Effect of longitudinal magnetic fields on a simulated in‐line 6 MV linac

Aubin, J St.; Santos, D. M.; Steciw, S; Fallone, B. G.

doi:10.1118/1.3481513

Cited by 30 publications

(42 citation statements)

References 16 publications

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“…In category 2, the linac and the MR (biplanar, open, or split-field) rotate in unison around the patient. 1 In this design, the linac and MR magnets are stationary with respect to each other 1,5,6 which eliminates any eddy currents induced in the linac as the gantry rotates, and allows for an unattenuated treatment beam. There are two possible linac-to-MR configurations in this design which allows for the B 0 field to be either perpendicular or parallel to the axis of the treatment beam.…”

Section: Introductionmentioning

confidence: 99%

“…In Ref. 6, it was shown that when a 0.011 T longitudinal magnetic field is present at the electron gun cathode, only a 16 6 1% target current loss 6 occurs. The lost target current is caused by magnetic deflection in the nonparallel component of the electron trajectories as the electron gun focuses the electron beam to the anode.…”

Section: Introductionmentioning

confidence: 99%

“…The lost target current is caused by magnetic deflection in the nonparallel component of the electron trajectories as the electron gun focuses the electron beam to the anode. 6 This paper will focus on the recovery of the lost target current that was caused by the magnetic deflections of the electrons in the electron gun when a 6 MV in-line linac is integrated with an MR imager in the parallel configuration. The focus of this work is founded on previous work that showed the electron gun is the most sensitive part of the linac to longitudinal magnetic fields and changes in its output current has a significant effect in the linac's target current loss.…”

Section: Introductionmentioning

confidence: 99%

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Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system

et al. 2012

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A ≥99% original target current is recovered with passive shield thicknesses >0.75 mm. An active shield consisting of two current rings of diameter of 110 mm with 625 and 430 A-turns fully recovers the loss that would have been caused by the magnetic fields. The minimal passive or active shielding requirements to essentially fully recover the current output of the linac in our parallel-configured linac-MR system have been determined and are easily achieved for practical implementation of the system.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system

et al. 2012

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“…Since MRI-linac radiotherapy requires that an electromagnetically sensitive electron accelerator functions within the magnetic fringe field of an MRI scanner, the emergence of MRI-linac radiotherapy has driven increased efforts in linear accelerator simulation amongst the medical physics research community. Operation of the electron gun in external magnetic fields is one of the most sensitive and studied aspects of integrated MRI-linac systems, [1][2][3][4][5] and is the focus of this paper. An electron gun is used to inject a steady stream of electrons at kilovoltage (kV) energies into an accelerating waveguide, where they are accelerated to megavoltage (MV) energies.…”

Section: Introductionmentioning

confidence: 99%

“…Further, no technical details on medical triode guns can be found in the literature. Therefore, the purpose of this work is two-fold; (1) to present an accurate model of a medical triode electron gun such that it may be utilized by future researchers and (2) to test the sensitivity of this gun in magnetic fields, in particular to assess whether different operating modes show different sensitivity to magnetic fields.…”

Section: Introductionmentioning

confidence: 99%

Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI‐linac therapy

et al. 2016

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Purpose: MRI-linac therapy is a rapidly growing field, and requires that conventional linear accelerators are operated with the fringe field of MRI magnets. One of the most sensitive accelerator components is the electron gun, which serves as the source of the beam. The purpose of this work was to develop a validated finite element model (FEM) model of a clinical triode (or gridded) electron gun, based on accurate geometric and electrical measurements, and to characterize the performance of this gun in magnetic fields. Methods: The geometry of a Varian electron gun was measured using 3D laser scanning and digital calipers. The electric potentials and emission current of these guns were measured directly from six dose matched true beam linacs for the 6X, 10X, and 15X modes of operation. Based on these measurements, a finite element model (FEM) of the gun was developed using the commercial software /. The performance of the FEM model in magnetic fields was characterized using parallel fields ranging from 0 to 200 G in the in-line direction, and 0-35 G in the perpendicular direction.Results: The FEM model matched the average measured emission current to within 5% across all three modes of operation. Different high voltage settings are used for the different modes; the 6X, 10X, and 15X modes have an average high voltage setting of 15, 10, and 11 kV. Due to these differences, different operating modes show different sensitivities in magnetic fields. For in line fields, the first current loss occurs at 40, 20, and 30 G for each mode. This is a much greater sensitivity than has previously been observed. For perpendicular fields, first beam loss occurred at 8, 5, and 5 G and total beam loss at 27, 22, and 20 G. Conclusions: A validated FEM model of a clinical triode electron gun has been developed based on accurate geometric and electrical measurements. Three different operating modes were simulated, with a maximum mean error of 5%. This gun shows greater sensitivity to in-line magnetic fields than previously presented models, and different operating modes show different sensitivity. C 2016 American Association of Physicists in Medicine. [http://dx

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Technical Note: Experimental characterization of the dose deposition in parallel MRI‐linacs at various magnetic field strengths

et al. 2019

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Purpose Dose deposition measurements for parallel MRI‐linacs have previously only shown comparisons between 0 T and a single available magnetic field. The Australian MRI‐Linac consists of a magnet coupled with a dual energy linear accelerator and a 120 leaf Multi‐Leaf Collimator with the radiation beam parallel to the magnetic field. Two different magnets, with field strengths of 1 and 1.5 T, were used during prototyping. This work aims to characterize the impact of the magnetic field at 1 and 1.5 T on dose deposition, possible by comparing dosimetry measured at both magnetic field strengths to measurements without the magnetic field. Methods Dose deposition measurements focused on a comparison of beam quality (TPR20/10), PDD, profiles at various depths, surface doses, and field size output factors. Measurements were acquired at 0, 1, and 1.5 T. Beam quality was measured using an ion chamber in solid water at isocenter with appropriate TPR20/10 buildup. PDDs and profiles were acquired via EBT3 film placed in solid water either parallel or perpendicular to the radiation beam. Films at surface were used to determine surface dose. Output factors were measured in solid water using an ion chamber at isocenter with 10 cm solid water buildup. Results Beam quality was within ±0.5% of the 0 T value for the 1 and 1.5 T magnetic field strengths. PDDs and profiles showed agreement for the three magnetic field strengths at depths beyond 20 mm. Deposited dose increased at shallower depths due to electron focusing. Output factors showed agreement within 1%. Conclusion Dose deposition at depth for a parallel MRI‐linac was not significantly impacted by either a 1 or 1.5 T magnetic field. PDDs and profiles at shallow depths and surface dose measurements showed significant differences between 0, 1, and 1.5 T due to electron focusing.

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Effect of longitudinal magnetic fields on a simulated in‐line 6 MV linac

Cited by 30 publications

References 16 publications

Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system

Magnetic shielding investigation for a 6 MV in-line linac within the parallel configuration of a linac-MR system

Performance of a clinical gridded electron gun in magnetic fields: Implications for MRI‐linac therapy

Technical Note: Experimental characterization of the dose deposition in parallel MRI‐linacs at various magnetic field strengths

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