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

Whelan, Brendan; Holloway, Lois; Constantin, Dragoş; Oborn, Bradley M; Bazalova‐Carter, Magdalena; Fahrig, Rebecca; Keall, Paul J.

doi:10.1118/1.4963216

Cited by 10 publications

(10 citation statements)

References 25 publications

(42 reference statements)

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“…The diode electron gun consists of a cathode, focusing electrode, and anode, and the triode gun has a grid electrode in addition to the cathode, electrode, and anode; thus, the amount of current emitted from the electron gun to the accelerating tube can be easily adjusted by controlling the pulse width and pulse repetition frequency (PRF) of the grid voltage. In addition, control of the high voltage is unnecessary; thus, reliable and accurate control is possible 20 . Because the dose rate of the linac is correlated with the beam energy and beam current, to precisely control the beam current produced by the linac during the radiotherapy process, a triode electron gun was chosen.…”

Section: Methodsmentioning

confidence: 99%

Medical X‐band linear accelerator for high‐precision radiotherapy

et al. 2021

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Purpose Recently, high‐precision radiotherapy systems have been developed by integrating computerized tomography or magnetic resonance imaging to enhance the precision of radiotherapy. For integration with additional imaging systems in a limited space, miniaturization and weight reduction of the linear accelerator (linac) system have become important. The aim of this work is to develop a compact medical linac based on 9.3 GHz X‐band RF technology instead of the S‐band RF technology typically used in the radiotherapy field. Methods The accelerating tube was designed by 3D finite‐difference time‐domain and particle‐in‐cell simulations because the frequency variation resulting from the structural parameters and processing errors is relatively sensitive to the operating performance of the X‐band linac. Through the 3D simulation of the electric field distribution and beam dynamics process, we designed an accelerating tube to efficiently accelerate the electron beam and used a magnetron as the RF source to miniaturize the entire linac. In addition, a side‐coupled structure was adopted to design a compact linac to reduce the RF power loss. To verify the performance of the linac, we developed a beam diagnostic system to analyze the electron beam characteristics and a quality assurance (QA) experimental environment including 3D lateral water phantoms to analyze the primary performance parameters (energy, dose rate, flatness, symmetry, and penumbra) The QA process was based on the standard protocols AAPM TG‐51, 106, 142 and IAEA TRS‐398. Results The X‐band linac has high shunt impedance and electric field strength. Therefore, even though the length of the accelerating tube is 37 cm, the linac could accelerate an electron beam to more than 6 MeV and produce a beam current of more than 90 mA. The transmission ratio is measured to be approximately 30% ~ 40% when the electron gun operates in the constant emission region. The percent depth dose ratio at the measured depths of 10 and 20 cm was approximately 0.572, so we verified that the photon beam energy was matched to approximately 6 MV. The maximum dose rate was measured as 820 cGy/min when the source‐to‐skin distance was 80 cm. The symmetry was smaller than the QA standard and the flatness had a higher than standard value due to the flattening filter‐free beam characteristics. In the case of the penumbra, it was not sufficiently steep compared to commercial equipment, but it could be compensated by improving additional devices such as multileaf collimator and jaw. Conclusions A 9.3 GHz X‐band medical linac was developed for high‐precision radiotherapy. Since a more precise design and machining process are required for X‐band RF technology, this linac was developed by performing a 3D simulation and ultraprecision machining. The X‐band linac has a short length and a compact volume, but it can generate a validated therapeutic beam. Therefore, it has more flexibility to be coupled with imaging systems such as CT or MRI and can reduce the bore size of the gantry. In add...

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Section: Methodsmentioning

confidence: 99%

Medical X‐band linear accelerator for high‐precision radiotherapy

et al. 2021

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“…The fringe field affects the radiation beam generation and transport in the linac (8,9,39). In particular, (1) it may deflect the electrons produced by the electron gun and reducing the stream of electrons injected to the waveguide and hence the beam output and (2) it may shift the incidence of the electron beam on the target and lead to the deformation of the resulting photon beam profiles and output loss as the beam passes through the primary collimator.…”

Section: Magnetic Shielding Optimizationmentioning

confidence: 99%

“…Furthermore, the hybrid nature of the MRI-linac treatment units requires the assessment of their concurrent functionalities, for instance dose deposition during imaging, congruence of the imaging and radiation isocentres, RF interference or gantry movement effect on the magnetic field homogeneity (7). And finally, the presence of the magnetic field also affects the radiation beam generation (8,9) and the dose deposition (10,11).…”

Section: Introductionmentioning

confidence: 99%

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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“…1(b) contains an additional DC acceleration section. It enables pre-acceleration of the electron beam and focusing of the emitted electrons to achieve a smaller radius and divergence since an optimal focusing electrode can be used [16]. Another advantage is the design of the cathode and the acceleration cells can be decoupled.…”

Section: Configurations Of the Rf Gunmentioning

confidence: 99%

Electron Injector Based on Thermionic RF-Modulated Electron Gun for Particle Accelerator Applications

Zhang

Adam

Militsyn

et al. 2020

IEEE Trans. Electron Devices

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In this paper, the design and simulation of an electron injector based on a thermionic RF modulated electron gun for particle accelerator applications is presented. The electron gun is based on a gridded thermionic cathode with the geometry based on a Pierce-type configuration. Both theory and numerical simulation were used to explore the relationship between the bunch length and charge. The reasons for the pulse widening were also analyzed. The beam dynamics simulations showed that a minimum pulse length of 106 ps could be achieved with a bunch charge of 33 pC when the driving RF frequency was 1.5 GHz. The average transverse emittance was about 17 mm⸱mrad from the particle-in-cell simulations. Operating at a higher RF frequency did not significantly reduce the micro-pulse length.

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

Cited by 10 publications

References 25 publications

Medical X‐band linear accelerator for high‐precision radiotherapy

Medical X‐band linear accelerator for high‐precision radiotherapy

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

Electron Injector Based on Thermionic RF-Modulated Electron Gun for Particle Accelerator Applications

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