44th Annual Meeting of the AAPM, July 15th, 2002
CE-IMRT 1: Quality Assurance of
IMRT Delivery Systems - Elekta
Acknowledgements
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M.B. Sharpe, Ph.D.
Department of Radiation Oncology
William Beaumont Hospital
Royal Oak, MI, 48073-6769
Kevin Brown – Elekta Oncology Systems
Geoff Budgell – Christie Hospital
Ted Thorson – Elekta Oncology Systems
Peter Williams – Christie Hospital
John Wong – William Beaumont Hosptial
msharpe@beaumont.edu
William Beaumont Hospital is customer and collaborator of
ADAC Laboratories and Elekta Oncology Systems
MBS June 4th, 2002
Outline
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Introduction
Introduction
Anatomy of an Elekta Accelerator and MLC
MLC Calibration
Beam Stability
Dosimetry of Small Fields
Summary
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Step-and-shoot approach with MLC.
Non-uniform intensity profiles generated with
a series of discrete, uniform beam segments.
MLC leaves are stationary when beam is on.
Beam is off when leaves are moving.
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Introduction
Elekta Linear Accelerators
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Considerations:
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Geometric calibration and constraints of MLC.
Beam stability for short irradiation times (few MUs).
Dosimetry of small fields (i.e., as small as 1x1 cm2).
Small fields offset from central axis.
Traveling wave accelerator
Mounted on drum structure.
Magnetron RF supply.
Diode electron gun: no grid.
Integrated MLC
Fewer segments and MLC backup jaws allow less
concern for inter-leaf leakage, tongue & groove
effects, and curved leaf face.
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Fast Tuning Magnetron (FasTraq)
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New magnetron with
instantaneous
frequency capture
Reduces initial start up
and inter segment times
Standard on new
machines
Available as upgrade
Elekta MLC
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40 leaf pairs (10mm pitch)
Replaces upper jaws
Backup jaws (30mm W)
40 x 40 cm2 field size
Total of 32.5 cm leaf travel
12.5 cm travel over central axis
Target
Primary
collimator
Filter
Ion chamber
W edge
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Opposing leaf-pairs do not touch
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Optical tracking (real time).
Integral auto-wedge
Reflector
K
Leaf
Leaf
Y back-up
diaphragm
X Diaphragms
Adaptor Ring
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Optical Leaf Tracking System
Elekta MLC
End View
Side View
direction of
leaf motion
ILLUMINATION SYSTEM
Reflected Light
ILLUMINATION SYSTEM
Incident Light
Camera
Camera
Camera
Camera
75/25Beam
75/25Beam Splitter
Splitter
75/25Beam
75/25Beam Splitter
Splitter
Wedge
Wedge
Mirror
Mirror
Adjustable
Adjustable
Mirror
Mirror
Adjustable
Adjustable
Mirror
Mirror
Projector
Projector
Assembly
Assembly
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Optical Leaf Tracking System
Wedge
Wedge
Mirror
Mirror
Adjustable
Adjustable
Mirror
Mirror
Adjustable
Adjustable
Mirror
Mirror
Projector
Projector
Assembly
Assembly
Leafbank
Leafbank
Leafbank
Leafbank
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Minimum Leaf Gap Requirement
Desired Field
MLC Conformation
central
axis
“Flag pole”
(10-12% transmission)
Area to be exposed
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MLC
X-Diaphram
Y-Diaphram
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Considerations:
0.08
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0.06
0.04
Series1
Series2
0.02
Series3
40
37
34
31
28
25
22
19
16
13
7
0.00
10
Deviation (cm)
MLC calibration
MLC reproducibility
• Beam stability for short irradiation times (few MUs).
• Small fields (i.e., as small as 1x1 cm2).
• Fields offset from central axis.
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Repositioning accuracy
1
IMRT QA and Commissioning
-0.02
-0.04
Leaf number
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An Clinically “Accepted” MLC
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0.5 mm Gap: -4.56%
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1mm Gap: -9.01%
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No Gap, No Overlap
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Beam Symmetry and Flatness
Beam Stability: Dose Rate
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With step-and-shoot delivery, there is the
potential for short irradiation times (MUs).
Dose rate stability influences the treatment
precision.
Measure dose per MU versus total MU.
Check short, and long term stability.
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For > 2MU, dose rate is within +/-2% (2s).
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Angle
Change
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1.04
1.01
1.000
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0.998
0.996
0.994
Stability of flatness and symmetry affects dose
rate for small fields directed off the central axis.
For an open 20x20cm2 field, measure profiles
for irradiations ranging from 1 to 100 MU.
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0.992
0.990
100 MU/min
200 MU/min
0.988
1.00
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1.002
Output factor
1.02
Beam Stability: Flatness, Symmetry
Fast Tuning Magnetron
SL20 - 18MV
Initial
2 weeks later
6 months later,
(new monitor chamber)
0.986
400 MU/min
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Sun Nuclear Profiler (46 diodes, 10 profiles/sec).
Flatness is +/-3% if more than 5MU delivered.
Symmetry +/-3% if more than 4MU delivered.
0.984
1
0.99
1
10
MU
100
10
100
1000
Number of MU
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Beam Stability: Flatness, Symmetry
Symmetry
0.10
5
Flatness
vs time (s)
after beam on
3
2
GUN-TARGET DIRECTION
CROSS-PLANE DIRECTION
0.05
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1
0
0.00
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0.16 0.32 0.48 0.64 0.8 0.96 1.12 1.28 1.44 1.6 1.76 1.92 sec
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0.16 0.32 0.48 0.64 0.8 0.96 1.12 1.28 1.44 1.6 1.76 1.92
1.8
-0.05
0.10
Flatness
Relative Dose Rate
Energy
Change
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Beam Stability: Dose Rate
1.03
Position
Change
1.6
1.4
Symmetry
vs time (s)
after beam on
0.05
1.2
1
0.8
0.6
0.00
0.4
0.2
-0.05
1
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10
Total MUs Delivered
100
0
sec
4
Relative Output in Water (6MV)
Relative Dose Rate vs Field Size
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Collimation of head scatter
affects dose rate “in-air”.
Dose rate in vivo is further
affected by photon scatter
and e- transport.
Beam weights calculated
by inverse planning must
be adjusted to account for
head scatter.
Courtesy of
Wolfram Laub
head scatter
Relative Output Factor
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1.1
1.0
0.9
phantom scatter & e-- transport
0.8
0.7
"In-Air"
In-Phantom
0.6
0.5
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2
4
6
8
10
Side Length of Square [cm]
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Output Factors for Small Fields
Output Factors for Small Fields
1.1
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SL20 -18MV
10x10cm 2
Profiles measured in water (10cm depth) using a
pinpoint ion chamber (0.015cm3).
Calculated with pencil-beam convolution and
extrafocal source model.
Verified radiation field size and offsets.
Calculations & measurement agree to within 1% on
average, 2.5% max.
1.0
Relative Dose Rate
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0.9
5x5cm 2
0.8
2x2cm 2
0.7
MEASURED: GUN-TARGET PLANE
MEASURED: CROSS PLANE
CALCULATED
0.6
2
1x1cm 2
0.5
-7.5
-5.0
-2.5
0.0
2.5
5.0
7.5
Distance Off-Axis [cm]
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Typically measure in water
(10cm depth) using a pinpoint
ion chamber (0.015cm3) and
beam scanning system
Scan profiles to verify leaf
positioning radiation field size
and offsets
%Change in Relative Ouput
Output Factors for Small Fields
20
6MV
10
0
Nominal Field Size
1cm
2cm
5cm
10cm
-10
-20
-3
(a)
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IMRT QA with EPID (iViewGT)
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-1
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Flat-panel imager
41cm2 a-Si panel
26cm2 field at isocentre
15cm offset in all quadrants
16 bit gray scale resolution
3 frames per sec readout
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Primary function is to verify
patient position
Deviation from Field Size [mm]
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iViewGT: IMRT QA Possibilities
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Small MU
verification
Dosimetry QA
QA beam flatness and
symmetry
MLC calibration
QA leaf position
accuracy
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40 mu
___ 20x2 MU
integrated
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Small MU
verification
Courtesy of Geoff Budgel, Ph.D.
MLC Calibration
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Extremely time-consuming using film,
especially if adjusting gain & offset.
Possibly requires tighter tolerances / more
frequent checking for IMRT
iViewGT has 0.25 mm resolution at isocentre
– ought to be sufficient for MLC calibration
20x2 mu
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Courtesy of Geoff Budgel, Ph.D.
MLC gain and offset calibration
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IMRT requires 10 cm over-travel measurements for both leaf banks
and jaws
Requires accurate definition of central axis of rotation
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Matched step & shoot fields
Film (40x32cm)
iViewGT (24x24cm)
Central
24 leaves
-12 cm -10 cm
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12 cm
10 cm
Courtesy of Geoff Budgel, Ph.D.
Courtesy of Geoff Budgel, Ph.D.
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Spot the errors
Subtraction images
0.5 mm
0.5 mm
1 mm
0.3 mm
1 mm
0.3 mm
0.7 mm
0.3 mm
0.7 mm
0.3 mm
0.5 mm
0.5 mm
0.2 mm
0.2 mm
1 mm
0.7 mm
Courtesy of Geoff Budgel, Ph.D.
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Effect of set-up on error detection
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No positional change
Re-set to same position
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Head angle 0.7 deg
Courtesy of Geoff Budgel, Ph.D.
Conclusions
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No
errors
1 mm
0.7 mm
Low MU images and template matching
enable easy patient position verification
Require access to raw images for dosimetry
applications
Looks promising for QC applications and
MLC calibration
With
errors
No positional change
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Re-set to same position
Head angle 0.7 deg
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Courtesy of Geoff Budgel, Ph.D.
Summary
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Commissioning:
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Quality Assurance:
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Range of clinical operation to 1x1 cm2 fields, and
1-2 MU irradiations
Verify beam stability for short exposures
Enhance monthly QA to test MLC operation,
accuracy and precision.
A-Si EPID: Intergated QA device.
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