Outline
Dosimetry Metrology for
IMRT
Jean M. Moran, Ph.D.
•
•
•
•
•
Acceptance testing
Detectors for commissioning
Phantoms
Dosimetry analysis tools
Commissioning tests
– Varying complexity and geometry
AAPM Summer School
June 2003
• Potential Pitfalls
• Summary
The Department of Radiation Oncology
University of Michigan
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Acceptance
Collimator Designs
Exit Window
• Prior to purchase and installation,
review manufacturer’s acceptance
tests
• If needed, adapt tests and tolerances
with manufacturer in purchase order
• Test basic functionality of equipment
• Tests may be dependent on the MLC
design
Exit Window
Upper
Jaws
MLC
Diaphragm
Upper
Jaws
Lower
Jaws
MLC
Lower
Jaws
MLC
Isocenter
Isocenter
Elekta
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Exit Window
Isocenter
Siemens
Varian
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Physical MLC Characteristics
Field
Elekta
Siemens
Varian
40x40 cm 2
40x40 cm 2
(40x27 cm 2)
40x40 cm 2
Rounded
Divergent
Rounded
Width
Leaf ends
Effect of Rounded Leaf Ends & Linear Motion
Linear leaf motion
Uncorrected
Corrected
Cold
Xmlc
Leaf width
1 cm
1 and 6.5 cm
0.5 and 1 cm
Length
32.5 cm
31 cm
16 cm (14.5
cm Carriage)
Inter -
No
No
Yes
digitation
=1 cm gap
Uniform
Correction
Hot
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Xlight
Xrad
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Graves 2001
Leaf position offset = Xrad -Xlight
1
MLC Transmission
Static MLC Testing
• Carriage skew
– Orientation of MLC carriages in the accelerator
head
– Determined by measurement with feeler
gauges for very small field size
– Important to prevent collisions especially for
systems with interdigitation of leaves
– Verify with film
• Dependent on
system design
• Values range from
1.5 to 3% for midleaf
and interleaf
transmission
• Alignment of each leaf bank to central axis
• Leaf position reproducibility and accuracy
Film exposed under closed leaves
Evaluate compared to open field
– Verify with graph paper
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Tongue and Groove Effect
Uniform Square Field Illustration of TG Effect
Caused by overlap of leaves and leaf design
?
Area in Red Never
Directly Exposed
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+
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Cold Spots from T&G Effect
~25% cold spot where
leaves overlap
70
square field
small leaves
60
large leaves
Dose (cGy)
50
40
30
20
10
0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Position (cm)
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1.0
1.5
2.0
2.5
DMLC and/or SMLC Mode Tests
• Leaf speed
• Dose rate evaluation
• Leaf position tolerance and
reproducibility
• Leaf acceleration and deceleration
• Rounded leaf tip transmission
• Beam stability (output, flatness,
symmetry, linearity)
• Interrupted treatments
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2
Equipment and Software for IMRT
Commissioning
Example Test: FenceTest
Multiple incorrect leaf positions
• Ideal measurement system:
• All leaves move
across field and
deliver dose to
small field gap (0.1
or 0.2 cm)
• Sensitive to errors
of 1 mm in leaf
position
– Excellent spatial resolution, accuracy, tissue
equivalent response, provide 3-D data,
portable to multiple phantoms, and easy to use
• Multiple detectors required
– Verifying IMRT fields is more complex than
static fields
– Profiles along major axes in a water phantom
are insufficient for characterizing systems
– Profiles are also time-consuming to obtain in
water phantom for IMRT delivery
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Equipment
Detector Characterization
• Linearity
• Depth dose and profiles in water
– Ion chamber – required for absolute dose
measurements
– Ion chamber or linear diode array
– Note: Depth dose curve measurements must
be made with a repeat delivery for each
datapoint on the curve (delivery stability)
• Detector arrays are useful for profile
measurements in water but 2-D detectors
are still required
• 2-D measurement methods
• Energy dependence
• Stem and cable effects
• Angular response
• Calibrated if for absolute
measurements
• Small field detectors
required for small field
characterization
– Sensitive to position
– Detector should be smaller
than homogeneous region of
dose to be measured
– Film
– Detector arrays
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1-D Detector Characteristics
Detector
1-D Detectors
Measurement
Volume (cm 3)
Sensitive
Area
(cm2 )
Diameter
(cm)
Thickness
(cm)
Effective Point of
Measurement (cm)
Micro -chamber
0.009
0.24
0.6
NA
p-type Si diode
0.3
0.49
0.4
Stereotactic
diode
NA
0.011
0.015
0.010
Pinpoint
chamber
Dose
(cGy)
70
60
50
40
30
20
10
DETECTOR
DISADVANTAGES
0.2
Micro -chamber
Poorer resolution than diodes
0.06
0.6
p-type Si diode
0.45
0.006
0.07
Stereotactic
diode
0.2
NA
0.06
Pinpoint
chamber
Over-respond to low energy photons
Martens et al. 2000
MOSFET
NA
0.04
NA
0.1
NA
MOSFET
Non-linear dose response for <30 cGyChuang et al 2002
Diamond
0.0019
0.056/0.073
0.73
0.026
0.1
Diamond
< resolution than diodes, expensive, Rustgi et al, Laub et al
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3
MOSFET System
MOSFET Consistency
4.0
• Excellent spatial
resolution
MOSFET1
MOSFET2
MOSFET3
3.0
2.0
• Automatic and
immediate readout
1.0
Bias Box
0.0
• Can be re-used
immediately
• Linear dose response
Reader
-1.0
-2.0
-3.0
MOSFET
• Response
independent of depth
-4.0
0
C. Chuang, UCSF
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TLDs
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2
4
6
8
10
12
14
16
Number of Measurements
18
20
Cynthia Chuang, UCSF
TLD Holder for Phantom Measurements
• TLDs must be characterized
– Time consuming
• Reusable
• Achievable accuracy: 2-3%
• Automatic reader required for multiple
TLD measurements
D.A. Low et al. “Phantoms for IMRT Dose Distribution Measurement and
Treatment Verification, Int J Radiat Oncol Biol Phys 40: 1231-1235 (1998).
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2-D Detectors
• Film
Radiographic Film
• Advantages
– Radiographic: XV and EDR
– Radiochromic
• Beam imaging system, CCD, SLIC,
AMFPI
– Excellent spatial resolution
– Readily available
– Less expensive than other 2-D systems
• Disadvantages
– Over-response to low energies
– Dependent on QA of film batch
– EPID systems attached to gantry
– Investigated more for pre-treatment QA
currently
– Dependent on processor and digitizer QA
– Sensitive to storage conditions
– Need to measure the response to dose for
each experiment
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4
XV vs EDR Film
Depth Dependence of EDR Film
6MV-EDR2-Depth corrected
6MV-EDR2-Regular
3.5
1400
Optical Density
Net Optical Density
3.0
2.5
2.0
XV2, 6 MV
1.5
XV2, 15 MV
1.0
1000
800
600
400
EDR2, 6 MV
200
EDR2, 15 MV
0.5
0
0
0.0
0.0
100.0
200.0
300.0
400.0
500.0
Chetty and Charland “Investigation of Kodak EDR film for
megavoltage photon beam dosimetry” PMB 47: 3629-3641 (2002).
Radiochromic Film: Advantages
• Decreased sensitivity to low energy
photons compared to radiographic
film
• No processing
– Film changes color with irradiation
• Insensitivity to visible light
• High spatial resolution
Niroomand-Rad et al. “Radiochromic film dosimetry: recommendations of
AAPM Radiation Therapy Committee Task Group 55,” Med Phys 25: 2093-2115.
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50
100 150
200 250 300 350
Dose (cGy)
600.0
Dose (cGy)
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1200
Dogan et al “Comparative evaluation of Kodak EDR2 and XV2 films for verification
of intensity modulated radiation therapy,” PMB 47: 4121-4130 (2002).
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Radiochromic Film: Disadvantages
• Non-uniform film response
– Can be minimized by using double-exposure
technique
• Response dependent on time and
temperature
• More expensive than radiographic film
• Digitizer response is dependent on the
light source of the digitizer and may need
to be modified
Niroomand-Rad et al. “Radiochromic film dosimetry: recommendations of
AAPM Radiation Therapy Committee Task Group 55,” Med Phys 25: 2093-2115.
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Active Matrix Flat Panel Dosimeter
(AMFPD)
Example: SMLC Delivery
AMFPD
Film
512 x 512 pixels
0.508 mm pitch
26.0 x 26.0 cm2
a-Si array
Direct Detection
-No fluorescent screen
Preamplifiers
AD converters
Non-commercial system
AMPFD at 10 cm depth in solid water
El-Mohri, et al. “Relative dosimetry using active matrix flat-panel
imager (AMFPI) technology,” Medical Physics 26, 1999.
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184 mu
600 mu/min
d=10 cm
5
Gel Dosimetry: Advantages
Example: SMLC Delivery
4
Film
AMFPD
160
120
80
40
0
184 mu
600 mu/min
d=10 cm
Position (cm)
2
0
150
130
-2
20 30 50 70
80
100 120
-4
-4
-2
0
2
• Obtain 3-D information in one
irradiation
• Gels can be prepared with different
density therefore ideal for
heterogeneous measurements
• Gel can be used in containers of
different shapes
• Ideal for anthropomorphic phantoms
4
Position (cm)
B4
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Gel Dosimetry: Disadvantages
• Sensitive to many factors such as time,
preparation, temperature
• Optical reader requires cylindrical container
for gel
• MR time is often limited and expensive
(unless dedicated scanner)
– Long scan times are required to increase
accuracy of readout
– E.g. 5% accuracy over 10 hr scan time (Gum et al
2002)
• Interface of gel and container results in less
accurate readout at edges of the gel
• Not ready for routine use in the community
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Gel Dosimetry
MR Dose Maps IMRT Plan
Slice 1
Slice 2
Slice 3
Dose %
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Dose
Deviation %
0 20 40 60 80 100 120
-60 -40 -20 0 30 40 60
Gum et al. “Preliminary study on the use of an inhomogeneous anthropomorphic
Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT
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treatment
Considerations for Phantoms
• Fiducials for reproducible setup of
phantom and detectors
• Flexibility to accommodate different
detectors
• Simple vs. anthropomorphic
• Homogeneous or heterogeneous
MR Dose Maps
- IMRT Plan
Water Tank
• Restricted to gantry at 0 degrees
– Unless mylar window for 90 degrees
• Flexibility in chamber position
• Important for basic depth dose and
profile measurements
• Output, flatness, symmetry, and
linearity assessment
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6
Water-equivalent Plastics
Simple Geometric Phantom
• Flat phantoms with custom chamber
inserts
20 cm
– Accommodate film at multiple depths
– Detector position only varies with depth
• Cylindrical phantoms (plastic or water
filled)
– Ion chamber at single position
– Possibly hold films
• Can be customized to hold various
detectors and have multiple positions
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Van Esch et al. “Acceptance tests and QC procedures for the clinical implementation of
IMRT using inverse planning and the sliding window technique: experience from five
radiotherapy departments,” Radiother Oncol 65: 53- 70 (2002).
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Cylindrical Phantom:
Ion Chamber and MOSFETs
IMRT Verification Phantom
Calc. 1.37 Gy
Meas. 1.42 Gy
Diff –3.52%
D.A. Low et al. “Phantoms for IMRT Dose Distribution Measurement and
Treatment Verification, Int J Radiat Oncol Biol Phys 40: 1231-1235 (1998).
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C. Chuang, UCSF
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Spiral Phantom for Dosimetric Verification
Calc. 0.81 Gy
Meas. 0.78 Gy
Diff –3.45%
RPC Head Phantom
Target Volumes
Removable Dry
Insert
Water
Water
Critical Structure
Paliwal et al “A spiral phantom for IMRT and tomotherapy treatment
delivery verification” Medical Physics pp. 2503-2507 (2000).
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TLDs in Target Volumes
Radiochromic film through multiple plans
Delivery is required by RTOG for participation in IMRT trials
7
Anthropomorphic Phantom for Gel Dosimetry
Dosimetry Analysis Software
• Transfer patient fluence maps and beam
geometry to phantom geometry
• 2-D dose difference displays with
colorwash
• DVHs
• Highlight of differences
• Gamma analysis
– % agreement and distance-to-agreement
Gum et al. “Preliminary study on the use of an inhomogeneous anthropomorphic
Fricke gel phantom and 3D magnetic resonance dosimetry for verification of IMRT
treatment plans” PMB 47: N67-N77 (2002).
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Dosimetric Analysis:
Overlay of Isodose Lines
Phantom Measurements
• Calculate IMRT plan for each field in
measurement geometry
• Film measurement at depth for individual
field at gantry=0
70
60
50
20
10
70 cGy
60 cGy
50 cGy
20 cGy
10 cGy
Film at depth in
water equivalent
plastic
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Dosimetric Analysis:
Dose Difference Display
Dosimetric Analysis : Dose Color Wash
Calculation
Individual Field
Film Data
Individual Field
Dose
(cGy)
70
60
50
40
30
20
10
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Calcs
Film
Intensity Map
Calcs
Film
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cGy
cGy
cGy
cGy
cGy
Intensity Map
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+/- 10%
Differences due to tongue -and-groove
8
Dosimetric Analysis:
Dose Difference and DTA
Dosimetric Analysis: Gamma Evaluation
Dose
Reference
Both
3%/3 mm
5%/5 mm
Evaluated
Unmod
Spatial
3% Dose
Difference
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3 mm
DTA
D. A. Low – Washington University
D. A. Low – Washington University
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Simple Geometry Tests
• Leaf position reproducibility in
dynamic or step-and-shoot mode
• Effect of gravity on leaf position
accuracy and reproducibility
• Sweeping gap test
• Fence test
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Simple Geometry Tests
• Leaf speed stability
• Leaf acceleration/deceleration
• Output checks for small to large fields
– Including smallest field size – 1 x 1 cm 2
• MLC limits (field size restrictions)
• Depth dose curve measurements
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Simple Geometric Tests
Geometric Tests
L. Xing et al. “Dosimetric verification of a commercial inverse
treatment planning system” PMB 44: 463-478 (1999).
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9
Geometric Tests
Acceptance tests and quality control (QC) procedures for the clinical
implementation of intensity modulated radiotherapy (IMRT) using inverse
planning and the sliding window technique: experience from five
radiotherapy departments Van Esch et al Radiotherapy Oncology 65: 5355
70JMM03
(2002).
Complex Cases – Simple Geometry
Evaluate dose across field as a function of regional beamlet intensity
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Complex Geometry:
Anthropomorphic phantom
Potential Pitfalls
• Limit number of monitor units per
segment to verified Linac behavior
• Limit field width for IMRT
• Overshoot-undershoot phenomenon
• Incorrect tolerance value
• Integration with record-and-verify
system
M. A. MacKenzie et al. “Dosimetric verification of inverse planned step and
shoot MLC fields from a commercial planning system,” J Appl Clin Med Phys
3: 97-109 (2002)
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Potential Pitfalls: Prior to Measurements
Detector Issues
• Verify all equipment is functioning
properly
Volume
– Film processor, digitizer
– Detectors, cables, electrometers (automatic
leakage correction)
– TLD reader, ovens
• Input/output to treatment planning system
• Standardize measurement setup when
possible
• Monitor software and hardware changes
and QA
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Dose (cGy)
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10
Detector Position
– Example: CAX of Varian MLC is a junction
of four 0.5 cm wide leaves
Beam 1 Ratio 9/21/02
1.20
Ratio
(Calculation/Measurement)
• Small ion chambers are very sensitive
to position
• Position should be considered with
respect to MLC design
Ratio Calc/Ion Chamber - CAX
Beam 1 Ratio - 9/25/02 20 seg
Beam 1Ratio 9/25/02 100 seg
Beam 2 Ratio 9/21/02
1.15
1.10
1.05
Beam 2 Ratio 9/25/02 20 seg
Beam 2 Ratio 9/25/02100 seg
1.00
0.95
0.90
regular
spike=cax spike=high spike=low
Spikiness values on part of field
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Ratio Calc/Ion Chamber -0.5, -0.5 cm from CAX
Effect of Leaf Position Offset on IMRT
Ratio (Calculation/Measurement)
SMLC Single Field 30 Fractions
1.20
Beam 1 Ratio - Off-axis - 20 seg
Beam 1 Ratio off-axis - 100 seg
Beam 2 Ratio Off-axis - 20 seg
Beam 2 Ratio Off-axis-100 seg
1.15
1.10
1.05
1.00
0.95
0.90
0.85
regular
spike=cax
spike=high
spike=low
Spikiness values on part of field
Cadman et al “Dosimetric considerations for validation of a sequential IMRT
Process with a commercial treatment planning system” PMB: 3001-3010 (2002).
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Effect of Leaf Position Offset on IMRT
Overshoot Phenomenon:
Example Varian System SMLC Delivery
Planned
No leaf offset correction
-3-12% errors
Given
With leaf offset corrected
+/- 5%
Planned
Given
Planned
Given
Cadman et al “Dosimetric considerations for validation of a sequential IMRT
Process with a commercial treatment planning system” PMB: 3001-3010 (2002).
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Ezzell and Chungbin, “The overshoot phenomenon in step -and -shoot IMRT
Delivery,” J Appl Clin Med Phys 2: 138-148 (2001).
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11
MLC Dynamic Log File (Dynalog)
System Communication Delay
Accelerator
Control Console
MLC
Workstation
Accelerator
Control
•
•
•
•
•
MLC
Controller
Varian 21 EX
Expected and actual leaf positions
Beam hold-off events
Position tolerance setting
Recorded approximately every 55 ms
• Information can be compared to imported
2-D information
~ 0.055 second
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DMLC: Effect of Incorporating Machine
Limitations
B1
Deviation in Leaf Trajectory
BEFORE
AFTER
44 mu, tolerance 0.1 mm
no gap: 137 beam hold-offs
54 mu, tolerance 0.25 mm
gap 1.1 mm: No beam hold-offs
Position (cm)
-3.0
-3.5
Expected Trajectory
Actual Trajectory
MLC Beam Hold-Off
-4.0
-4.5
-5.0
1.0
Hold0.0
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Time (sec)
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Summary
• Basic MLC characteristics should be
tested in static mode prior to IMRT testing
• IMRT tests should be specific to delivery
mode and device
• Be aware of potential issues with delivery
systems that may need further
investigation
• Multiple detectors and phantoms are
typically required for IMRT commissioning
• Quantitative dose analysis tools are
required for proper evaluation of delivery
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Case1, B1
Summary
• Measurements may show dosimetric –
mechanical differences that planning
systems may not model at this time
• Need to know the limits of the
mechanical systems and combination
of software + delivery
• Continued need to improve software
for delivery system, measurement
devices, phantoms, and dose analysis
tools
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12
Acknowledgements
University of Michigan
Dale Litzenberg
Jeff Radawski
Tim Nurushev
Benedick Fraass
Dan Low – Washington University
Cynthia Chuang - UCSF
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13