Spiraling contaminant electrons increase doses to surfaces outside the photon beam of an MRI-linac with a perpendicular magnetic field

Hackett, S.; Asselen, Bram van; Wolthaus, J.; Bluemink, J.J.; Ishakoglu, K; Kok, J G M; Lagendijk, J J W; Raaymakers, B W

doi:10.1088/1361-6560/aaba8f

Cited by 47 publications

(65 citation statements)

References 31 publications

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“…The number of SCEs is proportional to the photon fluence, and the effect is as such directly proportional to the field size. The proportionality is fairly linear, as observed by Hackett et al ,. in the relation between the maximum dose and the side of the square of the field size.…”

Section: Resultssupporting

confidence: 92%

“…These results match well to the previously measured values of about 2.9%, 2.5%, and 2.2% (± 0.2% uncertainty) for the same respective panel positions. Although the comparison at further panel positions is made with the measurements from Hackett et al ,. the reasonable agreement shown in the previous section between the MC and the updated experimental results could suggest that, at further panel positions, there is a reduced dependence on the details of the beam model.…”

Section: Resultsmentioning

confidence: 99%

“…The experiments of Hackett et al 17 were repeated for the 10 cm panel position. Experiments were performed on the clinical Elekta Unity MR-linac system at UMC-Utrecht which corresponds the beam model of the phase space files used in the MC simulations.…”

Section: B Experimental Setup For 10 CM From Isocentermentioning

confidence: 99%

“…Experiments were performed on the clinical Elekta Unity MR-linac system at UMC-Utrecht which corresponds the beam model of the phase space files used in the MC simulations. The experiments of Hackett et al 17 were made on a prototype MR-linac system. Measurements were made with EBT3 model GAFCHROMIC films (Ashland Inc, Bridgewater, NJ) from the same lot (#06131702).…”

Section: B Experimental Setup For 10 CM From Isocentermentioning

confidence: 99%

“…Recently, Hackett et al 17 explored the out-of-field dose contribution due to spiralling contaminant electrons (SCE) in the Unity MR-linac installed at the University Medical Center Utrecht. Electrons are generated in the air, 18,19 away from the linac head.…”

Section: Introductionmentioning

confidence: 99%

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Monte Carlo simulations of out‐of‐field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field

et al. 2019

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PurposeThe purpose of this study was to evaluate the potential skin dose toxicity contribution of spiralling contaminant electrons (SCE) generated in the air in an MR‐linac with a 0.35 or 1.5 T magnetic field using the EGSnrc Monte Carlo (MC) code. Comparisons to experimental results at 1.5 T are also performed.MethodsAn Elekta generated phase space file for the Unity MR‐linac is used in conjunction with the EGSnrc enhanced electric and magnetic field transport macros to simulate surface dose profiles and depth‐dose curves in panels located 5 cm away from the beam edge and positioned either parallel or perpendicular to the magnetic field. Electrons generated in the air will spiral along the magnetic field lines, and though surface doses within the field will be reduced, the electrons can contribute to out‐of‐field surface doses.ResultsSurface dose profiles showed good agreement with experimental findings and the maximum simulated doses at surfaces perpendicular to the magnetic field were 3.77 ± 0.01% and 3.55 ± 0.01% for 1.5 and 0.35 T. These results are expressed as a percentage of the maximum dose to water delivered by the photon beam. The surface dose variations in the out‐of‐field region converge to the 0 T doses within the first 0.5 cm of material. An asymmetry in the dose distribution in surfaces positioned on either side of the photon beam and aligned parallel to the magnetic field is determined to be due to the magnetic field directing electrons deeper into, or localizing them to the surface of, the measurement panel.ConclusionsThese results confirm the SCE dose contribution in surfaces perpendicular to the magnetic field and show these doses to be of the order of a few percentage of the maximum dose to water of the beam. Good agreement in the dose profiles is seen in comparisons between the MC simulations and experimental work. The effect is apparent in 0.35 and 1.5 T magnetic fields and dissipates within the first few millimeters of material. It should be noted that only SCEs from beam anteriorly incident on the patient will influence the patient surface dose, and the use of beams incident over different angles will reduce the dose to any particular patient surface.

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Section: Resultssupporting

confidence: 92%

Section: Resultsmentioning

confidence: 99%

Section: B Experimental Setup For 10 CM From Isocentermentioning

confidence: 99%

Section: B Experimental Setup For 10 CM From Isocentermentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Monte Carlo simulations of out‐of‐field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field

et al. 2019

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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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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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Spiraling contaminant electrons increase doses to surfaces outside the photon beam of an MRI-linac with a perpendicular magnetic field

Cited by 47 publications

References 31 publications

Monte Carlo simulations of out‐of‐field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field

Monte Carlo simulations of out‐of‐field skin dose due to spiralling contaminant electrons in a perpendicular magnetic field

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

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