Experimental characterization of magnetically focused electron contamination at the surface of a high‐field inline MRI‐linac

Roberts, Natalia; Patterson, Elizabeth; Jelen, Urszula; Causer, Trent; Holloway, Lois; Liney, Gary; Lerch, Michael L. F; Rosenfeld, Anatoly B.; Cutajar, Dean L; Oborn, Bradley M; Metcalfe, Peter E

doi:10.1002/mp.13847

Cited by 17 publications

(15 citation statements)

References 48 publications

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“…Inside the MRI linac the secondary electrons produced in the surrounding material are affected by the Lorentz Force depending on the field orientation. 24,25 The isolation and quantification of density perturbation effects on orientation will also be investigated in relation to the impact of magnetic field in future work.…”

Section: F | Tpr 2010mentioning

confidence: 99%

“…Despite these limitations for dosimetry applications the microDiamond remains an area of interest for QA with its response having recently been characterized for use in an MRI linac 24 and for the MRI-associated surface dose from electron contamination. 25 One of the key advantages of the PTW microDiamond is its high spatial resolution when operating in edge-on orientation, 21,26,27 however, the IAEA TRS-483 CoP requirement of a face-on orientation for all QA measurements means that this micron scale spatial resolution is unrealized. The edge-on and face-on orientation are referred to by the terms perpendicular and parallel orientation in the CoP, respectively.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

Brace

Alhujaili

Paino

et al. 2020

J Applied Clin Med Phys

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Purpose: The PTW microDiamond has an enhanced spatial resolution when operated in an edge-on orientation but is not typically utilized in this orientation due to the specifications of the IAEA TRS-483 code of practice for small field dosimetry. In this work the suitability of an edge-on orientation and advantages over the recommended face-on orientation will be presented. Methods: The PTW microDiamond in both orientations was compared on a Varian TrueBeam linac for: machine output factor (OF), percentage depth dose (PDD), and beam profile measurements from 10 × 10 cm 2 to a 0.5 × 0.5 cm 2 field size for 6X and 6FFF beam energies in a water tank. A quantification of the stem effect was performed in edge-on orientation along with tissue to phantom ratio (TPR) measurements. An extensive angular dependence study for the two orientations was also undertaken within two custom PMMA plastic cylindrical phantoms. Results: The OF of the PTW microDiamond in both orientations agrees within 1% down to the 2 × 2 cm 2 field size. The edge-on orientation overresponds in the build-up region but provides improved penumbra and has a maximum observed stem effect of 1%. In the edge-on orientation there is an angular independent response with a maximum of 2% variation down to a 2 × 2 cm 2 field. The PTW microDiamond in edge-on orientation for TPR measurements agreed to the CC01 ionization chamber within 1% for all field sizes. Conclusions: The microDiamond was shown to be suitable for small field dosimetry when operated in edge-on orientation. When edge-on, a significantly reduced angular dependence is observed with no significant stem effect, making it a more versatile QA instrument for rotational delivery techniques.

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Section: F | Tpr 2010mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

Brace

Alhujaili

Paino

et al. 2020

J Applied Clin Med Phys

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“…Electron focusing effect, modeled (42) and observed experimentally (16,32), was quantified and methods to minimize it have been investigated. Surface dose enhancement around the central axis was observed, which was dependent on the field size and reached 400-600% for 10 × 10 cm 2 fields and more than 1,000% for largest fields, relative to the dose at 10 cm depth.…”

Section: Discussionmentioning

confidence: 99%

“…As their sensitivity to radiation dose decreases over large voltage ranges, the readout was corrected by taking a reference reading at the beginning and end of each set of measurements. The MOSkin TM detectors have been recently shown to agree with EBT3 films in 1 T inline magnetic field and were deemed suitable for relative dose measurements (32). For both, microDiamond and MOSkin TM measurements customized solid water blocks were used adapted to host the detectors and to enable measurements depth variation ( Figures 2D-F).…”

Section: Phantoms and Detectorsmentioning

confidence: 99%

“…Increased dose in the initial few centimeters around the beam central axis due to the contaminant electron focusing in the fringe field of the magnet has been previously both, modeled (11,42) and observed experimentally on an earlier (16) and the current prototype (32). To characterize this effect in detail for the current system and to explore possible methods to mitigate it (offaxis irradiation, use of bolus), microDiamond and MOSkin TM detectors were used.…”

Section: Depth Dose Characteristicsmentioning

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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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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Experimental characterization of magnetically focused electron contamination at the surface of a high‐field inline MRI‐linac

Cited by 17 publications

References 48 publications

Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

Evaluation of the PTW microDiamond in edge‐on orientation for dosimetry in small fields

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

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

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