AAPM Summer School on IMRT Colorado Springs 2003
Commissioning and Quality Assurance
For IMRT Treatment Planning
M.B. Sharpe, Ph.D.
Department of Radiation Oncology
Princess Margaret Hospital
University of Toronto
Toronto, Ontario, CANADA
M5G 2M9
michael.sharpe@rmp.uhn.on.ca
IMRT: A Clinical Perspective
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Unique power to create and manipulate dose gradients
Detailed quantitative treatment objectives
Collimator movement during treatment.
Increased number of treatment monitor units
Need to control and determine treatment margins
through objective measurement of set-up uncertainties
internal organ motion.
All aspects of the radiotherapy process should be reexamined under more stringent requirements for
accuracy and precision.
AAPM Summer School 2003
IMRT: A Technical Perspective
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IMRT is an extension of “3D Conformal” practices.
Existing recommendations for RTP QA are
applicable.
Quality of IMRT/RTP plans depends on “up- and
down- stream” quality processes; i.e., QA of CT, MR,
as well as R&V, accelerator.
QA procedures for planning are linked closely with
commissioning, where base-line performance criteria
are established.
AAPM Summer School 2003
Commissioning and QA
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Information management (networking)
MLC: alignment, position, leaf speed, leaf leakage
Small-field dosimetry
Beam stability for short irradiations
Because of inter-play between dose rate and leaf
velocity, tolerances are stricter for dynamic MLC.
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Disease-specific commissioning
• feasibility studies
• inter- & intra- fraction setup variation & organ motion, and
potential interplay with multi-segments delivery
• procedure validation
AAPM Summer School 2003
Commissioning and QA
• ISO Précis:
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“Everything required to make
everything right”.
• Components:
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General validation
Procedure validation
Routine checks (Monitor for
change/deviations)
• Confidence comes from
evidence.
• Errors often come from
unforeseen events.
AAPM Summer School 2003
RTP Commissioning & QA
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Accept hardware and software.
Measure and enter basic dosimetric data, machine
geometry, other operational parameters.
Tune algorithm for best performance in anticipated
clinical situations.
Verify dose algorithms and associated configuration
parameters.
Configure import interfaces for imaging systems.
Verify image quality and geometry after data transfer.
Configure export interfaces to treatment machines
Learn how to interact with the system and apply it to
clinical cases.
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IMRT Planning System QA
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Geometry
• Image and Structure Import
• Multiplanar reconstructions
• Beam and DRR geometry, display, and export
Image Segmentation
• 3-D display
• Automatic tools - auto-contouring,auto-margin, etc
• Dose volume histograms (DVH)
Dose Calculation Algorithms
Plan Evaluation Tools
AAPM Summer School 2003
Image and Structure Import
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Vendor-specific (proprietary) interfaces have
given way to the DICOM Standard.
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DICOM main capabilities, including:
• Network Information Transfer
• Open Media Interchange
• Integration within the Healthcare Environment
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Open standards should be endorsed, used, and
reinforced.
AAPM Summer School 2003
Image and Structure Import
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IMRT is driven by volumetric segmentation.
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Greater need to assure diligent care and accuracy of
the segmented structures.
Segmentation guidelines outlined by TG 53
The interaction of segmentation with the processes
of IMRT depends on
• staff awareness,
• adequate time
• Resources for adherence to clinical protocols
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Patient Orientation
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Evaluating Transfer of Contours
Images may (or may not) be manipulated,
e.g.,
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Contours and IMRT
resample on import into RTP system.
Slices reordered or mirrored.
Examine effect of slice thickness and image
resolution on target volumes and their effects
on contour position.
Potential error in the position of the isocentre
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Slice Thickness & Image Resolution
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Image & Contour Import QA
CT Slice Thickness 2.5mm, DFOV 25 cm, 5 mm phantom grid
Phantom with 5 mm grid
spacing
CT scan with progressively
larger Display Field of View
Draw contours to coincide
with the grid.
Transfer images and
contours from Virtual
Simulator to RTP system.
Measure mismatch of the
contour with the image grid.
After DicomRT transfer
CT Sim
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Image & Contour Import QA
CT Slice Thickness 2.5mm, DFOV 50 cm, 5 mm phantom grid
Image & Contour Import QA
DFOV cm
25
40
Area cm2
Target 1
6.57
6.61
6.45
6.6
Area cm2
Target 2
6.12
6.10
6.08
6.2
Area cm2
Ext. cont.
150.16
152.03
151.03
154
Contour
Shift mm
<1
1
2
0
After DicomRT transfer
CT Sim
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Calculated
value
50
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Van Dyk & Craig
Auto Segmentation: External Surface
Beam Geometry Phantoms
Dependence of Segmentation
on Auto threshold values.
Threshold
values
Contour
Area cm2
From
Image
Actual
-150
150.56
6.92
7
-500
152.54
6.97
7
-800
153.57
6.99
7
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Isocentre
depth cm
Rotatable component
• Beam geometry
• Gantry rotation
• Couch rotation
Body component
• Quantify volumes
• Contour expansion
• CT number conversion
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Reconstructed Image Verification
e
bl
Ta
m
n
io
ot
Oblique CT reconstruction
Digitally reconstructed radiograph
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CT HU’s to Electron Density Conversion
Dose Calculation Algorithms
Needs:
System A
System B
System C
Battista and Bronskill
2000
1500
General
Flexible
Accurate
Fast
CT Number
1000
500
0
-500
-1000
0.0
0.5
1.0
1.5
Relative Electron Density
2.0
2.5
electron density phantom
AAPM Summer School 2003
• Relate dose calculation in patient to beam
calibration conditions
AAPM Summer School 2003
Dose Calculation Methods
Data measured in water
Dose: Convolution/Superposition
D( x , y, z ) =
∫ ∫ ∫ Φ ( x' , y' , z')K( x − x' , y − y' , z − z')dxdydz
Parameterize water data
Reconstitute water data
Calculate corrections to
water data
Calculate dose directly
based on beam and
phantom configurations
“Correction” based methods “Model” based methods
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Dose: Convolution/Superposition
D( x , y , z ) =
A Simple Algorithm Check
∫ ∫ ∫ Φ ( x' , y' , z)K ( x − x' , y − y')dxdy
• 20 X 20 cm2 field, 18MV
• 50 X 50 X 50cm3 water phantom
• 200cGy to 22cm depth
z
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Introduce air inhomogeneities,
1cm wide mediastinum, 2cm surface layer
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Contour correction: 1cm2 wide “spike”
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Contour correction: 25cm2 wide “spike”
AAPM Summer School 2003
A Simple Algorithm Check: MU’s
System A
System B
homo/hetero
homo/hetero
242.7 / 242.0
244 / 244
246.8 / 260.7
244 / 244
321.7 / 321.0
244 / 244
279.7 / 278.8
244 / 244
Dose Computation: Why Model Based?
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IMRT = Σ small fields
Dose = function(penumbra+leakage+head scatter)
Need accurate treatment head model to get this right
IMRT
IMRT beam profile
AAPM Summer School 2003
XRT
XRT beam profile
AAPM Summer School 2003
IMRT Can Include Small Fields
IMRT Can Include Small Fields
Intensity Profile
possibly unconstrained
intensity levels
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Accuracy of dose model
at small field sizes is a
consideration
Convolution-superposition
or Monte Carlo desirable
Measurement
Conventional RTP
Reconstituted from
Beam Segments
Intensity Grouping
includes MLC constraints
limit delivery to a few
discrete intensity levels
AAPM Summer School 2003
IMRT
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Boyer et al,
al, Med. Phys. 24: 757
Extend the Beam Model
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X-Ray Field Edge
Minimum Equivalent Square
• XRT: ~4x4cm2
• IMRT: ~0.5x1cm2 - 2x2cm2
How to get small field data?
• Direct measurement – difficult, errors
• Extrapolation of existing data – assumes an underlying
X-Ray Field
Edge = 1 HVL
physical model
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Either case will have uncertainty; evaluate its impact
on a clinical IMRT distribution generated by the
inverse planning system
AAPM Summer School 2003
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Boyer et al,
al, Med. Phys. 24: 757
Light and X-Ray Field Edges
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Curved Leaf Face Correction
Without correction
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Curved Leaf Face Correction
With correction
Density Scaling Approximation
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terma and kernel are computed for water and scaled
by the average density computed along raylines.
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Calculated Data
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Inhomogeneity Corrections
Woo, Cunningham et al 1990
Sharpe, Battista 1996, Superposition
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White et al (1996)
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Dose Calculation
Dose Calculation: Primary Rays Only
Beam j
φkj represents intensity of
ray k from beam j.
1
Primary
only
φk-1j
φkj
M
di
3D
scatter
Number of step&shoot fields >120
for tolerance level = 7%
φk+1j
AAPM Summer School 2003
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Dose Calculation: Primary + Scatter
Dose Algorithms Continue to Evolve
Monte
Carlo
Number of step&shoot fields <50
for tolerance level = 5%
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Dose Algorithms and IMRT
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5 mm
1
3
10 mm
Actual
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Big
Large
Small
Big
Large
100
80
60
40
20
0
2
Small
120
Medium
3 mm
Medium
140
1 mm
dose computation and dose
evaluation.
The volume chamber is
mounted sagittally, replacing
the grid in Lucy phantom.
Scans performed with
varying slice thickness and
volumes are contoured and
compared with actual
volumes
Big
• Interplay of imaging and
Large
Effect of Slice Thickness
Small
Effect of Slice Thickness
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Contours are visualized and used for dose
scoring.
For IMRT, fractional organ volumes are part
of the prescription.
Verify volume reporting.
Medium
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Dose algorithms use
during optimization may
(or may not) be the
same in different modes
or modules, e.g.:
• 3DCRT
• IMRT
• Post-IMRT evaluation.
IMRT is still a maturing
technology
Normalized Volume
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Plan Evaluation
CT
Small volume
1.75
MR
CADPLAN
V o lu m e D e lin e a tio n M o d a lity
Big volume
Large volume
12.25
126.75
Medium volume
5.25
Actual
volume in cc
Slice
thickness in mm
AcQSim CT
5
10
1
3
5
10
1
3
5
10
1
3
5
10
1.74
1.8
1.79
2.08
5.53
5.48
5.72
6.46
12.0
12.6
12.6
12.8
125.7
124.7
122.9
132.5
AcQSim
MR
Cadplan
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1.75
2.09
2.38
-
5.04
5.77
6.94
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12.1
13.6
13.5
-
124.4
124.5
145.3
1.92
1.84
1.76
1.17
5.82
5.26
4.96
4.46
13.8
12.1
12.2
12.2
-
132.0
128.1
123.6
1
3
AAPM Summer School 2003
Van Dyk & Craig
Effect of Slice Thickness
Dose Volume Histograms
• MRI scans performed for one patient using three slice thicknesses.
• Volumes contoured for each of the scans.
• An IMRT plan generated using 3mm slice thickness
• Re-calculated for other image sets.
Critical Structures in Head and Neck Treatment
100
100
3mm
6mm
12mm
PTV
60
Nodal Bed
40
20
0
40
Spinal Cord
80
% Volume
% Volume
80
45
50
55
60
Dose (Gy)
65
70
75
3mm
6mm
12mm
60
Calculated
Measured
40
20
0
0
60
20
40
60
80
100
120
Dose (%)
40 Rt.Parotid
20
AAPM Summer School 2003
80
Volume (%)
Target Structures for Head and Neck Treatment
100
0
Lt.Parotid
0
10
20
30
Dose (Gy)
40
50
60
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Volume Analysis for DVH
Polystyrene Cylinder
Plan Evaluation Tools for IMRT
Polystyrene Cube
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150
140
130
120
110
2
1.5
1
0.5
0
A
B
C
A
D
Lucite Cube
B
C
D
Air Wedge
60
40
20
40
30
20
10
0
B
C
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D
B
C
visualization
Check for simultaneous DVH tally
Behavior may depend on organ
“priority”
System may allow overlap, and
attempt to resolve competing
objectives in overlapping
Region of
structures
overlap
IMRT Commissioning:
Optimization
Manual Weights
RT
25.0
AP
25.0
LT
25.0
PA
25.0
Preserve simple symmetry
overcome contour variations
deal with skin “flash”
inhomogeneity corrections
AAPM Summer School 2003
IMRT Commissioning:
Optimization
AAPM Summer School 2003
OAR
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Test inverse-planning algorithms
Test MLC sequencing
Can the system be driven to sensible
solutions?
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PTV
PTV
D
Maximum variation +42% to -44%
Optimization
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0
A
Points belonging two
structures:
• Check if permitted
• Check for simultaneous
IMRT Weights
RT
24.52
AP
25.48
LT
24.83
PA
25.16
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Unopposed Fields
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(Van Esch et al, 2002).
Contour Correction, Single Field
Deliver uniform dose to a plane
Dose Ratio P1 /P2
OPEN: 0.76
20: 0.68
TAR10
IMRT intensity P1 /P2 =0.73
Heterogeneities and Surfaces
(a)
(b)
a)
b)
c)
d)
20cm
Different doses
Equal dose
Contour variation
Heterogeneities
20cm
10cm
P1
P2
(c)
(d)
target regions for uniform dose
air or cork heterogeneities
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(Van Esch et al, 2002).
Equal depth, Different doses
Routine QA
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Frequency of RTP QA
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Summary
• Assess software upgrades and
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patches.
Perform QA prior to release of any
software
Refer to AAPM TG53 and TG40
• When developing and validating
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techniques, patient-specific QA is
required.
Learn what is necessary from
patient-specific QA, and
generalize.
AAPM Summer School 2003
Monitor software and hardware modifications at all
stages (CT, MR, CT-Sim, RTP, R&V, Linac).
Verify patient geometry routinely
Identify several cases of increasing complexity as
“standards”.
Capture base-line output during commissioning.
Repeat plans “de-novo” at regular intervals, and post
software modification.
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DICOM dose not guarantee problem-free image and
structure transfer.
Calculation and evaluation of dose-volume objectives
depend on image slice thickness and resolution.
Specify and monitor protocols to assure consistent
image data from all sources.
Dose calculations continue to evolve, with increasing
accuracy, but may differ for regular and IMRT
treatment planning.
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Summary
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It is difficult to “decouple” all components of
IMRT planning software.
IMRT is a technical evolution.
The dependence of IMRT on segmentation
requires adherence to clinical protocols
Commission planning system, and validate
each treatment planning procedure.
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