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

Precision delivery features

Image guided capabilities
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History of Radiation Delivery

Orthovoltage: 1920-1950

High energy systems: 1930-1950




Van de Graff
Resonant transformer
Betatron
Radioactive isotope units



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Van De-Graaff
Radium
Cesium
Cobalt 60
Resonant Transformer
History of Radiation Delivery

Linear Accelerators: 1950 to present
 Traveling wave systems
 Standing wave systems
 Microtron
 Reflexotron
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Linear Accelerator System Overview
Shielding
Electron
Gun
Bending
System
Accelerator Wave Guide
HV Pulse
Control
System
µ Wave Pulse
Dosimetry
Collimation
RF
Power
Source
HV Pulse
Modulator
X-Ray or Electron Beam
for Treatment
AC
Line Power
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Support Structure
Basic Accelerator Technology
Microwave
Power sources
Acceleration structures
Beam transport systems
Support structures
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Power Source Options
Klystron
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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
Klystron
HV Pulse
Operational Trade-offs
Characteristics

Amplifier

Phase and power amplitude
stability

Longer life (6-10 yrs)

High power applications

Fixed PRF
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
Initial Cost

Additional Components
 RF Driver
 Rotating RF joint
 Oil tank
 Solenoid
 Power supplies
 (T drive)

Size

Replacement time

Higher operating voltages
Power Source Options
Magnetron
+
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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
Magnetron
Characteristics
HV Pulse
Operational Trade-offs

Self-oscillator

Shorter life (5-8 yrs)

Low initial cost

Frequency stability

Small size

Subject to RF reflections

Short replacement time

Lower power operation

Simple RF system

Fewer components

Variable PRF
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Accelerator Structures
HV Pulse
Short section
SW accelerator
Short section - TW accelerator
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Long section - SW accelerator
Accelerator Options
HV Pulse
TW ACCELERATOR STRUCTURE
SW ACCELERATOR STRUCTURE

Moderate shunt impedance, longer structure for
equivalent energy gain

High Shunt Impedance, shorter structure for equivalent
energy gain

Short fill time

Circulator not required

Longer fill time

High accelerating beam capacity

Circulator required

Spectrum insensitive to accelerating field

Low accelerating beam capacity

Bunching less sensitive to accelerating field

Spectrum sensitive to accelerating field

Generally low vacuum requirement

Bunching highly sensitive to accelerating field

High vacuum requirement
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Electron Beam Bending - 90
o
HV Pulse
Magnet Pole
Energy Spread
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Position Change
Angular Change
Achromatic Magnet Designs
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HV Pulse
Bending system trade-offs
HV Pulse
h1
r1
h2 > h1
r2 > r2
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h2
r2
Support System Designs
 Stand
and Gantry
 Compact packaging
 Support for beamstopper
 Requires shorter accelerator
structure
 Single unit floor imbedded
baseframe
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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
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Enhanced delivery technology
1980-90
 Multileaf
collimation
 Information
 Computer
technology
Control systems
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Multileaf Collimation
Trade-Offs

Leakage

Geometry

Field Size Capability

Number of leaves

Leaf pitch
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Multileaf Collimation
Geometry
Internal
External
Internal
•Lower collimator replacement
•Moveable carriage & leaves
•Upper collimator replacement
•Full thickness leaves
•Backup diaphragms
•Backup diaphragms
•Focus - Double, straight leaf
ends
•Focus - divergence + leaf ends
•Focus - divergence + leaf
ends
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Double Focus Collimation
Geometric Trade-Off
3.0
mm
error
3.8
mm
error
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Multileaf Field Sizes
Maximum Leaf Travel
Comparisons (1 cm leaves)
40 cm
1 cm leaf
area
12.5 cm
A
Max Field
Size
40
} x 40
Max Field Size
Max Field Size
1 cm leaves
1 cm leaves
Single Field
Single Field No center leaf gap
40
40
32.5cm
40
40
32.5
10cm
B
} x 27
30 cm
C
14.5 cm
27
40
14.5 cm
27
} x 40
30
40
40
40
40
40
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40
29
14.5
Multileaf Collimation
Leaf Pitch Trade-Off
Upper Collimator
Replacement
Lower Collimator
Replacement
External Collimator
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Multileaf Collimation
MicroMLC
• Maximum field size: 72 x 63 mm
•Number of leaves: 40 per side
•Leaf thickness: 1 mm
•Material: tungsten
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• Maximum field size: 100 x 100
mm
•Number of leaves: 26 per side
•Leaf thickness: 5.5, 4.5, 3 mm
•Material: tungsten
Clinical Setups
Distance
a – gantry to isocentre
b – head to isocentre
c – head size
d – floor to isocentre
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Varian – Clinac
EX
104cm
41cm (30 with
wedge)
90cm
131cm
ELEKTA PRECiSE
124cm
SIEMENS PRIMUS
97.3cm
45cm
62cm
124cm
43cm
75cm
133cm
Expanding Data Requirements
For Treatment Delivery
115,000
120,000
100,000
Number of Data Elements
Basic Data Parameters
X-Jaw
Y-Jaw
Collimator Rotation
Gantry Rotation
Blocks
Wedges
80,000
60,000
40,000
Expanded Parameters
Couch Positions (4)
20,000
Asymmetric Jaws (4)
MLC Leaf Positions (80) 0
Patient Coordinates (4-6)
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23,000
7
14
28
44
52
91
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Control of Radiation Delivery
Hardware
Electronics
Linac
Linac
MLC
MLC
Linac
MLC
Linac
MLC
Control
Control
Control
Control
Console
User
Interface
User
Interface
Linac
MLC
Linac Common MLC
Control Memory Control
User
Interface
Added
GUI
Record/Verify
DB
Evolving Strategies in Radiation Oncology - 28
Treatment Planning
Image Guidance Technology

Electronic portal imaging

Motion management
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Advanced Image Guidance
Varian Trilogy
Elekta Synergy
Evolving Strategies in Radiation Oncology - 30
Image Guidance - Components
Solid state imaging panel
Clearance
90cm
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Kilovoltage X-ray source
Volume Data Acquisition
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Reconstructed Volume Image
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Transverse view
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CT section sequence
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Comparison to Planning CT
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Target Volume Definition
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3D Volume information
Rando Head Phantom
Note resolution in
all dimensions
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Synergy “double-exposed”
Cone Beam CT
Evolving Strategies in Radiation Oncology - 39
3.5 cGy skin dose, 3.0 cGy prostate dose
Synergy “double-exposed” Cone Beam CT - Patient 1
3.5 cGy skin dose, 3.0 cGy prostate dose
Evolving Strategies in Radiation Oncology - 40
Patient 2

Clear delineation of borders of the bladder
(B), prostate (P), seminal vesicles (SV), and
rectum (R) with modest increase in imaging
dose.
B
P
R
SV
3.8 cGy skin dose;
3.5 cGy isocenter dose
Evolving Strategies in Radiation Oncology - 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
Evolving Strategies in Radiation Oncology - 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
Evolving Strategies in Radiation Oncology - 43
With thanks to the
Elekta Synergy™ Research Group
NKI, Amsterdam, Netherlands
Christie, Manchester, UK
Princess Margaret, Toronto, Canada
William Beaumont, Royal Oak, USA
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Evolving Strategies in Radiation Oncology - 45