Tools for IMRT QA
N. Dogan, Ph.D
Department of Radiation Oncology
Virginia Commonwealth University
Medical College of Virginia Hospitals
Richmond, VA, USA
Objectives
• To identify the QA tasks involving
IMRT
• To describe the QA tools for all
aspects of IMRT process
• To explain the limitations of the current
IMRT QA tools
• To compare the IMRT QA tools and
techniques
N. Dogan /July 2005
N. Dogan
QA tasks for IMRT
• Machine QA- Acceptance and routine QA of
the MLC for IMRT delivery - dosimetric and
geometric characteristics
• Algorithm QA for IMRT - QA of planning
system and data consistency with machine
• Patient Specific QA – prove plan works
91D and 2D dosimetry of treatment components such
IM beams and segments
93D dosimetry of entire treatment delivery
• Post Treatment QA
• Log-file analysis
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IMRT QA Tools
• Detectors
• Phantoms
• Scanners
• Dosimetric Analysis Tools
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Detector Requirements for IMRT QA
• Geometric and dosimetric accuracy
• Volumetric simultaneously integrating dosimeter
•
•
•
•
•
to faithfully quantify the dose delivered over the
total time of treatment
Good spatial resolution, tissue equivalent
response
Ability to provide 3-D information
Portability to multiple phantoms
Ease of use
Sufficiently large dynamic range and be
insensitive to photon energy spectrum and dose
rate response which is independent of the
energy spectrum
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IMRT QA Tools
Detectors
• Many of them available for IMRT
measurements
• Necessary to characterize the detector
response for both static and dynamic fields
for linearity
• Need to be calibrated for absolute
measurements
• Need to determine stem and cable effects
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IMRT QA Tools
Detectors
• Need to determine energy dependence
and angular response
• Small field detectors required for small
field characterization
9Sensitive to position
9Detector should be smaller than
homogeneous region of dose to be
measured
• Assess electrometer response
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IMRT QA Tools
Detectors, cont.
Dose
(cGy)
70
60
50
40
30
20
10
• Need to determine necessary
resolution
9 depends on the resolution of the
beamlet grid that is used for
planning and sequencing fields for
delivery
9 Chambers with the smaller
volumes are more sensitive to
position and will have a higher
response when positioned at an
opposing leaf pair junction and
between adjacent leaves
More stable
measurement
point
Poor detector
position
Courtesy of Jean Moran, UofM
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IMRT QA Tools
1-D and 2-D Detectors
• Ion chamber (1-D)
• TLDs and MOSFETs (1-D)
• Detector arrays (2-D)
• Film (2-D)
9Radiographic
9Radiochromic
• Gels (3-D)
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IMRT QA Tools
Small 1-D Detectors
Volume
(cm3)
Diameter
(cm)
Disadvantages
Microchamber
0.009
0.6
Poorer resolution than diodes
Pinpoint
chamber
0.015
0.2
Detector
Over-respond to low energy
photons
Martens et al. 2000
p-type Si
diode
0.3
0.4
Stereotactic
diode
NA
0.45
MOSFET
NA
NA
Non-linear dose response for <30
cGy
Diamond
0.0019
0.73
< resolution than diodes, dose
rate dependence, expensive
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IMRT QA Tools
Ion Chamber
• Advantages
9Available in different shape and sizes
9Dosimetric response is well understood.
9Absolute dose measurements – theory is well
establish, they can be used as a benchmark
standard
9Easy to calibrate
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IMRT QA Tools
Ion Chamber, cont.
• Disadvantages
9Only one measurement point for each
irradiation – does not yield sufficient information
to evaluate the dose throughout the target and/or
critical structures
9Volume averaging – the measurements are to
be considered as an average throughout the
chamber’s active volume - does not yield
significant errors if the ion chamber is placed in a
low dose-gradient region even for relatively large
chambers
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IMRT QA Tools
Ion Chamber volume averaging, cont.
Micro cham: 0.009cc
PTW: 0.125cc
Farmer:0.65cc
D.A. Low et al. “Ionization chamber volume
averaging effects in dynamic intensity
modulated radiation therapy beams, Med.
Phys.30(7): 1706-1711 (2003
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IMRT QA Tools
TLDs
• Advantages
9Multiple measurement points in a single
irradiation
9Reusable
9Easy to use in multiple phantoms
9Small size and versatility in placement
9Readily available readout equipment
9Achievable accuracy: 2-3%
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IMRT QA Tools
TLDs
• Disadvantages
9Requires calibration to determine calibration
factor for each TLD chip
9Requires calibration of subset of TLD chips for
each measurement
9TLD reader response and oven temperature
should me routinely monitored to maintain
consistent TLD response
9Automatic reader recommended for IMRT field
verification due to large number of TLDs
required for verification in a plane (60 or more)
– inefficient for routine IMRT QA
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IMRT QA Tools
TLDs, cont.
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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IMRT QA Tools
MOSFET systems
• Advantages
9Excellent spatial resolution – small size (~0.04mm2)
9Multiple detectors can be irradiated simultaneously
9Automatic and immediate readout
9Can be re-used immediately
9Linear dose response > 30 cGy
9Response independent of depth
9Commercially available phantoms to accommodate
the small detectors
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IMRT QA Tools
MOSFET systems
• Disadvantages
9Decrease linearity for < 30 cGy – limited to
high dose applications
9Over-response for the phantom scatter
factor for small fields
9Specific application and measurement
conditions should be carefully assessed
and the detector should be used in the
appropriate dose range
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IMRT QA Tools
MOSFET systems
Bias Box
Reader
MOSFET
TNRD50 system
Courtesy of Cynthia Chuang, UCSF
An axial image of MOSFET
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phantom
IMRT QA Tools
MOSFET systems
MOSFET Linearity
Mosfet Consistency
1400
4.0
MOSFET1
MOSFET2
MOSFET3
MOSFET4
1200
MOSFET1
MOSFET2
MOSFET3
3.0
2.0
1000
1.0
800
0.0
-1.0
600
-2.0
400
-3.0
-4.0
200
0
2
4
6
8
10
12
14
16
18
20
Number of Measurements
0
0
100
200
300
400
Radiation (cGy)
Courtesy of Cynthia Chuang, UCSF
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IMRT QA Tools
MOSFET systems
Percent Depth Dose Comparison
1.2
Angular Dependence
Ion Chamber
MOSFET
1
2.0
1.5
Percentage (%)
1.0
0.5
0.8
0.0
-0.5
0.6
-1.0
-1.5
0.4
-2.0
-2.5
0.2
-3.0
0
0
20
40
60
80
100
120
140
160
180
Degrees
0
5
10
15
20
25
30
35
Depth (cm)
Courtesy of Cynthia Chuang, UCSF
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IMRT QA Tools
Cal. 1.64 Gy
Meas. 1.72 Gy
Diff 4.6 %
Cal. 0.70 Gy
Meas. 0.68 Gy
Diff - 2.8 %
Courtesy
of
Cynthia
Chuang,
UCSF
Calc. 2.18 Gy
Meas. 2.09 Gy
Diff –4.35%
Calc. 1.37 Gy
Meas. 1.42 Gy
Diff –3.52%
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Calc. 0.81 Gy
Meas. 0.78 Gy
Diff –3.45%
N. Dogan
Current IMRT QA Tools
2-D Detectors
• Film
9 Radiographic
9 Radiochromic
• Beam imaging system, CCD, SLIC, AMFPI
• 2-D Detector arrays
9 Diode array (Mapcheck)
9 Ion chamber
• Active matrix flat panel detector (AMFPD)
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IMRT QA Tools
Radiographic Film
• Advantages
9Readily available (XV, EDR2, …)
9Can be cut into any desired shape
9Excellent spatial resolution (<1mm)
9Less expensive than other 2-D systems
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IMRT QA Tools
Radiographic Film, cont.
• Disadvantages
9Over-response to low energy x-rays – high
atomic number of the active material – not
good for absolute dosimetry
9Dependent on QA of film batch
9Dependent on processor and digitizer
9Sensitive to storage conditions
9Need to measure the response to dose for
each experiment – H&D curve each time
9Proper normalization is critical
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Current IMRT QA Tools
Radiographic Film, cont.
• Other issues
9Store in a cool and dry place
9Make sure that the temperature for the
film processor is stable
9Film digitizer pixel spacing, integrity of
OD, beware of artifacts
9Verify spatial and optical density
accuracy
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IMRT QA Tools
Rapid Film Calibration
120
MU
240
MU
90
MU
210
MU
60
MU
180
MU
30
MU
150
MU
• Multiple dose levels per
•
•
•
film-3x3 cm2 fields of
different dose levels
Step-and-shoot or SMLC
delivery
Different dose values
required for XV and EDR2
film (15 -120MU for XV
and 30-240MU for EDR2)
Saves both time and film
Childress et al Med Phys 29(10), 2002.
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IMRT QA Tools
XV vs. EDR Film
XV
3.5
1.6
1.4
3.0
EDR
2.5
2.0
XV2, 6 MV
1.5
XV2, 15 MV
1.0
Co60-EDR2
6MV-EDR2
10MV-EDR2
18MV-EDR2
1.2
Optical Density
Net Optical Density
Depth-corrected H&D
EDR2, 6 MV
1
0.8
0.6
0.4
EDR2, 15 MV
0.5
0.2
0
0.0
0.0
100.0
200.0
300.0
400.0
500.0
600.0
Dose (cGy)
Chetty and Charland 2002
PMB 47: 3629-3641
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0
50
100
150
200
250
300
350
Dose (cGy)
Dogan et al. 2002
PMB 47: 4121-4130
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IMRT QA Tools
Depth-corrected H&D curves
XV
1.6
6MV-EDR2-Depth corrected
6MV-EDR2-Regular
EDR
1.2
1.4
Optical Density
Optical Density
1.4
1
0.8
0.6
0.4
18MV-EDR2-Depth corrected
18MV-EDR2-Regular
1.6
1.2
1
0.8
0.6
0.4
0.2
0.2
0
0
0
50
100 150
200 250
Dose (cGy)
300 350
0
50
100
150
200
250
300
350
Dose (cGy)
Dogan et al. 2002 PMB 47: 4121-4130
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IMRT QA Tools
Ion Chamber
Film- depth corrected H&D
Film regular H&D
Dogan et al. 2002 PMB
47: 4121-4130
(a)
(b)
Ion chamber and EDR2 film depth-dose curves for a) 6 x 6 cm2, b) 14 x 14 cm2 films for 10 MV
beam. Films were positioned parallel to the beam and OD to dose conversion was done using
regular and depth-corrected H&D curves.
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IMRT QA Tools
Childress et al. Med. Phys. 32(2) 2005
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IMRT QA Tools
As compared to XV film, EDR2 film
• has less dependence on the
processor, field size
• less response to low energy photons
• have better reproducibility and
agreement with ion chamber
measurements
• can be used to measure a complete
fraction of an IMRT treatment
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IMRT QA Tools
Radiographic Film: 2-D Dosimetric Measurements
Intensity map
from Opt System
Calculated
Leaf
Sequencer
Calc-Meas
Courtesy of Jean Moran, UM
Measured
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IMRT QA Tools
Radiographic Film: Routine DMLC QA
• Using radiographic films
9Intensity-modulated
pattern field
9Check leaf position,
acceleration, motion
stability
9Check for hot and cold
density
9Visual check
DMLC field 14x14 cm2
at SSD =100 cm, 2 cm
separated strips
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IMRT QA Tools
Film – Processor issues
• Should do routine maintenance and quality
•
•
•
•
assurance – verify spatial intensity,
characteristic response, noise due to large
changes in optical density ( Dempsey et al,
Med Phys, 26; 1721-1731, 1999).
Should be warmed up prior to use
Should have appropriate amount of chemicals
- Several films should be run in advance
Should have stable temperature
Should have a consistent rate of feeding into
the processor
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IMRT QA Tools
Film – Other Issues
• Accurate positioning of the film in the
phantom – for the registration with
treatment planning system
• Minimized errors by using a solid-water
slab designed for film
• Have pins between slabs that puncture the
film
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IMRT QA Tools
Radiochromic Film (RCF)
• Advantages
9No significant energy dependence –
decreased sensitivity to low-energy photons
9Insensitivity to visible light
9Very high spatial resolution - well-suited for
measurements in high-dose gradient fields
9Self-developing – no developer or fixer is
required
9Easy to handle
9Tissue equivalent
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IMRT QA Tools
Radiochromic Film, cont.
• Disadvantages
9Takes a couple of hours for the color change to
stabilize, and it may be necessary to wait up to two
days before evaluating the film
9Sensitive to the air temperature and humidity
9Ultraviolet light may cause a color change without
exposure to ionizing radiation
9Size, availability, and cost
9Non-uniform response to radiation – double exposure
technique minimizes this effect
9Issues with thermal history, wavelength dependence,
and local sensitivity of the film
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IMRT QA Tools
RCF (Gafchromic HS and MD55-2) vs radiographic
films (XV and EDR2)
O. Zeidan et al., Med. Phys., 31
(10):2730-2737 (2004)
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IMRT QA Tools
RCF profiles vs. Ion chamber
J. Dempsey et al., Med. Phys., 27 (10):2462-2475 (2000)
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IMRT QA Tools
RCF – digitizer issues
• Response of the digitizer
• Light source characteristics
• Design
Gluckman et al, Med Phys, 29(8); 1839-1846, 2002.
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IMRT QA Tools
Other 2-D systems
• Beam imaging system, CCD, SLIC,
Amorphous silicon flat panel detector
(AMFPD)
9 EPID systems attached to gantry
9 Investigated more for pre-treatment QA currently
• 2-D Detector arrays
9 Diode array (e.g; MapCheck)
9 Ion chamber (e.g; LA48 linear array)
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IMRT QA Tools
EPID Systems
•
•
•
•
Charged coupled device (CCD) camera systems
Scanning liquid ion chambers (SLICs)
Amorphous silicon flat panel detector (AMFPD)
Active matrix flat panel imagers (AMFPIs)
Patient or
Phantom
Transit Dosimetry
Pre-Tx 2-D
Measurements
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Film
Replacement
N. Dogan
IMRT QA Tools
EPID Systems
• aS500 EPID
9 1 mm copper plate
9 Phosphor scintillating layer
(Kodak Lanex Fast B –
Gd2O2S:Tb, 70 mg/cm3)
9 Array of photodiodes
9 Amorphous Silicon panel Æ
each pixel consists of:
¾Light sensitive photodiode
¾Thin film transistor
9 16-bit ADC
Munro et. al, Med. Phys. 25, 1998
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IMRT QA Tools
EPIDs
Advantages
• Many centers have installed EPIDs and being
primarily used for patient-specific
pretreatment field verification and MLC QA
9 Logical extension to investigate dosimetric applications
• Mounted to linear accelerator - known
geometry with respect to the beam
9 Detector sag must be accounted for at different gantry
angles
9 Positioning reproducibility important
• Real time digital evaluation
9 No processor, data acquisition takes less time
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IMRT QA Tools
EPIDs - Challenges
• EPIDs were primarily designed for patient
localization
9High resolution, good contrast images
9Additional dose to the patient should be minimized
• The conversion of imager response to
dose is complex
9Imaging system dependent
• Other problems
9Ghosting
9Lag
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IMRT QA Tools
EPIDs – Dose determination
• Imager response must be calibrated to a
standard
• Absolute calibration to ion chamber at a
point over a ROI
9E.g. ion chamber in a mini-phantom or slab at
same SDD as EPID
• 2-D calibration to actual beam distribution at
the imager plane
9Can be measured with film or a diode array
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IMRT QA Tools
Factors for EPID Response
• Water-equivalent depth of the
detector
• Field size dependence and scatter
properties within the imager
• Short- and long-term reproducibility
• Dose rate
• Energy dependence
• Spatial integrity
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IMRT QA Tools
EPID: DMLC measurements
Overall: Good agreement
10 MV
+
Predicted
EPID
Ion Chamber
25 MV
Discrepancies in the penumbra region (up to 10%)
Pasma Med Phys 26: 2373-2378 (2376) 1999
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IMRT QA Tools
Linear Diode Array in water vs. CCD
Without
short range
penumbra
correction
Courtesy of Jean Moran, UofM
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IMRT QA Tools
Dose Determination using EPID (SLIC)
Chang et al., Int J Radiat Oncol Phys 47: 231-240 (p. 233)
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IMRT QA Tools
Calculation vs. measured using AMFPD for DMLC
Calculated
(Calculated – Measured)
Agreement : Within +/- 2 cGy
Courtesy of Jean Moran, UofM
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IMRT QA Tools
• EPIDs can provide a much-needed
replacement for pre-tx QA film dosimetry
9Only if proper QA of the EPID is established
9Need better understanding of regions where EPIDs
are inadequate for dosimetry
9Systems must be verified at more centers against
accepted QA methods such as film and ion chamber
9Additional software is required before more facilities
can do proper validation of the methods (Software
must be commissioned)
9Can be part of a comprehensive QA program in
conjunction with other methods such as
computational checks (monitor programs, log file
analysis, etc.)
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IMRT QA Tools
Gel Dosimeters
• Advantages
93-D information in one irradiation
9Energy and dose-rate independent
9High sensitivity and linear response
9Cumulative
9Gel density can be changed - Ideal for
anthropomorphic phantoms
9Near tissue equivalent
9Multiple readout techniques (MR, optical-CT)
9New gel formulations and readers
commercially available
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IMRT QA Tools
Gel Dosimeters
• Disadvantages
9Sensitive to time, preparation, temperature
9Cylindrical container required for optical readers
- less accurate readout at gel/container
interface
9MR time is often limited and expensive - long
scan times for accurate readout, e.g. 5%
accuracy over 10 hr scan time (Gum et al.
2002)
9Relative dosimeter -require cross-calibration
technique – batch to batch they are different
9Cost
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IMRT QA Tools
Gel dosimetry
8cm
• In-house optical CT scanner – cost is less
• Oldham and Kim, Med. Phys. 31 (5), 1093-1104.
• Upgraded motors, motion control, and user interface. (Pacific
Scientific: step motors. National Instruments: motion control and
Labview.)
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IMRT QA Tools
Gels: Optical Density to Dose Calibration
• 6 Beam calibration irradiation
• BANG gel phantom diameter 17.4cm
Courtesy of Mark Oldham, Duke University
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IMRT QA Tools
Gel Dosimeters
• Five Field Prostate IMRT
Courtesy of Mark Oldham, Duke University
• Re-computed for a 3 L BANG gel dosimeter.
•Dmax scaled to 1.8 Gy to fit dynamic range of optical scanner
• BANGkitTM from MGS Research. Optical-CT @ 1x1x3mm, 5hours
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IMRT QA Tools
Gel Dosimeters
Isodose comparison: Pinnacle (red), Gel-dosimetry (black)
Courtesy of Mark Oldham, Duke University
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IMRT QA Tools
Gel Dosimeters
Gum, et al. “Preliminary study on the use of an inhomogeneous
anthropomorphic Fricke gel phantom and 3D magnetic resonance
dosimetry for verification of IMRT plans ,” Phys Med Biol 47; N67-77 2002.
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IMRT QA Tools
Phantoms for IMRT Measurements
• multiple phantoms for commissioning
• Fiducials for reproducible setup of
phantom and detectors
• User-customized for different detectors –
allow special holders
• Simple vs. anthropomorphic
• Homogeneous or heterogeneous
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Current IMRT QA Tools
Simple Geometric Phantoms
• Water tank
9 Accommodate different ion chambers
9 Use for measurements of depth dose and profiles
9 Output, flatness, symmetry, and linearity assessment
• Cylindrical mini-phantom
9 Use with ion chamber to assess dependence of output on gantry
angle
• Water-equivalent plastics: slab w/ custom
chamber inserts
9 1-D and 2-D measurements
9 Detector position can be varied with depth
• Cylindrical phantoms (plastic or water filled)
9 Straightforward geometry
9 Ion chamber at single position
9 Plastic phantoms may hold films
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IMRT QA Tools
Water-equivalent square IMRT Verification
Phantom
D.A. Low et al. “Phantoms for IMRT Dose Distribution Measurement and
Treatment Verification, Int J Radiat Oncol Biol Phys 40: 1231-1235 (1998).
N. Dogan /July 2005
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Current IMRT QA Tools
A Cylindrical Phantom containing movable ion
chamber
L. Xing et al. “Dosimetric verification of a commercial inverse treatment planning
system, Phys. Med. Biol. 44: 463-478 (1998).
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IMRT QA Tools
A cylindrical Plastic Phantom
Detector
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IMRT QA Tools
Plastic Cylindrical Phantom with MOSFETs
Calc. 1.37 Gy
Meas. 1.42 Gy
Diff –3.52%
Calc. 0.81 Gy
Meas. 0.78 Gy
Diff –3.45%
Courtesy of Cynthia Chuang, UCSF
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IMRT QA Tools
Spiral Phantom
Paliwal et al “A spiral phantom for IMRT and tomotherapy treatment delivery
verification” Med Phys (2000).
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IMRT QA Tools
Anthropomorphic: RPC Head Phantom
Target Volumes
Removable Dry
Insert
Water
Critical Structure
Water
Courtesy of Jean Moran, UofM
TLDs in Target Volumes
Radiochromic film through multiple plans
Delivery is required by RTOG for participation in IMRT trials
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IMRT QA Tools
Dosimetric Analysis Tools
• Provide a comprehensive and
quantitative comparison between two
dose distributions
• Different ones available
• Important to know the limitations
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IMRT QA Tools
Dosimetric Analysis Tools
• Overlay of isodoses
• 2-D dose difference displays with
•
•
•
•
colorwash
Dose difference histograms
Distance-to-agreement (DTA)
Gamma evaluation
Normalized agreement test (NAT)
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IMRT QA Tools
Isodose lines and Dose Difference Display
70 cGy
60 cGy
50 cGy
20 cGy
10 cGy
Calcs
Film
+/- 10%
Courtesy of Jean Moran, UofM
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IMRT QA Tools
Dose difference display
• Useful in shallow dose gradients
• Overly sensitive in steep dose gradients
– e.g.; a small spatial shift (due to
experimental measurement errors)
between two dose distributions yield
large dose differences
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IMRT QA Tools
Dose difference histogram and profiles
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IMRT QA Tools
Distance to Agreement (DTA)
• Is the distance between a reference point
•
•
•
and the nearest point in the compared
dose distribution that exhibits the same
dose
Is not overly sensitive in steep dose
gradients
In shallow dose gradients, a large DTA
value may be computed even for relatively
small dose differences
May be hard to interpret
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IMRT QA Tools
Combination of dose difference and DTA
• Identify regions where the dose difference
•
•
and DTA are simultaneously by greater
than a pre-selected criteria – points that fail
both criteria are identified on a composite
distribution
The display of the dose difference may
emphasize the impression of failure in high
dose gradient region
Provides no information on the magnitude
of the failure
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IMRT QA Tools
Gamma Analysis- Generalization of composite distribution
• Measures the closest distance between each reference point
and evaluated dose distribution after scaling by ∆D and ∆d
G G
Γ ( re , r r ) =
G G
G G
r 2 ( r e , r r ) δ 2 ( re , r r )
+
∆d 2
∆D 2
G G
G
γ ( r r ) = m i n {Γ ( re , rr ) } ∀ {re }
G G
r(re , rr ): spatial distance between evaluated and reference dose
points
∆D : Dose difference criteria
Low et al, Med Phys 30(9) 2455-64 (2003).
∆d : DTA
• The point with the smallest deviation from reference point is a
quantitative measure of the accuracy of the correspondence ->
the quality index, γ (rr) of the reference point
γ (rr) ≤ : 1 ->correspondence is within the specified acceptance
criteria
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IMRT QA Tools
Dose Difference and DTA
Dose Difference and DTA Analysis
Summary
Dose Diff and DTA criteria : 2% of
Dmax and 2mm
Points Checked = 5348
Points Passed DTA = 5312
Points Passed DD = 4363
Points Passed Either = 5343
Points Passed Both = 4332
99.3269 % of the points passed DTA
81.5819 % of the points passed DoseDiff
99.9065 % of the points passed either
Either
81.0022 % of the points passed Both
Dose Difference Statistics Summary
Mean Dose Diff = 0.488805 0.877915
DTA Summary
Mean DTA = 0.0477486 0.0747123
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IMRT QA Tools
Gamma Analysis
Gamma Analysis Summary
Dose Diff and DTA criteria : 2% of
Dmax and 2mm
Points Checked = 5348
Points Passed = 5348
100 % of the points passed Gamma
Gamma Statistics Summary
GammaBar = 0.0406743 0.0620086
Dose Diff and DTA criteria : 3% of
Dmax and 3mm
Points Checked = 5348
Points Passed = 5348
100 % of the points passed Gamma
Gamma Statistics Summary
GammaBar = 0.0271162 0.0413391
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IMRT QA Tools
Normalized Agreement Test (NAT)
• Is based on a 2D array of
•
calculated image of NAT
values derived from
comparisons of measured and
computed doses.
Assumes that two dose
distribution images are
registered each other and NAT
is calculated using
N A T = D sc a le × (δ − 1 )
N A T in d e x
A ve( N A T )
=
A v e ( D sc a le )
δ : lesser of Abs(∆D/ ∆Dm) or
∆d/ ∆dm
Dscale: Di /Dmax
• NATindex represents the
average deviation from the
∆Dm and ∆dm criteria for
every dose pixel, ignoring the
ones less than the set criteria
N. Childress et al, “The design and testing of noval clinical parameters for
dose comparison,” Int J. rad. Oncol Biol Phys 56(5) 1464-1479 (2003).
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IMRT QA Tools
NAT Index
N. Childress et al, “The design and testing
of noval clinical parameters for dose
comparison,” Int J. rad. Oncol Biol Phys
56(5) 1464-1479 (2003).
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IMRT QA Tools
Other Analysis Tools
• MU check software
9In-house dose calc
9Commercial packages (e.g; Radcalc)
9Monte Carlo (e.g; Peregrine, EGS4, …) –
Patient QA
• Software for Post-treatment QA
9Analysis of IMRT delivery log files (e.g; inhouse analysis software, Argus IMRT QA
package)
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IMRT QA Tools
MC verification
Superposition
Monte Carlo
∆=10%
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Summary
• Multiple detectors and phantoms are
typically required for IMRT QA
• Quantitative dose analysis tools are
necessary for proper evaluation of delivery
- identify the cause of discrepancies
between delivery and measurements
• Treatment planning vendors are starting to
provide dosimetric evaluation tools
• Aware of the limitations of each tool
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Summary
• Verify that all equipment is functioning
properly
9Film processor, digitizer
9Detectors, cables, electrometers (automatic leakage
correction)
9TLD reader, ovens
• Input/output to treatment planning system
• Standardize measurement setup when
possible
• Monitor software and hardware changes
and QA
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Summary
• Measurements may show dosimetric
differences that planning systems may not
model at this time – curved leaf ends
• Need to know the limits of the mechanical
systems and interactions with controller and
accelerator software for delivery
• Continued need for improvements to
software for delivery system, measurement
devices, phantoms, and dose analysis tools
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N. Dogan
Acknowledgements
Jean Moran – U of Michigan
Cynthia Chuang – UCSF
Mark Oldham – Duke University
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N. Dogan