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

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

5. Early high energy acceleration units
Resonant Transformer
Van De-Graaff

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

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

8. Power Source Options Klystron
HV Pulse
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

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

10. Power Source Options Magnetron
HV Pulse
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 +

11. Magnetron Characteristics Operational Trade-offs Self-oscillator
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

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

13. Accelerator Options TW ACCELERATOR STRUCTURE SW ACCELERATOR STRUCTURE
HV Pulse
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

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

15. Achromatic Magnet Designs
HV Pulse

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

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

18. 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

19. Enhanced delivery technology
Multileaf collimation
Information technology
Computer Control systems

20. Multileaf Collimation Trade-Offs
Leakage
Geometry
Field Size Capability
Number of leaves
Leaf pitch

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

22. Double Focus Collimation Geometric Trade-Off
3.0 mm
error
3.8 mm

23. Comparisons (1 cm leaves)
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 A
12.5 cm
} x 40 40 40
32.5cm
32.5
10cm B
} x 27
30 cm 27 27 C
14.5 cm 40
} x 40
14.5 cm 40 30 40 40 40 40 40 29
14.5

24. Multileaf Collimation Leaf Pitch Trade-Off
Upper Collimator
Replacement
Lower Collimator
Replacement
External Collimator

25. Multileaf Collimation MicroMLC
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

26. Clinical Setups

27. 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)

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

29. Image Guidance Technology
Electronic portal imaging
Motion management

30. Advanced Image Guidance
Varian Trilogy
Elekta Synergy

31. Image Guidance - Components
Solid state imaging panel
90cm
Clearance
Elekta is very proud of its patient clearance - 45cm from the front of the head to the isocentre, the largest in the industry.
All these extra parts are further from the isocentre than the head is, and can fold away for patient set-up (see later slide).
The 90cm diameter has an important competitive element - TomoTherapy’s HiArt product has a bore of just 85cm diameter.
Kilovoltage X-ray source

32. Volume Data Acquisition

33. Reconstructed Volume Image

34. Transverse view

35. CT section sequence

36. Comparison to Planning CT

37. Target Volume Definition

38. 3D Volume information Rando Head Phantom Note resolution in
all dimensions

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

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

41. 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

42. 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

43. 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

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