The dose response of PTW microDiamond and microSilicon in transverse magnetic field under small field conditions

Blum, Isabel; Tekin, Tuba; Delfs, Björn; Schönfeld, Ann-Britt; Kapsch, Ralf‐Peter; Poppe, Björn; Looe, Hui Khee

doi:10.1088/1361-6560/ac0f2e

“…When the output correction factors k were applied, the spread of the corrected values was considerably reduced for the smallest field sizes at 5 cm and 10 cm depth. Moreover, at 0.35 T, Blum et al [ 14 ] found small field output correction factors close to the free magnetic field values for the PTW 60019 and 60023. That’s why, even though the output correction factors were published for measurements without magnetic field at 10 cm depth, they seem to be applicable at low magnetic field at 5 cm depth pending the publication of new MR specific values.…”

Section: Discussionmentioning

confidence: 70%

“…Generally, diodes are not recommended for output factors (OF) measurements in the presence of a magnetic field [ 13 ]. The diode non-water equivalent density components have been identified as the major contributors to their behaviour in magnetic field and a strong dependence of the small field output correction factors with the magnetic field strength has been found [ 14 ]. Woodings et al compared the PTW 60019 microdiamond detector to a small ionization chamber and they considered it suitable for small field measurements and MR-linac commissioning.…”

Section: Introductionmentioning

confidence: 99%

MR compatible detectors assessment for a 0.35 T MR-linac commissioning

Chea,

Croisé,

Huet

et al. 2024

Radiat Oncol

0

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Purpose To assess a large panel of MR compatible detectors on the full range of measurements required for a 0.35 T MR-linac commissioning by using a specific statistical method represented as a continuum of comparison with the Monte Carlo (MC) TPS calculations. This study also describes the commissioning tests and the secondary MC dose calculation validation. Material and methods Plans were created on the Viewray TPS to generate MC reference data. Absolute dose points, PDD, profiles and output factors were extracted and compared to measurements performed with ten different detectors: PTW 31010, 31021, 31022, Markus 34045 and Exradin A28 MR ionization chambers, SN Edge shielded diode, PTW 60019 microdiamond, PTW 60023 unshielded diode, EBT3 radiochromic films and LiF µcubes. Three commissioning steps consisted in comparison between calculated and measured dose: the beam model validation, the output calibration verification in four different phantoms and the commissioning tests recommended by the IAEA-TECDOC-1583. Main results The symmetry for the high resolution detectors was higher than the TPS data of about 1%. The angular responses of the PTW 60023 and the SN Edge were − 6.6 and − 11.9% compared to the PTW 31010 at 60°. The X/Y-left and the Y-right penumbras measured by the high resolution detectors were in good agreement with the TPS values except for the PTW 60023 for large field sizes. For the 0.84 × 0.83 cm2 field size, the mean deviation to the TPS of the uncorrected OF was − 1.7 ± 1.6% against − 4.0 ± 0.6% for the corrected OF whereas we found − 4.8 ± 0.8% for passive dosimeters. The mean absolute dose deviations to the TPS in different phantoms were 0 ± 0.4%, − 1.2 ± 0.6% and 0.5 ± 1.1% for the PTW 31010, PTW 31021 and Exradin A28 MR respectively. Conclusions The magnetic field effects on the measurements are considerably reduced at low magnetic field. The PTW 31010 ionization chamber can be used with confidence in different phantoms for commissioning and QA tests requiring absolute dose verifications. For relative measurements, the PTW 60019 presented the best agreement for the full range of field size. For the profile assessment, shielded diodes had a behaviour similar to the PTW 60019 and 60023 while the ionization chambers were the most suitable detectors for the symmetry. The output correction factors published by the IAEA TRS 483 seem to be applicable at low magnetic field pending the publication of new MR specific values.

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“…Alternative dosimeters to standard reference ion chambers such as smaller volume chambers [ 72 ], microDiamond and microSilicon type detectors [ 73 ] have also been evaluated. Similar to conventional linacs, the output factors for small fields and the response of detectors under non-equilibrium conditions is an emerging area of research [ 74 ].…”

Section: Future Researchmentioning

confidence: 99%

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Begg

¹

,

Jelen

²

,

Moutrie

³

et al. 2023

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Consistency and clear guidelines on dosimetry are essential for accurate and precise dosimetry, to ensure the best patient outcomes and to allow direct dose comparison across different centres. Magnetic Resonance Imaging Linac (MRI-linac) systems have recently been introduced to Australasian clinics. This report provides recommendations on reference dosimetry measurements for MRI-linacs on behalf of the Australiasian College of Physical Scientists and Engineers in Medicine (ACPSEM) MRI-linac working group. There are two configurations considered for MRI-linacs, perpendicular and parallel, referring to the relative direction of the magnetic field and radiation beam, with different impacts on dose deposition in a medium. These recommendations focus on ion chambers which are most commonly used in the clinic for reference dosimetry. Water phantoms must be MR safe or conditional and practical limitations on phantom set-up must be considered. Solid phantoms are not advised for reference dosimetry. For reference dosimetry, IAEA TRS-398 recommendations cannot be followed completely due to physical differences between conventional linac and MRI-linac systems. Manufacturers’ advice on reference conditions should be followed. Beam quality specification of TPR20,10 is recommended. The configuration of the central axis of the ion chamber relative to the magnetic field and radiation beam impacts the chamber response and must be considered carefully. Recommended corrections to delivered dose are $${k}_{{Q}_{msr}{Q}_{0}}^{{f}_{msr}{f}_{ref}}$$ k Q msr Q 0 f msr f ref , a correction for beam quality and $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr , for the impact of the magnetic field on dosimeter response in the magnetic field. Literature based values for $${k}_{\overrightarrow{B},{Q}_{msr}}^{{f}_{msr}}$$ k B → , Q msr f msr are given. It is important to note that this is a developing field and these recommendations should be used together with a review of current literature.

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“…Subsequent measurements were then performed with B=0.35 and 1.4 T without changing the collimator setting in between. More details on the measurement setup can be found in Blum et al (2021). 31021 chamber (middle row) and the microSilicon (lower row).…”

Section: Test Datasetsmentioning

confidence: 99%

Model-based machine learning for the recovery of lateral dose profiles of small photon fields in magnetic field

Looe

¹

,

Blum

²

,

Schönfeld

³

et al. 2022

Self Cite

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Objective:To investigate the feasibility to train artificial neural networks (NN) to recover lateral dose profiles from detector measurements in a magnetic field. Approach:A novel framework based on a mathematical convolution model has been proposed to generate measurement-less training dataset. 2D dose deposition kernels and detector lateral fluence response functions of two air-filled ionization chambers and two diode-type detectors have been simulated without magnetic field and for magnetic field B = 0.35 and 1.5 T. Using these convolution kernels, training dataset consisting of pairs of dose profiles D(x,y) and signal profiles M(x,y) were computed for a total of 108 2D photon fluence profiles ψ(x,y) (80% training / 20% validation). The NN were tested using three independent datasets, where the second test dataset has been obtained from simulations using realistic phase space files of clinical linear accelerator and the third test dataset was measured at a conventional linac equipped with electromagnets. Main results:The convolution kernels show magnetic field dependence due to the influence of the Lorentz force on the electron transport in the water phantom and detectors. The NN show good performance during training and validation with mean square error (MSE) reaching a value of 1e-6 or better. The corresponding correlation coefficients R reached the value of 1 for all models indicating an excellent agreement between expected D(x,y) and predicted D_pred (x,y). The comparisons between D(x,y) and D_pred (x,y) using the three test datasets resulted in gamma indices (1mm/1% global) < 1 for all evaluated data points. Significance:Two verification approaches have been proposed to warrant the mathematical consistencies of the NN outputs. Besides offering a correction strategy not existed so far for relative dosimetry in a magnetic field, this work could help to raise awareness and to improve understanding on the distortion of detector’s signal profiles by a magnetic field.

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The dose response of PTW microDiamond and microSilicon in transverse magnetic field under small field conditions

Cited by 6 publications

References 38 publications

MR compatible detectors assessment for a 0.35 T MR-linac commissioning

MR compatible detectors assessment for a 0.35 T MR-linac commissioning

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

Model-based machine learning for the recovery of lateral dose profiles of small photon fields in magnetic field

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