OR: Accelerator Fundamentals: Role and Impact on IMRT

Functional Requirements for IMRT

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

Timothy J. Waldron, M.S.

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Accelerating Structures: Traveling-Wave

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

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Implementation of Second Generation IMRT » Tomotherapy

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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.

Accelerating Structures: Traveling-Wave

Accelerating Structures: Traveling-Wave e-

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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.

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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

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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.

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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.

Accelerating Structures: Standing-Wave e-

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Accelerating Structures: Energy Control

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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.

Energy Control: Acceleration Per Cavity

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

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

Energy Control: Length of Accelerator/# of Cavities (1) E1

A1

E1

E2 < E1 A2 = A1/2

E2 < E1

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

Electron Injection z

Energy Switch “out”

E1

Energy Switch “in”

E2 < E1

» Controllable source of electrons to be accelerated (Thermionic emission). » Provide initial velocity to electrons for capture by oscillating electric fields. z

Diode Electron Gun ACCELERATING STRUCTURE

FOCUSING (V0 - HV)

Gun/Injector Functions

Both Diode and Triode designs are currently in use.

Triode Electron Gun (gridded) GRID (+ inj. on/ - inj. off) FOCUSING ( - HV)

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FILAMENT

ACCELERATING STRUCTURE

FILAMENT

CATHODE (-HV)

CATHODE (- V0 - HV)

Triode Electron Gun (gridded)

GRID (+ inj. on)

GRID (- inj. off) FOCUSING ( - HV)

FILAMENT

CATHODE (- V0 - HV)

Triode Electron Gun (gridded)

ACCELERATING STRUCTURE

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FOCUSING ( - HV)

FILAMENT

ACCELERATING STRUCTURE

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CATHODE (- V0 - HV)

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Basic Beam Parameters z

Basic Beam Parameters z

Amplitudes -RF

» 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 cathodedriven current further increases temperature.

» 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-up period for RF system to stabilize frequency.

Dose Rate, Beam Control z

Dose Rate, Beam Control

Beam Pulse = Coincident RF + Injection

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» Options depend upon gun/accelerator type. z

» 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).

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Intra-segment time Time required for beam production to restabilize after beam suppression between IMRT segments. Linac and control system each contribute some component.

First-Generation IMRT Implementations

Elekta SL-25/Precise z

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» Elekta SL-25/Precise » Siemens Primus/Oncor » Varian 21EX/Millenium

Run-up time At 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.

Control Options via:

Existing Radiotherapy delivery systems adapted/modified for IMRT. z System Overview, Overall Control Architecture, Beam Control, IMRTspecific parameters for:

Amplitudes -Gun Current

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Travelling-Wave accelerator Diode (nongridded) Injector/Gun Energy Control via RF frequency and beam loading 80-leaf MLC replaces upper jaws

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Elekta Control Architecture I/O

Elekta/Precise Control System Architecture -Overall

Control Area -Redundant, 1/2 shown

CONTROL AREA 16 -HT & RF

Treatment

Display (NT) Processor

MULTIBUS 2

Remote

BACKPLANE

Terminal Unit (RTU) x2

Eurocard Cardrack/Backplane

High Voltage Microwav eHardwar e/Circuits

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

Terminal

Debug Terminal

CONTROL AREA 12 -Radiation Head Control Mode Remote Selection Terminal BeamUnit Modifier Controls/ (RTU) x2 Circuits

Control Processor (RMX)

FULL-DUPLEX

1/2 Serial Link to Control System

DAISY-CHAIN

Remote

CONFIGURATION

Terminal Unit (RTU) x2

Interface & Motor Controls Hardware/ Circuits

Elekta Control System: MLC

Control Processor (RMX)

MLC Head Electronics

Head Control RTU (Area 12)

Video Digitizer Card

FULL-DUPLEX

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SERIAL BUS IN DAISY-CHAIN CONFIGURATION

Elekta/Precise Beam Control (IMRT)

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Signal Conditioning Card (SCC)

DIE #2 (ICCA B only)

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.

Analog Output 8 bit DA 8 channels

Control Area Specific Circuit Boards (Dosimetry in RHCA, HV Supply Control in HTCA)

Other Machine Hardware

Analog Output 12 bit DA 8 channels

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.

Siemens Primus/Oncor z

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Digital Input Encoder (DIE) 32 Inputs 8 Outputs via FPLAs

Aux PSU PCB DC Power Supply

Elekta Beam Control z

MULTIBUS 2 BACKPLANE

Relay Output Card (ROC)

Analog Input PCB (12 Bit 10V AD)

CONTROL AREA 72 -Interface Cabinet

SERIAL BUS IN

CCD Camera

Multiplexer Terminal Unit (MTU)

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Standing-Wave Accelerator Triode (gridded) Gun Injector Energy 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

Siemens Control System: MLC

Function

Function

Function

Function

Controller

Controller

Controller

Controller

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1

2

3

MOTORS

DOSE 1

DOSE 2

BEAM

“High Speed” Serial Comm

Console PC w/ Serial Interface Processor

FULL-DUPLEX SERIAL BUS DAISY-CHAIN CONFIGURATION

Function

Function

Function

Function

Controller

Controller

Controller

Controller

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6

5

4

I/O

INTER

LIGHTS

HAND

-LOCKS

BMSHLD

CONTROL

Leaf Bank B Drives + Feedback X 29

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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.

Varian 21-EX z z z

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Standing-Wave Accelerator Triode Gun Injector Energy control via RFpower-per cavity/beam loading, energy switch for low-X. 120 leaf Tertiary MLC with rounded leaf ends.

Console PC w/ Serial Interface Processor

Leaf Bank A Drives + Feedback X 29 Multiplexer I/O

Hardware Lines to Motors and I/L Controllers

Siemens Beam Control (IMRT)

Siemens Beam Control z

Function Controller Comm Chain

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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 PAUSE to suppress dark current by adjusting PFN to “IPFN” (80% of nominal) value. Intrasegment time is