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
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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
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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
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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
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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.
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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
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Basic Beam Parameters
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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
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Dose Rate, Beam Control
Beam Pulse = Coincident RF + Injection
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» Options depend upon gun/accelerator type.
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» 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
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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
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Digital Input
Encoder (DIE)
32 Inputs
8 Outputs via
FPLAs
Aux PSU
PCB DC
Power
Supply
Elekta Beam Control
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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
0
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
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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
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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 <1 second for linac,
additional time for control system.
Varian Control Architecture Overall
STD BUS BACKPLANE
Comm
Console
PC
Clinical
Keyboard
Processor
Control
Processor
Control
Linear
Accelerator
Hardware and Inroom circuits
Timer
Input /
Output
Signal
Conditioning
Backplane
Signal
processing /
scaling and
distribution
Varian
Cardrack
Machine
parameter
control and
interlock
circuits
Common
RAM
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Varian Control System: MLC
Varian Beam Control
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Console Electronics
MLC
Workstation
PC
Carriage
A
Carriage
B
Leaf
Drive/
Feedback
I/O
Leaf
Drive/
Feedback
I/O
X 60
X 60
Control
Timer
Hardware
Interface
Lines:
Beam
Holdoff
MLC I/L
RS232
Serial
Comm
Comm
Processor
Signal Conditioning
I/O
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MLC
Controller
Full-Duplex Optic
Fiber Link X 2
RS-422 Serial
Comm
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Injection 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 (IMRT)
Varian Beam Control (2)
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Dose rate is controlled by selecting injector
pulses to be coincident or not out of a 6pulse 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 mSec
sampling 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).
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Second-Generation IMRT
Implementation
Tomotherapy Hi-Art
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Radiotherapy delivery system designed
specifically for IMRT delivery.
z System Overview, Overall Control
Architecture, Beam Control, IMRTspecific parameters.
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.
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Helical Tomotherapy
Standing-Wave
Accelerator (Single
photon)
Triode (gridded)
Gun Injector
64 Leaf Interlaced
Binary MLC
MVCT Detector
Array
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The Geometry of
TomoTherapy
Hi-Art Control Architecture Overall
6 MV Linac
Binary MLC
85 cm Gantry Aperture
Approximately 85 cm
LINAC
GANTRY
ANGLE
40 cm MVCT FOV
HARDWARE
SYNC
5 mm X 6.1 mm X 6.1 mm
Minimum Voxel size at Isocenter
STC
MLC/JAWS
Approximately 50 cm
5 - 40 mm Selectable
Slice Thickness
64 MLC Beamlets - 6.25 mm
average width at isocenter
COUCH
DETECTOR
ARRAY
Operator
Workstation/
Console
OBC
SLIP
RINGS
MVCT Detector System
DRS
Continuous Rotation
To
External
Data
Server
Courtesy of Tomotherapy, Inc.
Hi-Art Interlock Sub-system
Hi-Art Beam Control (IMRT)
LINAC
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“HARDWARE” I/L
STC
HIGH
VOLTAGE
ENABLE
GANTRY
DOOR
I/L
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S.W. SAFETY
TASK
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OBC
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Operator
Workstation
/Status
Console
COMMUNICATIONS
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 is
dephased/non-coincident with RF.
COUCH
Hi-Art Beam Control (IMRT)
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Beam delivery is hardware-synchronized pulseby-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
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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 Systems
Calvin J. Huntzinger of Varian Medical
Systems
David C.Murray of Tomotherapy Inc.
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Thank you!
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