFET dosimeter characterization in MR‐guided radiation therapy (MRgRT) Linac

Yadav

Hallil²,

Tewatia

et al. 2019

J Applied Clin Med Phys

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Purpose: With the increasing use of MR-guided radiation therapy (MRgRT), it becomes important to understand and explore accuracy of medical dosimeters in the presence of magnetic field. The purpose of this work is to characterize metal-oxide-semiconductor field-effect transistors (MOSFETs) in MRgRT systems at 0.345 T magnetic field strength.Methods: A MOSFET dosimetry system, developed by Best Medical Canada for invivo patient dosimetry, was used to study various commissioning tests performed on a MRgRT system, MRIdian ® Linac. We characterized the MOSFET dosimeter with different cable lengths by determining its calibration factor, monitor unit linearity, angular dependence, field size dependence, percentage depth dose (PDD) variation, output factor change, and intensity modulated radiation therapy quality assurance (IMRT QA) verification for several plans. MOSFET results were analyzed and compared with commissioning data and Monte Carlo calculations.Results: MOSFET measurements were not found to be affected by the presence of 0.345 T magnetic field. Calibration factors were similar for different cable length dosimeters either placed at the parallel or perpendicular direction to the magnetic field, with variations of less than 2%. The detector showed good linearity (R 2 = 0.999) for 100-600 MUs range. Output factor measurements were consistent with ionization chamber data within 2.2%. MOSFET PDD measurements were found to be within 1% for 1-15 cm depth range in comparison to ionization chamber.MOSFET normalized angular response matched thermoluminescent detector (TLD) response within 5.5%. The IMRT QA verification data for the MRgRT linac showed that the percentage difference between ionization chamber and MOSFET was 0.91%, 2.05%, and 2.63%, respectively for liver, spine, and mediastinum. Conclusion: MOSFET dosimeters are not affected by the 0.345 T magnetic field in MRgRT system. They showed physics parameters and performance comparable to TLD and ionization chamber; thus, they constitute an alternative to TLD for realtime in-vivo dosimetry in MRgRT procedures. Magnetic resonance image-guided radiation therapy (MRgRT) has gained impetus recently. ViewRay ® (ViewRay Inc., Oakwood, OH) is one of the first MRgRT treatment modalities with an onboard MR scanner attached to a teletherapy 60 Co beam. 1 The MRIdian ® Linac system which combines MR imaging and a 6 MV beam is the latest system developed by ViewRay. MRgRT offers many advantages including real-time imaging, accuracy in treatment delivery, higher soft tissue contrast, and no additional imaging dose. 2 Though there are significant advantages of these hybrid treatment systems with onboard MR scanners, these present a unique challenge to dosimetric measurements with some of the conventional radiation detectors, due to possible magnetic field susceptibility. Several dosimeters are used for quality assurance in radiotherapy; these include ionization chambers (IC), thermoluminescent dosimeters (TLD), film, and active dosimeters such as diodes and metal-oxide-semic...

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

Cited by 50 publications

References 12 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

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

MOSFET dosimeter characterization in MR‐guided radiation therapy (MRgRT) Linac

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