Beam characterisation of the 1.5 T MRI-linac

Woodings, S.; Bluemink, J.J.; Vries, J.H.W. De; Niatsetski, Yury; Veelen, Bob van; Schillings, J.; Kok, J G M; Wolthaus, J.; Hackett, S.; Asselen, Bram van; Zijp, H M van; Pencea, S; Roberts, David A.; Lagendijk, J J W; Raaymakers, B W

doi:10.1088/1361-6560/aab566

Cited by 63 publications

(103 citation statements)

References 27 publications

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“…Chen et al investigated the dosimetric effect of ERE at air-tissue and lung-tissue interfaces during intensity modulated radiation therapy (IMRT) [5]. Woodings et al conducted a comprehensive dosimetric characterization of the Elekta 1.5T MR-Linac beam for clinical application [6]. To address the unintended skin dose increase by ERE, Oborn et al [7] suggested the use of 1 cm-thick bolus significantly mitigates the exit dose increase.…”

Section: Introductionmentioning

confidence: 99%

Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System

Han¹,

Wen²,

Lee³

et al. 2018

IJMPCERO

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Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective radiation treatments compared to conventional IGRT by using real-time on-board MR imaging. Under the influence of a magnetic field, however, secondary electrons exiting a surface can be forced in a circular path and re-enter the medium, resulting in dose increase at a beam-exit surface, called the electron return effect (ERE). The purpose of the study is to compare the exit skin dose computed by Monte Carlo dose calculation with measurements using an adult anthropomorphic phantom and to measure the effect of skin dose reduction by adding 1 cm-thick bolus. Method: The plan was compared with measurements using an adult anthropomorphic phantom combined with radiochromic films and thermoluminescent dosimeters. We also measured the skin dose reduction by adding 1 cm-thick bolus on the frontal surface of the phantom. Results: We found that 1 cm-thick bolus reduced the skin dose by up to 20% both in measurements and calculations. The plan was found to overestimate the measured skin dose by about 10% and there was no significant difference in the bolus effect between the breast skin and the skin (without breast attachment) doses. Conclusion: In conclusion, we confirmed the ERE effect on the anthropomorphic phantom under the magnetic field and the exit skin dose reduction by adding a bolus. Skin dose measurements using anthropomorphic phantom may be helpful to evaluate more realistic skin dose and the bo-How to cite this paper:

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Section: Introductionmentioning

confidence: 99%

Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System

Han¹,

Wen²,

Lee³

et al. 2018

IJMPCERO

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“…In this work, as an approximate estimate the value of d max measured off-axis was reported and amounted to 1.47 cm at SID of 2.4 m and 1.45 cm at SID of 1.8 m for ∼10 × 10 cm 2 fields. For comparison, a reduced d max of 1.3 cm was reported for a 7 MV FFF beam of a commercial 1.5 T transverse system (28).…”

Section: Discussionmentioning

confidence: 99%

“…Penumbra values could be measured with films, offering high spatial resolution, only for a subset of fields required by IEC standard. For an ∼10 × 10 cm 2 field the penumbra was 1.35 cm in the leaf motion direction (x) and 0.86 cm in the direction perpendicular to the leaf motion (y) at SID of 2.4 m and 1.02 cm in the leaf motion direction and 0.74 cm in the direction perpendicular to the leaf motion at SID of 1.8 m. For comparison, penumbra values measured for the same field size on a commercial transverse 1.5 T system, equipped with an Elekta Agility MLC, with an SID of 1.4 m determined using artificial flattening were between 0.74 and 0.87 cm (28). No penumbra asymmetry and profile shift, characteristic for transverse systems (28), has been observed, which simplifies beam modeling in the TPS for inline systems.…”

Section: Discussionmentioning

confidence: 99%

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Dosimetric Optimization and Commissioning of a High Field Inline MRI-Linac

et al. 2020

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Unique characteristics of MRI-linac systems and mutual interactions between their components pose specific challenges for their commissioning and quality assurance. The Australian MRI-linac is a prototype system which explores the inline orientation, with radiation beam parallel to the main magnetic field. The aim of this work was to commission the radiation-related aspects of this system for its application in clinical treatments. Methods: Physical alignment of the radiation beam to the magnetic field was fine-tuned and magnetic shielding of the radiation head was designed to achieve optimal beam characteristics. These steps were guided by investigative measurements of the beam properties. Subsequently, machine performance was benchmarked against the requirements of the IEC60976/77 standards. Finally, the geometric and dosimetric data was acquired, following the AAPM Task Group 106 recommendations, to characterize the beam for modeling in the treatment planning system and with Monte Carlo simulations. The magnetic field effects on the dose deposition and on the detector response have been taken into account and issues specific to the inline design have been highlighted. Results: Alignment of the radiation beam axis and the imaging isocentre within 2 mm tolerance was obtained. The system was commissioned at two source-to-isocentre distances (SIDs): 2.4 and 1.8 m. Reproducibility and proportionality of the dose monitoring system met IEC criteria at the larger SID but slightly exceeded it at the shorter SID. Profile symmetry remained under 103% for the fields up to ∼34 × 34 and 21 × 21 cm 2 at the larger and shorter SID, respectively. No penumbra asymmetry, characteristic for transverse systems, was observed. The electron focusing effect, which results in high entrance doses on central axis, was quantified and methods to minimize it have been investigated. Conclusion: Methods were developed and employed to investigate and quantify the dosimetric properties of an inline MRI-Linac system. The Australian MRI-linac system has been fine-tuned in terms of beam properties and commissioned, constituting a key step toward the application of inline MRI-linacs for patient treatments.

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“…VMAT CBCT plans were based on a 10MV full-arc and IMRT MRI plans on seven 7MV flattening filter free (FFF) co-planar beams (30,75,135,181,225, 285, 330 degrees) applied to the same isocenter. For more details regarding the beam characterization of the MRL we refer to [31]. For these plans, optimization was done with a 1.5T magnetic field turned-on to realistically simulate an MRL treatment [19].…”

Section: Treatment Planningmentioning

confidence: 99%

Potential benefit of MRI-guided IMRT for flank irradiation in pediatric patients with Wilms’ tumor

et al. 2018

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Purpose/Objective: Flank irradiation for Wilms' tumor (WT) is currently performed at our institute using a cone-beam computed tomography-guided volumetric modulated arc (VMAT CBCT) workflow. By adding real-time magnetic resonance imaging (MRI) guidance to the treatment, safety margins could be reduced. The study purpose was to quantify the potential reduction of the planning target volume (PTV) margin and its dosimetric impact when using an MRI-guided intensity modulated radiation therapy (IMRT MRI) workflow compared to the VMAT CBCT workflow. Material/Methods: 4D-CT, MRI and CBCT scans acquired during preparation and treatment of 15 patients, were used to estimate both geometric, motion and patient setup systematic (P) and random (r) errors for VMAT CBCT and IMRT MRI workflows. The mean PTV (PTV mean) expansion was calculated using the van Herk formula. Treatment plans were generated using five margin scenarios (PTV mean ± 0, 1 and 2 mm). Furthermore, the IMRT MRI plans were optimized with a 1.5T transverse magnetic field turned-on to realistically model an MRI-guided treatment. Plans were evaluated using dose-volume statistics (p<.01, Wilcoxon). Results: Analysis of P and r errors resulted in a PTV mean of 5 mm for the VMAT CBCT and 3 mm for the IMRT MRI workflows in each orthogonal direction. Target coverage was unaffected by the margin decrease with a mean V 95% ¼100% for all margin scenarios. For the PTV mean , an average reduction of the mean dose to the organs at risk (OARs) was achieved with IMRT MRI compared to VMAT CBCT : 3.4 ± 2.4% (p<.01) for the kidney, 3.4 ± 2.1% (p<.01) for the liver, 2.8 ± 3.0% (p<.01) for the spleen and 4.9 ± 3.8% (p<.01) for the pancreas, respectively. Conclusions: Imaging data in children with WT demonstrated that the PTV margin could be reduced isotropically down to 2 mm when using the IMRT MRI compared to the VMAT CBCT workflow. The former results in a dose reduction to the OARs while maintaining target coverage.

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Beam characterisation of the 1.5 T MRI-linac

Cited by 63 publications

References 27 publications

Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System

Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System

Dosimetric Optimization and Commissioning of a High Field Inline MRI-Linac

Potential benefit of MRI-guided IMRT for flank irradiation in pediatric patients with Wilms’ tumor

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