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

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

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

Early high energy acceleration units Resonant Transformer Van De-Graaff

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

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

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

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

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 +

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

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

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

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

Achromatic Magnet Designs HV Pulse

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

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

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

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

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

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

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

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

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

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

Clinical Setups

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)

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

Image Guidance Technology Electronic portal imaging Motion management

Advanced Image Guidance Varian Trilogy Elekta Synergy

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

Volume Data Acquisition

Reconstructed Volume Image

Transverse view

CT section sequence

Comparison to Planning CT

Target Volume Definition

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

Synergy “double-exposed” Cone Beam CT 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

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

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

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

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