ESTR

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Poster presented at:

1929-EPBrendan Whelan DOI: 10.3252/pso.eu.ESTRO2016.2016

Physics track: Implementation of new technology, techniques, clinical protocols or trials (includingQA & audit)

Characterisation of a gridded electron gun in magnetic fields: implications for MRI-Linac therapy

Introduction & Purpose

Brendan Whelan1,2, Dragos Constantin3, Lois Holloway2, Magdalena Bazalova-Carter4, Brad Oborn5, Rebecca Fahrig3, Paul Keall1

E: brendan.whelan@sydney.edu.au

1. University of Sydney, Australia2. Ingham Institute & Liverpool Hospital, Australia3. Stanford University, USA

4. University of Victoria, Canada5. Illawarra cancer care centre, Australia

• A gridded (or triode)gun has an electricallyindependent controlgrid placed over thecathode surface

• Results in muchgreater operatingflexibility:

• Efficient and robust beam gating

• Continuously variable dose rate

• High quality injection beam over a wide range of beam energies and current

Gun solution in zero magnetic field:

Mode High Voltage (kV) (measured)

Grid Voltage (V) (measured)

Emission Current (mA)(measured)

Emission Current (mA)(simulated)

% Error (absolute mean)

6X 14.98±0.03 92.5±7.4 327.6±26.8 322±27 3.210X 9.98±0.03 41.6±3.6 127.3±9.5 123±11 5.115X 10.98±0.03 69.7±5.7 214.2±18.8 214±18 3.5

The aim of this work was to develop a validated model of a clinical gridded electron gun, and to characterise its performance in magnetic fields, as needed for MRI-Linac systems, which

are being developed by multiple groups

What is a gridded electron gun, and why is it important?

1. The majority of Linac vendors utilise gridded electron guns

2. However, no technical data on gridded guns exists in the literature

3. Previous studies characterising electron gun performance in magnetic fields focused on non gridded guns

The Electron gun is the source of the treatment beam, and one of the most sensitive Linac

components to external magnetic fields

• Geometry: 3D laser scan of a Varian highenergy gun, accurate to ±0.1 mm

• Boundary conditions: experimentallymeasured on 6 dose matched VarianTrueBeam Linacs

• Physics: Electrostatic system described byPoisson’s Equation (equation 1) solved usingFinite Element Method (FEM) in Opera 3D

• Thermion electron emission modelled using Child’s law (equation 2)

• The emitted current depends on the field,and vice versa. Therefore Iterative solvingtechnique utilised.

𝛁𝛁𝟐𝟐𝑽𝑽 = 𝒑𝒑𝜺𝜺𝟎𝟎

𝑱𝑱𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆𝒆 =𝟒𝟒𝜺𝜺𝟎𝟎𝟗𝟗

𝟐𝟐𝟐𝟐𝒆𝒆𝟎𝟎

𝑽𝑽𝟑𝟑/𝟐𝟐𝒆𝒆𝟐𝟐

Equation 2: Child’s Law for space charge limited thermionic electron emission

Equation 1: Poisson’s equation

Gun performance in magnetic fields:

Methods

Results

Conclusions

References1. Alberta Group. MRI Linac web page. 2016; Available from: http://www.mp.med.ualberta.ca/linac-mr/.2. Utrecht Group. MRI-Linac web page. 2016; Available from: http://www.umcutrecht.nl/en/Research/Research-centers/UMC-Utrecht-Center-for-Image-Sciences/Research-programs/MR-Radiotherapy/MRI-guided-Radiotherapy.3. Sydney Group. MRI-Linac webpage. 2016; Available from: http://sydney.edu.au/medicine/radiation-physics/research-projects/MRI-linac-program.php.4. ViewRay. Viewray webpage,. 2016; Available from: http://www.viewray.com/index.htm.5. B.-N. Lee et. al., Status of KAERI 6 MeV 9.3 GHz X-Band Electron Linac for Cancer Treatment System. 2014.6. T. Waldron. Function Requirements for IMRT. 2003; Available from: https://www.aapm.org/meetings/03SS/Presentations/Waldron.pdf.7. P. Freislederer, et. al., Characteristics of gated treatment using an optical surface imaging and gating system on an Elekta linac. Radiation Oncology, 2015. 10(1): p. 68.8. D. E. Constantin et. al., A study of the effect of in-line and perpendicular magnetic fields on beam characteristics of electron guns in medical linear accelerators. Medical physics, 2011. 38(7): p. 4174-4185.9. J. St. Aubin et. al., Effect of longitudinal magnetic fields on a simulated in-line 6 MV linac. Medical physics, 2010. 37(9): p. 4916-4923.10. J. St Aubin et. al., Effect of transverse magnetic fields on a simulated in-line 6 MV linac. Physics in medicine and biology, 2010. 55(16): p. 4861.

Phase space of different modes:

Experimental versus simulated data:

Key Findings:

Model Ref In-line fieldsensitivity

Perpendicularfield sensitivity

CathodeRadius

AnodeRadius

Cathode:anode (ratio)

HV

Litton [8] ~163G 2G 3.18 mm 2.29 mm 1.40 10 kV

Varian [8] ~141G 5G 2.73 mm 1.73 mm 1.60 9.5 kV

Alberta [9,10] 100G NA 2.31 mm 1.00 mm 2.31 30.6 kVThiswork 50G 7.5G 5.53 mm 2.62 mm 2.1 15 kV

Comparison with previous work:

• FEM model of a clinical gridded gun developedbased on highly accurate geometry measurements

• Validated using experimental data• First clinical gridded gun modelled in literature• Characterised in magnetic fields

• Different clinical modes use different gunsettings, and as such, exhibit differentsensitivity

• This gun is around twice as sensitive tooperation in external in-line magnetic fields asprevious work on diode guns. This is due toboth the geometry and operating voltage.

• This gun has a much larger foot print in space,which makes magnetic shielding more difficult

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