Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Pojtinger, Stefan; Nachbar, Marcel; Kapsch, Ralf‐Peter; Thorwarth, Daniela

doi:10.1016/j.phro.2020.10.005

“…The consistency of TPR 20,10 between G0 and G90 lends credence to measuring these factors from G0. This is in agreement with previous Monte Carlo simulations [31] which suggested that a larger TPR 20,10 change of 0.679 (6 MV linac) to 0.703 ( 7MV MRL) did not in uence for the PTW 30013 Farmer by more than 0.5 %.…”

Section: Discussionsupporting

confidence: 93%

Commissioning Measurements On An Elekta Unity MR-Linac

Powers

¹

,

Baines

²

,

Crane

³

et al. 2021

Preprint

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MR-guided radiotherapy technology is relatively new and commissioning publications, QA protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptions of vendor supplied phantoms facilitated determination of gantry angle accuracy and MV isocentre, whereas in-house developed phantoms were used for End-to-End (E2E) testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during IMRT commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was < 0.6 %. Beam quality as determined by TPR20,10 was 0.704 ± 0.002, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0 % (2.0 %, 2.0 mm). G90 output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During IMRT commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0 % (2.0 %, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The IMRT QA devices and techniques used in this work highlight issues with machine behaviour when appropriate gamma criteria are set.

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“…Ideally, reference dosimetry should be performed directly at isocentre from G90 and should this be achievable at 5.0 cm depth, with future vendor-designed 1D tanks, the corresponding factors should be correctly determined [21] and applied. The consistency of TPR 20,10 between G0 and G90 supports measuring these factors from G0, in agreement with previous Monte Carlo simulations [18].…”

Section: Discussionsupporting

confidence: 90%

“…With beam quality dependent factors for reference output, a determination of this angular dependence, or at minimum an assessment of the change in these factors with beam quality, should be evaluated. Previous Monte Carlo simulations [ 18 ] have suggested that a TPR 20,10 change of 0.679 (6 MV linac) to 0.703 (7 MV MRL) did not influence beam quality dependent factors for the PTW 30013 Farmer by more than 0.5%; however, it was felt that confirmation with measurement was warranted.…”

Section: Methodsmentioning

confidence: 99%

Commissioning measurements on an Elekta Unity MR-Linac

Powers

¹

,

Baines

²

,

Crane

³

et al. 2022

View full text Add to dashboard Cite

Magnetic resonance-guided radiotherapy technology is relatively new and commissioning publications, quality assurance (QA) protocols and commercial products are limited. This work provides guidance for implementation measurements that may be performed on the Elekta Unity MR-Linac (Elekta, Stockholm, Sweden). Adaptations of vendor supplied phantoms facilitated determination of gantry angle accuracy and linac isocentre, whereas in-house developed phantoms were used for end-to-end testing and anterior coil attenuation measurements. Third-party devices were used for measuring beam quality, reference dosimetry and during treatment plan commissioning; however, due to several challenges, variations on standard techniques were required. Gantry angle accuracy was within 0.1°, confirmed with pixel intensity profiles, and MV isocentre diameter was < 0.5 mm. Anterior coil attenuation was approximately 0.6%. Beam quality as determined by TPR20,10 was 0.705 ± 0.001, in agreement with treatment planning system (TPS) calculations, and gamma comparison against the TPS for a 22.0 × 22.0 cm2 field was above 95.0% (2.0%, 2.0 mm). Machine output was 1.000 ± 0.002 Gy per 100 MU, depth 5.0 cm. During treatment plan commissioning, sub-standard results indicated issues with machine behaviour. Once rectified, gamma comparisons were above 95.0% (2.0%, 2.0 mm). Centres which may not have access to specialized equipment can use in-house developed phantoms, or adapt those supplied by the vendor, to perform commissioning work and confirm operation of the MRL within published tolerances. The plan QA techniques used in this work can highlight issues with machine behaviour when appropriate gamma criteria are set.

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“…The correction for the magnetic field, , has a vector component over the B field due to the correction factor having a dependence on the configuration of the photon beam, magnetic field and chamber as described in the " Configuration of chamber long axis, radiation beam central axis and primary magnetic field " section. The correction factor is also dependent on beam quality [ 62 , 70 ]. corrects the measured charge in the magnetic field, , which is set up in the configuration specified by the correction factor.…”

Section: Reference Dosimetrymentioning

confidence: 99%

“…The correction factors presented are for the beam qualities of the MRIdian and Unity respectively. A Monte Carlo study showed a change in of less than 0.5% between beam qualities of values of 0.6790 (6 MV Linac) and 0.7028 (7 MV MRI-linac) [ 70 ]. From this it can be concluded that the change in is small within the beam quality variability of the systems.…”

Section: Reference Dosimetrymentioning

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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Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

Cited by 4 publications

References 21 publications

Commissioning Measurements On An Elekta Unity MR-Linac

Commissioning Measurements On An Elekta Unity MR-Linac

Commissioning measurements on an Elekta Unity MR-Linac

ACPSEM position paper: dosimetry for magnetic resonance imaging linear accelerators

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