July 30, 2008 Houston
Improvements in IMRT QA Processes –
Corrective Actions by Examples
Ying Xiao, Ph.D.
Problem: An Unacceptable IMRT QA Case
• Case
– Mock Head and Neck
• Treatment Planning System/Algorithm
– CMS/XiO 4.3/Superposition/Convolution
• Delivery System/Beam
– Elekta Synergy/MiniMLC/6 MV
• IMRT QA
Comparison between Planned and Measured 2D Dose
3 % Dose / 3 mm DTA
Gamma Index Pass Rate: 43.4%
What Could be Wrong?
• Treatment Planning System (50%)
– Inaccurate Data, Inaccurate/Insufficient Modeling, Software
Glitches, Complexity of the IMRT plan…
• Delivery System (20%)
– Output Inaccuracy, Beam Definition System Error (e.g. MLC
Error), Error in Patient Positioning System
• Measurement Process (30%)
– Suboptimal Measurement Techniques, Limitation/Inaccuracy in
Measurement Devices, Human Error in Execution Process
Investigation into Treatment Planning SystemIntegrity of Input Data
• Measurement Devices Appropriateness
– e.g. Profile Penumbra Measured with Micro-chamber?
• Consistency of Measured Data (Inter-Physicist Verification)
– Output Factors
– Small Field Measurements
– …
Ibbott GS, et all Challenges in credentialing institutions and participants in advanced technology multiinstitutional clinical trials. IJROBP. 2008;71:S71-S75
Markus Alber et al GUIDELINES FOR THE VERIFICATION OF IMRT, ESTRO
1
Dose Algorithms for IMRT
Investigation into Beam Modeling
Model Parameters: Pinnacle
1:
DD Energy spectrum
2:
build-up Electron contamination (max depth, electron
contamination surface dose,
depth coefficient, off axis coefficient, depth fraction and
scale fraction)
3:
horns Fluence increase and cone radius
4:
tail Jaw transmission, Gaussian height and width
5:
geometric penumbra Effective source size
•
•
•
•
Clarkson?
Pencil Beam?
Superposition/Convolution
Monte Carlo
Investigations into MLC Modeling Parameters
4, 5
4, 5
CMS/XIO 4.3
MLC leaf gap reduction (Vendor Specific)
A. Rangel et al, Phys. Med. Biol. 52 (2007) 6011–6025
2
Effects of MLC Sigma Perturbation
MLC Sigma: 0.1
IMRT QA Gamma Index
Pass Rate: 96.2%
MLC Sigma: 0.5
MLC Sigma: 0.2
IMRT QA Gamma Index IMRT QA Gamma Index
Pass Rate: 74.9%
Pass Rate: 93.2%
Effects of MLC Transmission Factor
Perturbation
MLC Transmission 1.5%
IMRT QA Gamma Index
Pass Rate: 96.2%
MLC Transmission 3%
IMRT QA Gamma Index
Pass Rate: 43.4%
Another institution: Source Size Adjustment
Improvements in Measurement Techniques
• Reduction in Source Size
– Sharper Penumbra
• Contour the Ion Chamber Active Volume for DVH Analysis
• Consider Couch Transmission
• IMRT QA Results (2D Planar Dose)
– Before Adjustment: Gamma Index Pass Rate 70%
– With Reduced Source Size: Gamma Index Pass Rate 95%
Test Case: Cshape Hard,
William Song, Jatinda Palta, U. of Florida
William Song, Jatinder R. Palta, University of Florida Health Science Center
3
Adjustment of IMRT Plan Complexity
Reported Improvements in IMRT QA by Reduction in IMRT Plan
Complexity
• Wayne State Univ School of Medicine
• Charleston Radiation Therapy Cons
Summary of Investigational Leads
• Treatment Planning
– Inaccurate Data, Inaccurate/Insufficient Modeling, Software
Glitches, Complexity of the IMRT plan…
• Delivery System
– Output Inaccuracy, Beam Definition System Error (e.g. MLC),
Error in Patient Positioning System
• Measurement Process
– Suboptimal Measurement Technique, Limitation/Inaccuracy in
Measurement Devices, Human Error in Execution Process
Summary
Superposition/Convolution Algorithm
• Using the test suite allowed some facilities to improve their
IMRT commissioning via
– Improvements in beam modeling
– Improvements in measurement techniques
• D(x, y, z)=
(x', y', z')K(x−x', y − y', z − z')dxdydz
Terma
Kernel
Dose
4
Effects of Model Parameters
Perturbation of Energy Spectrum
1
Spectrum used for Modeling 6MV beam from
ELEKTA
CMS/XiO Spectra Library
Effect of Spectra Perturbation on PDD
and IMRT QA
1
Spectrum used for Modeling 6MV beam from
Varian
CMS/XiO Spectra Library
Improvements in Measurement Techniques
Calculated
• Contour the Ion Chamber Active Volume for DVH Analysis
• Consider Couch Transmission
Measured
ELEKTA Spectrum, 2.4 cm x 2.4 cm
IMRT QA: 2D Dose Measured at 10 cm Depth
Gamma Index Pass Rate: 96.2%
Varian Spectrum, 2.4 cm x 2.4 cm
IMRT QA: 2D Dose Measured at 10 cm Depth
Gamma Index Pass Rate: 96.2%
• Film Measurement Tips
– Consistent Processing Condition
– Consistent Film Batch for H&D Curve and Dose Measurements
– Careful Film Placement (e.g. Reduce Air Gap)
– Utilization of Best OD Range (Typically 2-2.5)
– Better Calibration of Geometry, Consistency and Linearity of
Film Scanner
Gary A. Ezzell, Mayo Clinic Scottsdale
William Song, Jatinder R. Palta, University of Florida Health Science Center
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