IMRT QA: Multiple Institution
Planning and Delivery
Comparison
Ying Xiao, Ph.D.
Gary A. Ezzell, Ph.D.
Jatinder Palta, Ph.D.
RPC results - 2008
• 250 irradiations of H&N
phantom
• 28% had failed …
- 7% dose in low
gradient
- 4 mm DTA in high
gradient
TG-119
• Task group 119 was charged with
expanding on the “IMRT Guidance
Document” from 2003
- Chaired by Ying Xiao, PhD
• Motivated by on-going concerns about
practical IMRT commissioning and QA
Focus on IMRT commissioning
• Presumption is that the high failure
rate indicates inadequate
commissioning of the IMRT planning
and delivery system
• Need for better information about
when is the dosimetric accuracy “good
enough”
1
Participants
TG-119: What is achievable in practice?
• Define a suite of planning problems
• Define a measurement protocol
• Compare results from different
institutions that had passed the RPC
credentialing for IMRT
• Use the results to recommend
minimum criteria for acceptability
• Make the test suite available to others
Equipment
• Linacs
- 5 Varian, 2 Elekta, 2 Siemens,
1 Tomotherapy
• Planning systems
- 4 Pinnacle, 3 Eclipse, 1 CMS, 1
Tomotherapy, 1 In-house
Mayo Clinic Arizona
Thomas Jefferson University
Hospital
University of California, San
Francisco
University of Florida
Robert Wood Johnson
University Hospital
Virginia Commonwealth
University
Memorial Sloan Kettering
Cancer Center
Charleston Radiation
Therapy Consultants
Karmanos Cancer Center
Dosimetry tools
• Rectangular plastic phantom
• Composite irradiation
measurements
- Chamber of suitable size
- Film in coronal planes
- Gamma criteria: 3%, 3 mm
2
Dosimetry tools
• Individual field measurements
- Detector array, film, EPID
• Gamma analysis: 3%, 3 mm
Bands
Dose profile through central plane
Preliminary tests
• AP:PA
- Set chamber reading to dose conversion
- Test film dosimetry in simplest case
• Bands
- Set of overlapping AP:PA fields to
different doses
- Test measurement process
Mock clinical cases
• Model common clinical cases
• Use consistent approach
- 6 MV
- 7 – 9 coplanar fields, equally spaced
• Dose goals specified
• Confirm that plans are “similar”
3
Mock Head/Neck
Dose goals
PTV
Cord
Parotids
Mock Head/Neck
90% of volume to receive at least 50 Gy
99% of volume to receive at least 46.5 Gy
<= 20% of volume to receive more than 55 Gy
No part of volume to receive more than 40 Gy
50% of volume to receive less than 20 Gy
Beam arrangement
6 MV, 9 fields at 40o intervals from the vertical
Chamber measurement points
Isocenter, in the mid PTV and 4.0 cm posterior, mid spinal cord
Film planes
Isocenter, in the mid PTV and 4.0 cm posterior, mid spinal cord
Multi-Target
Mock Prostate
25 Gy
PTV
95% 75.60 Gy
5% > 83 Gy
Rectum
30% 70 Gy
10% > 75 Gy
Bladder
30% 70 Gy
10% > 75 Gy
50 Gy
12.5 Gy
Three cylindrical targets are stacked along the axis of
rotation. Each has a diameter of approximately 4 cm
and length of 4 cm
4
CShape
Target: 95%
Preliminary Results - Chamber
• (Measured – Plan) / Dose per fraction
• High dose region
- Average difference: 0.0%
- Standard deviation: 2.3%
• Low dose region
- Average difference: 0.4%
- Standard deviation: 2.1%
50 Gy; 10% > 55 Gy
Core: 5% > 25 Gy (easier version);
10 Gy (harder version)
Chamber – high dose region
Chamber – low dose region
Composite Ion Chamber Dosimetry
Low Dose Plane
0.070
0.060
0.050
0.040
0.030
0.020
0.010
0.000
-0.010
-0.020
-0.030
-0.040
-0.050
-0.060
-0.070
Prostate
Head/Neck
MultiTarget
CShape50
CShape20
Bands
A
B
C
D
E
F
G
H
I
J
Institution
(Meas-Plan)/Dose per fraction
(Meas-Plan)/Dose per fraction
Composite Ion Chamber Dosimetry
High Dose Plane
0.070
0.060
0.050
0.040
0.030
0.020
0.010
0.000
-0.010
-0.020
-0.030
-0.040
-0.050
-0.060
-0.070
Prostate
Head/Neck
MultiTarget
CShape50
CShape20
A
B
C
D
E
F
G
H
I
J
Person
95% confidence level: mean ± 1.96
= 0.045
95% confidence level: mean ± 1.96
= 0.044
5
Film measurements – composite
irradiations
• Gamma criteria: 3% dose, 3 mm DTA
• Only six institutions have reported
• High dose planes
- 96.6 ± 4.1% of points passed
• Low dose planes
- 96.3 ± 4.8% of points passed
• 95% confidence ~ 88% - 100%
Individual field measurements
• Gamma: 3 %, 3 mm DTA
- Absolute Dose, 10% Threshold, Van Dyk %
Difference, Apply Measurement Uncertainty
• Average pass rate: 97.9 ± 2.5%
• Confidence level: 93% – 100%
Individual field measurements
• 7 institutions reported
- 5 used diode array (MapCheck)
- 1 used film, 1 used EPID
• 5 with diode array sent plan and
measurement data to one institution
for consistent analysis
Minimum commissioning expectations
• Ion chamber in composite irradiation
- Mean < 3% agreement with calculated
(relative to prescription dose)
- 95% confidence level: 4.5%
• Film dosimetry in composite irradiation
- 90% of point in region of interest should
pass gamma of 3% / 3 mm
- With film normalized to chamber results
on the same plane
Draft conclusions
6
Minimum commissioning expectations
• Field-by-field measurements
- Careful! Gamma pass percentages
depend heavily on the
implementation
- With the choices used here, 90% of
points should pass a gamma of 3% /
3 mm
Draft conclusions
7