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Linear Accelerator Technology

Linear Accelerator Technology. State of the Art Platforms for Advanced Image Guidance. Theodore Thorson, Ph.D. Senior Advisor Elekta. Outline. Historical development of treatment delivery equipment Linear accelerator components, alternative engineering approaches & clinical influence

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Linear Accelerator Technology

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  1. Linear Accelerator Technology State of the Art Platforms for Advanced Image Guidance Theodore Thorson, Ph.D. Senior Advisor Elekta

  2. Outline • Historical development of treatment delivery equipment • Linear accelerator components, alternative engineering approaches & clinical influence • Precision delivery features • Image guided capabilities

  3. Resonant Transformer Van De-Graaff History of Radiation Delivery • Orthovoltage: 1920-1950 • High energy systems: 1930-1950 • Van de Graff • Resonant transformer • Betatron • Radioactive isotope units • Radium • Cesium • Cobalt 60

  4. History of Radiation Delivery • Linear Accelerators: 1950 to present • Traveling wave systems • Standing wave systems • Microtron • Reflexotron

  5. Shielding Bending System Electron Gun Accelerator Wave Guide µ Wave Pulse HV Pulse Dosimetry RF Power Source Collimation Modulator HV Pulse X-Ray or Electron Beam for Treatment Linear Accelerator System Overview Control System AC Line Power Support Structure

  6. Basic Accelerator Technology • Microwave Power sources • Acceleration structures • Beam transport systems • Support structures

  7. HV Pulse Power Source OptionsKlystron • Linear Amplifier • Requires frequency stabilized oscillator • High Voltage (140kV) • High Power (7+MW) • Typical 10,000 hr life • High cost • Electromagnet (solenoid) • Phase and power amplitude independent of reflections

  8. HV Pulse Klystron Operational Trade-offs • Initial Cost • Additional Components • RF Driver • Rotating RF joint • Oil tank • Solenoid • Power supplies • (T drive) • Size • Replacement time • Higher operating voltages Characteristics • Amplifier • Phase and power amplitude stability • Longer life (6-10 yrs) • High power applications • Fixed PRF

  9. HV Pulse Power Source OptionsMagnetron • Simple Oscillator • Low Power operation (3-6MW) • 5000 hr typical life • Low cost • Permanent or electromagnet • Low Voltage (45 kV) • Power amplitude phase and frequency dependent on reflections • Electromagnetic tuning +

  10. HV Pulse Magnetron Characteristics Operational Trade-offs • Self-oscillator • Low initial cost • Small size • Short replacement time • Simple RF system • Fewer components • Variable PRF • Shorter life (5-8 yrs) • Frequency stability • Subject to RF reflections • Lower power operation

  11. HV Pulse Accelerator Structures Short section SW accelerator Short section - TW accelerator Long section - SW accelerator

  12. TW ACCELERATOR STRUCTURE Moderate shunt impedance, longer structure for equivalent energy gain Short fill time Circulator not required High accelerating beam capacity Spectrum insensitive to accelerating field Bunching less sensitive to accelerating field Generally low vacuum requirement SW ACCELERATOR STRUCTURE High Shunt Impedance, shorter structure for equivalent energy gain Longer fill time Circulator required Low accelerating beam capacity Spectrum sensitive to accelerating field Bunching highly sensitive to accelerating field High vacuum requirement HV Pulse Accelerator Options

  13. HV Pulse Electron Beam Bending - 90o Magnet Pole Energy Spread Position Change Angular Change

  14. HV Pulse Achromatic Magnet Designs

  15. HV Pulse Bending system trade-offs h1 r2 r1 h2 > h1 h2 r2 > r2

  16. Support System Designs • Stand and Gantry • Compact packaging • Support for beamstopper • Requires shorter accelerator structure • Single unit floor imbedded baseframe

  17. Support System Designs • Drum and Arm • Small backwall to isocenter distance • High degree of patient access • Structural support for other components • Easy access for service • Easily accomodates longer accelerator structure

  18. Enhanced delivery technology 1980-90 • Multileaf collimation • Information technology • Computer Control systems

  19. Multileaf CollimationTrade-Offs • Leakage • Geometry • Field Size Capability • Number of leaves • Leaf pitch

  20. Internal • Lower collimator replacement • Full thickness leaves • Focus - Double, straight leaf ends • Internal • Upper collimator replacement • Backup diaphragms • Focus - divergence + leaf ends • External • Moveable carriage & leaves • Backup diaphragms • Focus - divergence + leaf ends Multileaf CollimationGeometry

  21. 3.0 mm error 3.8 mm error Double Focus CollimationGeometric Trade-Off

  22. Multileaf Field Sizes Max Field Size 1 cm leaves Single Field No center leaf gap Max Field Size 1 cm leaves Single Field Max Field Size Maximum Leaf Travel Comparisons (1 cm leaves) 40 cm 1 cm leaf area 40 40 40 } x 40 A 12.5 cm 40 40 32.5 32.5cm 10cm B } x 27 30 cm 27 27 40 C 14.5 cm } x 40 30 40 14.5 cm 40 40 40 40 29 40 14.5

  23. Multileaf CollimationLeaf Pitch Trade-Off Upper Collimator Replacement Lower Collimator Replacement External Collimator

  24. Multileaf CollimationMicroMLC • Maximum field size: 72 x 63 mm • Number of leaves: 40 per side • Leaf thickness: 1 mm • Material: tungsten • Maximum field size: 100 x 100 mm • Number of leaves: 26 per side • Leaf thickness: 5.5, 4.5, 3 mm • Material: tungsten

  25. Clinical Setups

  26. Expanding Data Requirements For Treatment Delivery • Basic Data Parameters • X-Jaw • Y-Jaw • Collimator Rotation • Gantry Rotation • Blocks • Wedges Expanded Parameters • Couch Positions (4) • Asymmetric Jaws (4) • MLC Leaf Positions (80) • Patient Coordinates (4-6)

  27. Control of Radiation Delivery MLC MLC Linac Linac Hardware MLC Linac Linac Electronics MLC Linac MLC Linac Control MLC Control Common Control Control Control Control Memory Console User Interface User Interface User Interface Added GUI Record/Verify DB Treatment Planning

  28. Image Guidance Technology • Electronic portal imaging • Motion management

  29. Advanced Image Guidance Varian Trilogy Elekta Synergy

  30. Solid state imaging panel Clearance 90cm Kilovoltage X-ray source Image Guidance - Components

  31. Volume Data Acquisition

  32. Reconstructed Volume Image

  33. Transverse view

  34. CT section sequence

  35. Comparison to Planning CT

  36. Target Volume Definition

  37. 3D Volume information Rando Head Phantom Note resolution in all dimensions

  38. Synergy “double-exposed” Cone Beam CT 3.5 cGy skin dose, 3.0 cGy prostate dose

  39. Synergy “double-exposed” Cone Beam CT - Patient 1 3.5 cGy skin dose, 3.0 cGy prostate dose

  40. Patient 2 B • Clear delineation of borders of the bladder (B), prostate (P), seminal vesicles (SV), and rectum (R) with modest increase in imaging dose. P R SV 3.8 cGy skin dose; 3.5 cGy isocenter dose

  41. 3D Patient Motion • Respiratory correlated CT (RCCT) • Serial or spiral CT with external surrogates • Imaging at many phases of breathing cycle • Free breathing cone-beam CT (no surrogates) 3D Motion • 3 images/sec, 1000 projections, 6 phases/cycle

  42. Summary • Beam bending systems should be achromatic - all manufacturers comply • All types of power sources are used and can be made to work in most applications • There is no ideal accelerator design • All designs include trade-offs, most are minor, but may have significance to clinical applications • All accelerator systems will work in a variety of clinical situations • Consider your clinical practice requirements and consider how various trade-offs affect your needs • Advanced image guidance will allow precision capabilities of treatment systems to achieve accuracy in delivery

  43. With thanks to the Elekta Synergy™ Research Group William Beaumont, Royal Oak, USA Princess Margaret, Toronto, Canada NKI, Amsterdam, Netherlands Christie, Manchester, UK

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