Linear Accelerator Fundamentals

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Functional Requirements for IMRT

Timothy J. Waldron, M.S.

OR: Accelerator Fundamentals: Role and Impact on IMRT

Short Review of Basic Concepts» Accelerating Structures» Electron Injection» Energy Control» Dose Rate/Beam Control

Implementation of First Generation IMRT Systems» Elekta» Siemens» Varian

Implementation of Second Generation IMRT» Tomotherapy

Accelerating Structures: Traveling-Wave

Circular transmission waveguide“Tube and Washer” slow wave structure decreases phase velocity of the RF to a useful level (< c).Washer spacing greater at proximal end, constant at distal end -electron transit time decreases, then is essentially constant as energy approaches c.


Packets of RF energy are injected at proximal end and extracted at distal end.Instantaneously, half of structure electric field is zero, no acceleration occurs.Electric field amplitude decreases along length of accelerator due to resistive losses.


Electrons are captured and accelerated by the differential electric field components of RF waves.Electrons accelerated downstream travel with RF wave groups.Output electron energy spectrum is primarily dependent upon RF frequency.

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Accelerating Structures: Standing-Wave

Series of coupled circular resonant cavities.Alternating “zero field” cavities propagate RF only, and so may be on or off of beam axis.Most proximal cavity (buncher) may be larger, but generally all accelerating cavities same size.


RF is injected at any point, not extracted per se.SW structure is a highly resonant “shorted” transmission line, RF propagates/reflects.After “fill time”, electric fields in structure establish standing wave pattern of apparently stationary nodes and modes of uniform amplitude.


Electron transit time approximately 1/2 RF wave time constant.Electrons “see” constant repelling electric field upstream and attracting electric field downstream. Resonant structure operates over narrow frequency range.

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Accelerating Structures: Energy Control

TW Accelerators» Accelerator length (power limited)» RF Frequency

SW Accelerators» RF Power/cavity.» Number of cavities (length).» Several techniques in use.

Energy Control: Acceleration Per Cavity

A1

A2 = A1/2 E2 < E1

E1

Energy Control: Length of Accelerator/# of Cavities (1)

E2 < E1

E1

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Energy Control: Length of Accelerator/# of Cavities (2)

Energy Switch “out”

E1

E2 < E1Energy Switch “in”

Electron Injection

Gun/Injector Functions» Controllable source of electrons to be

accelerated (Thermionic emission).» Provide initial velocity to electrons for

capture by oscillating electric fields.

Both Diode and Triode designs are currently in use.

Diode Electron Gun

FILAMENT

CATHODE (-HV)

FOCUSING (V0 - HV)

ACCELERATING STRUCTURE

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Triode Electron Gun (gridded)

FILAMENT

CATHODE (- V0 - HV)

GRID (+ inj. on/ - inj. off)

FOCUSING ( - HV)



FILAMENT

CATHODE (- V0 - HV)

GRID (- inj. off)

FOCUSING ( - HV)


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FILAMENT

CATHODE (- V0 - HV)

GRID (+ inj. on)

FOCUSING ( - HV)


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Basic Beam Parameters

Amplitudes -RF» RF Power level determines available work

to accelerate electrons.» RF Work in accelerator is shared between

accelerating electrons and resistive heating of structure.

» Resistive heating/cooling of accelerator impacts frequency characteristics.

» Initial beam-on may incorporate a run-upperiod for RF system to stabilize frequency.

Basic Beam ParametersAmplitudes -Gun Current» Governs fluence or “dose” per pulse.» Increasing gun/injector current “loads” RF,

result is decrease in average energy as available work is exceeded.

» Increased cathode/filament temperature increases emission (potential gun current).

» Increasing cathode voltage increases gun current. Backheating occurs as cathode-driven current further increases temperature.

Dose Rate, Beam Control

Beam Pulse = Coincident RF + Injection» Options depend upon gun/accelerator type.

Control Options via:» Coincidence/anti-coincidence of gun + RF to

control single beam pulse production.» Control repetition frequency of coincident

RF + gun to control pulse rate» Control fluence per pulse via gun current to

control “dose” rate. Calibration may be affected (recombination).

Dose Rate, Beam Control

Run-up timeAt initial beam on, the time necessary for RF

in accelerator to stabilize, and re-stabilize as beam is loaded with electrons. Depends strongly upon gun and accelerator design.

Intra-segment timeTime required for beam production to re-

stabilize after beam suppression between IMRT segments. Linac and control system each contribute some component.

First-Generation IMRT Implementations

Existing Radiotherapy delivery systems adapted/modified for IMRT.System Overview, Overall Control Architecture, Beam Control, IMRT-specific parameters for:» Elekta SL-25/Precise» Siemens Primus/Oncor» Varian 21EX/Millenium

Travelling-Wave acceleratorDiode (non-gridded) Injector/GunEnergy Control via RF frequency and beam loading80-leaf MLC replaces upper jaws

Elekta SL-25/Precise

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Elekta/Precise Control System Architecture -Overall

CONTROL AREA 16 -HT & RF

Remote

Terminal Unit

(RTU) x2Treatment

Terminal

Debug Terminal

Display (NT)

Processor

Control Processor

(RMX)

FULL-DUPLEX

SERIAL BUS IN

DAISY-CHAIN

CONFIGURATION

MULTIBUS 2

BACKPLANE

High Voltage

MicrowaveHardware/Circuits

Remote

Terminal Unit

(RTU) x2

Mode Selection

Beam-Modifier Controls/ Circuits

CONTROL AREA 12 -Radiation Head Control

Remote

Terminal Unit

(RTU) x2

Interface & Motor Controls

Hardware/Circuits

CONTROL AREA 72 -Interface Cabinet

Elekta Control Architecture -I/O

Eurocard Cardrack/Backplane

Control Area -Redundant, 1/2 shown

Analog Input PCB

(12 Bit 10V AD)

Signal Conditioning Card (SCC)

MultiplexerTerminal Unit

(MTU)

Digital Input Encoder (DIE)

32 Inputs 8 Outputs via

FPLAs

DIE #2 (ICCA B

only)

Relay Output Card (ROC)

Remote Terminal Unit (RTU) 1 of 2 -“A” or “B”

Aux PSU PCB DC

Power Supply

Analog Output 8 bit DA

8 channels

Analog Output

12 bit DA 8 channels

Control Area -Specific Circuit

Boards (Dosimetry in RHCA, HV

Supply Control in HTCA)

1/2 Serial Link to Control System

Other Machine

Hardware

Elekta Control System: MLC

CCD Camera

MLC Head Electronics

Head Control RTU (Area 12)

Control Processor (RMX)

Video Digitizer

Card

FULL-DUPLEX

SERIAL BUS IN

DAISY-CHAIN

CONFIGURATION

MULTIBUS 2

BACKPLANE

Elekta Beam Control

Diode gun and TW accelerator: Each microwave pulse synchronized with an injection pulse and intended to produce beam.Run-up 8-10 seconds, as cathode temp stabilizes and RF system tunes to proper frequency.Dose rate is controlled by adjusting the machine Pulse Repetition Frequency (PRF).For a nominal dose rate of 700 MU/minute, the SL-25 is pulsed at 400 PPS. Nominal output is approximately 0.03 MU/pulse.

Elekta/Precise Beam Control (IMRT)

Intra-segment state is achieved by setting gun filament to standby value and suppressing triggers to modulator and injection. Magnetron uses a solenoid linear actuator to drive the magnetron tuning plunger instead of a rotary gear/chain arrangement. Faster tuning reduces intra-segment time to 2-4 seconds.Reduced dose rates are selected by varying frequency of filament voltage (cathode temperature) at the nominal system PRF.

Siemens Primus/OncorStanding-Wave AcceleratorTriode (gridded) Gun InjectorEnergy control via RF-power-per cavity/beam loading.58 or 82-leaf double-focussed MLC replaces lower jaws.

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Siemens Control System Architecture -Overall

Function

Controller

0

MOTORS

Function

Controller

1

DOSE 1

Function

Controller

2

DOSE 2

Function

Controller

3

BEAM

Function

Controller

4

HAND

CONTROL

Function

Controller

5

LIGHTS

BMSHLD

Function

Controller

6

INTER

-LOCKS

Function

Controller

7

I/O

Console PC

w/ Serial

Interface

Processor

FULL-DUPLEX

SERIAL BUS

DAISY-CHAIN

CONFIGURATION

Leaf Bank A Drives + Feedback

X 29

Leaf Bank B Drives + Feedback

X 29

Multiplexer I/O

Console PC w/ Serial Interface Processor

“High Speed” SerialComm

Function Controller

CommChain

Hardware Lines to

Motors and I/L Controllers

Siemens Control System: MLC

Siemens Beam Control

Triode gun, SW accelerator.An injection pulse is produced in coincidence with each RF pulse.Dose rate is controlled by adjusting PRF of system.Dose rate servo takes input from Dose Channel 2, adjusts PRF to maintain specified rate.Run up 3-6 seconds, gun pulse is dephased/non-coincident with RF.

Siemens Beam Control (IMRT)

Machine enters a PAUSE state while beam shaping components are moved.PAUSE is achieved by “de-phasing” injection and RF so they are non-coincident.RF power may be reduced during PAUSEto suppress dark current by adjusting PFN to “IPFN” (80% of nominal) value.Intrasegment time is <1 second for linac, additional time for control system.

Varian 21-EX

Standing-Wave AcceleratorTriode Gun InjectorEnergy control via RF-power-per cavity/beam loading, energy switch for low-X.120 leaf Tertiary MLC with rounded leaf ends.

Varian Control Architecture -Overall

Console PC

Comm

Processor

STD BUS BACKPLANE

Common

RAM

Control

Timer

Clinical Keyboard

Control

ProcessorInput / Output

Signal Conditioning

Backplane Signal

processing / scaling and distribution

Varian Cardrack Machine parameter

control and interlock circuits

Linear Accelerator

Hardware and In-room circuits

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Varian Control System: MLC

MLC Workstation

PC

Carriage A

Leaf Drive/

Feedback I/O

X 60

Carriage B

Leaf Drive/

Feedback I/O

X 60

MLC Controller

CommProcessor

Signal Conditioning I/O

Console Electronics

Full-Duplex Optic Fiber Link X 2

RS-422 Serial Comm

Hardware Interface

Lines: Beam

HoldoffMLC I/L

RS-232

Serial Comm

Control

Timer

Varian Beam ControlInjection pulse is coincident with RF to produce a beam pulse, or delayed to not produce. Gun is pulsed continuously for constant temp/emission.Dark current is suppressed by ionic vacuum pumping in gun region, and a solenoid that encloses the accelerator.Microwave system and injector are pulsed at constant 360 PPS in lowX and 180 PPS in highX. Nominal max dose rate is 600 MU/min, the dose per pulse is approximately 0.03 and 0.06 MU/pulse for lowX and highX, respectively.Run-up is approximately 500 mSec.

Varian Beam Control (2)Dose rate is controlled by selecting injector pulses to be coincident or not out of a 6-pulse train. In the highest dose rate all 6 pulses are coincident.Dose rate servo delays pulses as needed to achieve specified rate over a 50 mSecsampling cycle (control window). Resolution of the dosimetry subsystem is 0.01 MU, but the overall resolution is 1 beam pulse (0.03 MU in lowX or 0.06 MU in highX mode).

Varian Beam Control (IMRT)

In IMRT, the Dynamic Beam Delivery servo auguments the dose rate servo: Injection pulses are delayed/coincident to produce beam or not based on a control window.The control window and beam holdoff are now a function of the status of the modulating/beam-shaping device (MLC position, gantry angle, jaw position or gating). Intrasegment time is 50-60 mSec. 50 mSec. from control system, 0-10 mSec. for linac.

Second-Generation IMRT Implementation

Radiotherapy delivery system designed specifically for IMRT delivery.System Overview, Overall Control Architecture, Beam Control, IMRT-specific parameters.

Tomotherapy Hi-Art

Helical TomotherapyStanding-Wave Accelerator (Single photon)Triode (gridded) Gun Injector64 Leaf Interlaced Binary MLCMVCT Detector Array

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Binary MLC

6 MV6 MV LinacLinac

85 cm Gantry Aperture

40 cm MVCT FOV

5 - 40 mm SelectableSlice Thickness

5 mm X 6.1 mm X 6.1 mmMinimum Voxel size at Isocenter

MVCT Detector System

Approximately 85 cm

Approximately 50 cm

64 MLC Beamlets - 6.25 mmaverage width at isocenter

Continuous Rotation

The Geometry ofTomoTherapy

Courtesy of Tomotherapy, Inc.

OBC

LINAC

MLC/JAWS

DETECTOR ARRAY

HARDWARE SYNC

SLIP RINGS

STC

DRS

GANTRY ANGLE COUCH

Operator Workstation/

Console

To External

Data Server

Hi-Art Control Architecture -Overall

OBC

STC

Operator Workstation

/Status Console

HIGH VOLTAGEENABLE

LINAC

DOOR I/L

“HARD-WARE” I/L

GANTRY S.W. SAFETY TASK

COUCH

COMMUNICATIONS

Hi-Art Interlock Sub-system Hi-Art Beam Control (IMRT)

Triode gun, SW accelerator.An injection pulse is produced in coincidence with each RF pulse.Dose rate is controlled by adjusting PRF of system.Run up 3-6 seconds, gun pulse isdephased/non-coincident with RF.

Hi-Art Beam Control (IMRT)

Beam delivery is hardware-synchronized pulse-by-pulse to gantry rotation (rather than MU delivery).Ionometric dosimetry system functions to maintain constant dose rate of 1000 cGy/min at a nominal PRF of 300 PPS, or approximately 3.33 cGy/pulse.“Traditional” flattening filter unnecessary, resulting in factor of 1-3 increase in dose rate.

Heartfelt Thanks

Frank Spitz of Thomas Jefferson University Hospital for education on Elekta systems.George Aleman of MD Anderson Cancer Center for review and details on Siemens digital control system.Jim Bilich of Siemens Medical SystemsCalvin J. Huntzinger of Varian Medical SystemsDavid C.Murray of Tomotherapy Inc.

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Thank you!

Documents

Linear Accelerator Fundamentals