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

Jelen, Urszula; Dong, Bin; Begg, Jarrad; Roberts, Natalia; Whelan, Brendan; Keall, Paul J.; Liney, Gary

doi:10.3389/fonc.2020.00136

Cited by 13 publications

(20 citation statements)

References 46 publications

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“…The shorter SSD of 1.8 m was chosen for this study as the relative surface dose compared to the extended SSD is larger and acts as a “worst case” scenario of electron contamination. 17 …”

Section: Methodsmentioning

confidence: 99%

“…The shorter SSD of 1.8 m was chosen for this study as the relative surface dose compared to the extended SSD is larger and acts as a "worst case" scenario of electron contamination. 17 Experimental depth dose measurements were acquired with EBT3 Gafchromic film (Ashland Inc, USA), a PTW microDiamond type 60019 detector, and MOSkin TM . Gafchromic EBT3 film was included in this study due to its high spatial resolution, making it useful for two-dimensional dose distribution analysis.…”

Section: Methodsmentioning

confidence: 99%

“…One approach to reduce high entrance dose was experimentally investigated and involved a 2‐cm‐thick acrylic beam spoiler/bolus placed 5 cm upstream from the surface of the phantom. 17 This method was used to treat live rats that were treated for brain tumors within the Australian MRI‐Linac. 21 Although this approach reduces surface dose, it also reintroduces electrons from the spoiler itself, leading to a higher surface dose well exceeding 100%, compared to normal x‐ray beam treatments that typically benefit from the skin sparing effect.…”

Section: Introductionmentioning

confidence: 99%

“…However, a high field inline configuration does give rise to a high surface dose due to electron focusing along the central magnetic axis that can exceed the dose at d max leading to a loss of skin sparing. [15][16][17] This becomes less pronounced for systems that have a lower magnetic field strength, such as the 0.5 T Aurora RT system (MagnetTx Oncology Solutions, Canada). 18,19 The interest in surface dose is motivated by skin reactions that arise immediately after radiotherapy and can continue for months and years.…”

Section: Introductionmentioning

confidence: 99%

“…With a magnetic field parallel or inline to the beam direction, these aforementioned dosimetric perturbations are reduced: no lateral dose shift, a reduction in penumbral width, and no ERE. However, a high field inline configuration does give rise to a high surface dose due to electron focusing along the central magnetic axis that can exceed the dose at d

_{\text{max}}

leading to a loss of skin sparing 15–17 . This becomes less pronounced for systems that have a lower magnetic field strength, such as the 0.5 T Aurora RT system (MagnetTx Oncology Solutions, Canada) 18,19 …”

Section: Introductionmentioning

confidence: 99%

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Characterizing magnetically focused contamination electrons by off‐axis irradiation on an inline MRI‐Linac

Patterson¹,

Oborn

Cutajar³

et al. 2022

J Applied Clin Med Phys

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The aim of this study is to investigate off -axis irradiation on the Australian MRI-Linac using experiments and Monte Carlo simulations. Simulations are used to verify experimental measurements and to determine the minimum offset distance required to separate electron contamination from the photon field. Methods: Dosimetric measurements were performed using a microDiamond detector, Gafchromic ® EBT3 film, and MOSkin TM . Three field sizes were investigated including 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm 2 . Each field was offset a maximum distance, approximately 10 cm, from the central magnetic axis (isocenter). Percentage depth doses (PDDs) were collected at a source-tosurface distance (SSD) of 1.8 m for fields collimated centrally and off -axis. PDD measurements were also acquired at isocenter for each off -axis field to measure electron contamination. Monte Carlo simulations were used to verify experimental measurements, determine the minimum field offset distance, and demonstrate the use of a spoiler to absorb electron contamination. Results: Off -axis irradiation separates the majority of electron contamination from an x-ray beam and was found to significantly reduce in-field surface dose. For the 1.9 × 1.9, 5.8 × 5.8, and 9.7 × 9.6 cm 2 field, surface dose was reduced from 120.9% to 24.9%,229.7% to 39.2%,and 355.3% to 47.3%,respectively.Monte Carlo simulations generally were within experimental error to MOSkin TM and microDiamond, and used to determine the minimum offset distance, 2.1 cm, from the field edge to isocenter. A water spoiler 2 cm thick was shown to reduce electron contamination dose to near zero. Conclusions: Experimental and simulation data were acquired for a range of field sizes to investigate off -axis irradiation on an inline MRI-Linac. The skin sparing effect was observed with off -axis irradiation, a feature that cannot be achieved to the same extent with other methods, such as bolusing, for beams at isocenter.

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Section: Methodsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

_{\text{max}}

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Characterizing magnetically focused contamination electrons by off‐axis irradiation on an inline MRI‐Linac

Patterson¹,

Oborn

Cutajar³

et al. 2022

J Applied Clin Med Phys

Self Cite

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Electron streaming dose measurements and calculations on a 1.5 T MR‐Linac

Patterson,

Powers,

Metcalfe

et al. 2024

J Applied Clin Med Phys

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PurposeTo evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)‐linac.MethodSkin dose due to the ESE on an MR‐linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x‐y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22 cm2 fields, gantry at 90°. Out‐of‐field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed ±20.0 cm from the isocenter along the x‐direction. A high‐resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07 mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1 cm dose grid with 0.2% statistical uncertainty (0.1 cm, 0.2%) and a 0.2 cm dose grid with 3.0% statistical uncertainty (0.2 cm, 3.0%).ResultsOSLD, film, the 0.1 cm, 0.2%, and 0.2 cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as –75.3%, –7.0%, –24.7%, and –41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements.ConclusionsThese results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

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Magnetic field induced dose effects in radiation therapy using MR‐linacs

et al. 2023

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MR-guided radiotherapy (MRgRT) is one of the most significant advances in radiotherapy in recent years. The hybrid systems were designed to visualize patient anatomical and physiological changes during the course of radiotherapy, enabling more precise treatment. However, before MR-linacs reach their full potential in delivering safe and accurate treatments to patients, the radiotherapy team must understand how a magnetic field alters the dosimetric properties of the radiation beam and its potential impact on treatment quality and clinical outcomes. This review aims to provide an in-depth description of the magnetic field induced dose effects for the two widely available systems, the 0.35 T and the 1.5 T MR-linacs. In MR-linac treatments, the primary photon beam passes through MR components that never exist in conventional linacs, which alter both in-field and out-of -field doses. More importantly, the interplay between the always-on magnetic field and the secondary electrons is not negligible. This interplay affects dose deposition in the patient, resulting in reduced in-field skin dose due to purged-out contaminant electrons, shortened build-up distance and a shifted crossline profile owing to asymmetric dose kernel. Especially two effects, namely, electron return effect (ERE) and electron stream effect (ESE), are not seen in conventional radiotherapy. This review also summarizes the clinical observations on the site-specific treatments influenced mostly by the magnetic field. In MR-linac treatment, the head and neck region is one of the most challenging sites as ERE occurs at low and high density tissue interfaces and around air cavities, generating hot and cold spots. In breast cancer treatment, consideration should be given to the increased in-field skin dose induced by ERE and the increased out-of -field dose caused by ESE for regions such as the ears, chin, and neck. In lung cancer treatments, tissue inhomogeneity combined with ERE will exacerbate target dose heterogeneity and increase or decrease interface dose. Lastly, treatment in the abdomen and pelvic region will be affected by the presence of gas pockets near the target. The review provides practical recommendations to mitigate these effects.

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

Cited by 13 publications

References 46 publications

Characterizing magnetically focused contamination electrons by off‐axis irradiation on an inline MRI‐Linac

Characterizing magnetically focused contamination electrons by off‐axis irradiation on an inline MRI‐Linac

Electron streaming dose measurements and calculations on a 1.5 T MR‐Linac

Magnetic field induced dose effects in radiation therapy using MR‐linacs

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