Dosimetry Techniques for IMRT
Daniel A. Low, Ph.D.
Department of Radiation Oncology
Mallinckrodt Institute of Radiology
Washington University School of Medicine
St. Louis, Missouri USA
Dose/MU Validation
• Dose and MUs are greatly interdependent in
IMRT
• “MU” validation requires either
– Direct measurement of dose using TPS MUs/fluences
• Time-intensive measurements
• Equipment/techniques also required for commissioning
• Currently most thorough method of validation
– Independent computation of dose
• Commercial and academic efforts still single-points
• Ideally, recompute entire 3D dose and compare (DVHs?)
Issues for Measurement-Based
Comparisons
• Quality assurance requires quantitative dose
measurements
• Independent registration of measurement
and calculation
• Techniques limited by IMRT dose delivery
– Temporal dose delivery
– Integrating dosimeters
Tools
• Dosimeters
• Phantoms
• Film Scanner
Dosimeters
• Integrating
–
–
–
–
TLD chips
Radiographic film
Radiochromic film
PAG gel (BANG-2)
• Non-Integrating
– Ionization Chamber
Ionization Chamber
• Inconveniences
– Acquire 1 measurement for entire IMRT delivery
– Relatively insensitive, large active volume
– May volume average (we have not yet seen this)
• Convenience
– Everyone has one
– Calibration straightforward
Ionization Chamber Volume
Averaging
4 mm
1
100
Dose (%)
0.96
60
40
0.94
20
0.92
0
0.9
-20
-15
-10
-5
0
Position (mm)
5
10
15
20
Ratio Ion Chamber/Film
0.98
80
Test of Ion Chamber Integration
1-D dry scanner
3 chambers
2 orientations
2 energies
Integration Accuracy
TLD Chips
• Larger number of simultaneous measurements
• Factor for each chip
–
–
–
–
Uniform irradiation of chip batch
Read out chips and re-anneal
Repeat until 3 measurements are made
In each case, the readings are compared against the
batch and ratio of chip to average used as factor
– Individual chips tracked
– Requires automated reader
• Calibration for each measurement (subset of
chips)
• 3% chip-to-chip reproducibility possible
TLD
Calibration
% Cal Dose Error (of 180)
300
10
High Cal doses
250
5
Dose (cGy)
200
150
0
100
-5
50
Low Cal Doses
0
0
200
400
600
800
1000
Corrected Reading
-10
1200
% Error
(of 180 cGy)
TLDs
200
150
Dose (cGy)
100
50
Measured
Calculated
0
-160 -140 -120 -100 -80 -60
Z (mm)
-40
-20
0
TLDs – Critical Structure
200
150
Dose (cGy)
100
Measured
Calculated
50
0
-80
-60
-40
-20
0
20
40
60
80
Low-Dose Results
100
80
Measured
Calculated
Dose (cGy) 60
40
20
0
-160 -140 -120 -100
-80
-60
Z (mm)
-40
-20
0
TLD Scatter Plots
Measured Doses
250
200
Measured
dose (cGy)
150
100
50
y = 16.038 + 0.96004x R= 0.99538
0
0
50
100
150
200
Calculated dose (cGy)
250
Histograms
30
Number of Measured Points
25
20
15
10
5
0
Fig 11b
-30
-20
-10
0
Measured/Calculated (%)
10
20
Radiographic Film
• Accuracy not yet quantified for high energy
photons
• Best we have for 2D dosimetry
• Proper processing and normalization critical
–
–
–
–
–
Same batch
Process at same time
H&D curve every time
Nonlinear fit necessary
Independent dose normalization desirable
New Film Option - Kodak EDR2
Optic a l Density vs Dos e for XV and ECL Film
3
6 MV XV
18 MV XV
6 MV ECL
18 MV ECL
2.5
Optical Dens ity
2
1.5
1
0.5
0
0
50
100
150
200
Dos e (cGy)
250
300
350
Radiographic Film
Radiographic Film
Discrepancy Analysis 1
• TPS:
–
–
–
–
Input data (penumbra, PDD, outputs, leaf offsets)
Accelerator model inaccurate
Dose calculation algorithm limitation
Leaf sequencing algorithm
• Experiment
– MLC information transfer
– Experimental setup
• Geometry
• Irradiation (wrong patient/field/MUs…) – >30 params for each
irradiation
• Bad HD curve
• Bad processing
Discrepancy Analysis 2
• Delivery
– Incorrect MLC calibration (readout vs position)
– Incorrect accelerator operation (e.g. sticking
leaf)
• Analysis
– Film scanning/readout
• Densitometer artifacts
• User-input data (film position, etc.)
• Incorrect registration
Future
• New dosimeters becoming available
– Radiochromic film
– PAG gel (BANG-2)
• Both are “research” densitometers
Radiochromic Film
Quantitative Tests
CAX Profiles
HDR (Steep Gradients)
Measurement Vials
Cubic Phantom
Fiducial Markers
Calibration Vials
Lucite Jig
Bang-2 Gel
600
900
1200
1400
300
Gamma (described later)
Another Experiment
900
600
1400
1200
300
Gamma again
Optical Readout
Phantoms
• Generally two types
– Anthropomorphic
• Internal heterogeneities are anatomically correct
• Heterogeneities may make dose measurements and
comparisons complicated
• Multiple dosimeter comparisons difficult
• Geometric alignment may be difficult
– Geometrically Regular
• Alignment straightforward
• Internal construction precise
• Multiple dosimeters possible
Assembly Screws Film Compression Screws
Talon
CT Pointer
Scribes
Spacers
WaterEquivalent
Plastic
Extraction
Tool
Chamber
Holders
Scribes
Ion Chamber
Cable
Film Dose Readout and
Comparison
• Goals:
– High resolution, multidimensional, quantitative
verification of delivered dose
– Efficient
– Limit cost (equipment and supplies)
• Comparisons:
– Hybrid plans
– Comparison tools
QA Process
Ion
Chamber
Msmts
Treatment
Plan
Phantom
Geometry
Phantom
Plan
Film
Exposure
Registration,
Comparison
Film
Scanner
QA
Report
Commercial Products –
Measurement vs Calculation
• Validation must compare calculation and
measurement
• Independent registration of measurement
and calculated doses
• Automated extraction of planar dose from
treatment plan
Dose Distribution Comparisons
• Traditional Tools
–
–
–
–
–
Point comparisons
Superimposed dose distributions
Dose difference
Distance-to-agreement
“Composite failure analysis”
• Additional Tool
– Multidimensional dose-difference and DTA
– “gamma”
Traditional Dose-Difference and
DTA
Calculation Point
δ
r r
Dm ( r m ), r m
y
∆ DM
∆ dM
r
Dc ( r c )
r r
δ ( rm , rc )
r
r
r rc − r m
rc
x
Gamma
Γ = general distance in criteria-normalized
dose and distance space
γ = minimum value of Γ for entire
“calculated” distribution
γ <= 1 passes
γ > 1 fails
• Allows γ histograms and statistical
evaluation
Gamma Method
r r
Dm ( r m ), r m
∆ DM
δ
r r Calculation Point
Γ (r m , r c )
r
Dc ( r c )
y
r r
δ ( rm , rc )
r
rc
r
rx
rc − rm
∆ dM
“Laboratory”
Conclusions
• Dosimetric consequences profound
• More quantitative approach for
measurements required
• Commonly used techniques may be
adequate – new Task Group for film
dosimetry
• Vendors are assisting: Dose distribution
input and comparison software