evaluation of an electron beam energy verification method using statistical process...
TRANSCRIPT
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Evaluation of an Electron Beam Energy Verification Method Using
Statistical Process Control
Stephanie A. Parker, MS, DABR
Novant Health GWSM
Ke rne rsvi l le Winst on-Sa lem Sa lisbury
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I have no conflicts of interest to disclose
Conflicts of Interest
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Background
Measurement Methods
Specification Limits
Statistical Process Control Techniques
Control Limits
Process Capability
Process Acceptability
Outline
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Electron Energy Constancy
Important component of routine linac QA
TG142 – Monthly Check
Challenging Measurement
Rapid falloff of depth dose beyond D-Max
Multiple electron energies per machine
Background
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Electron Energy Constancy
Measurement method using detector array and wedge shaped filter
Described by several authors
1991 - 2011
Automated feature of some array detectors
Background
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Electron Energy Constancy
Equipment may be limited at some facilities
Smaller Clinics
Satellite Facilities
Budget Constraints
Method described applicable to most array detectors
Background
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Background
Measurement Methods
Specification Limits
Statistical Process Control Techniques
Control Limits
Process Capability
Process Acceptability
Outline
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Measurement Method
Closely Followed Method Described by Watts in 1998
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Measurement Method
IBA Blue Phantom
Annual QA
Ep,0 Measured for Each Electron Beam
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Measurement Method
MapCheck2 Diode Array
Aluminum Wedge
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Measurement Method
Equipment Setup
MapCheck2
Leveled
Centered with CAX
100 cm SSD
20 cm x 20 cm Electron Cone
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Measurement Method
Equipment Setup
Aluminum Wedge
Wedge Direction Inplane
Toe Toward Gantry
Heel Right Angle on MapCheck2 Surface
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Measurement Method
Equipment Setup
Aluminum Wedge
Toe Aligned with 15 cm Field Edge Demarcation
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Measurement Method
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Measurement Method
Planar Fluences were Measured for Each Electron Energy
Only readings along center of detector in Y direction of interest
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Intercept of the tangent line to the 50% point on the “toe” end of profile calculated and recorded
Measurement Method
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Measurement Method
4 Linear Accelerators
1 Varian 21iX
2 Varian 21 EX
1 Varian 21 EX-S
5 Electron Energies per Linac
6, 9, 12, 16, 20 MeV
Total of 20 Electron Beams
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Measurement Method
y = -0.132x + 14.478R² = 0.9992
0
5
10
15
20
25
-60 -40 -20 0 20 40 60 80
Ep
,0 (
Me
V)
Array Detector Intercept
Most Probably Electron Energy at Surface (Ep,0) vs. Array Detector Intercept
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Efficiency
Same setup for each electron beam
Adds about 10 minutes to acquire fluences
Use same setup without wedge to measure profiles
Flatness/Symmetry
Measurement Method
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Background
Measurement Methods
Specification Limits
Statistical Process Control Techniques
Control Limits
Process Capability
Process Acceptability
Outline
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Specification Limits
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Need to correlate shift in PDD to change in Ep,0
Referred to TG-70
Specification Limits
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Specification Limits
Need to Know: Change in Ep,0 for a 2 mm change in R50
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Specification Limits
y = 2.5595x - 0.3241
R² = 1
0
5
10
15
20
25
30
0 2 4 6 8 10 12
Ep
,0(M
eV
)
R50 (cm)
Ep,0 vs R50
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Slope = 2.56 MeV/cm
Slope = 0.256 MeV/mm
2 mm Shift in R50 = 0.51 change in MeV
Spec = +/- 0.5 MeV
Specification Limits
y = 2.5595x - 0.3241
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Clinical Implementation
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Remaining Questions
Stability of Method Over Time
Reproducibility of Setup
Inter-User Variability
Turned to Statistical Process Control Techniques
A lot of options
Statistical Process Control Techniques
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Most Applicable
Control Limits & Control Charts
Process Capability
Process Acceptability
Statistical Process Control Techniques
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Control Charts
Originated with Walter Shewhart
1920’s
Bell Laboratories
Used to determine if Process:
Stable
Has Predictable Performance
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Shewhart Identified two sources of process variation
“Chance” Variation
Inherent in process
Stable over Time
“Assignable” Variation
Result of specific event outside system
Unstable over Time
Control Charts
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“Chance” Variation
Random Error
Common Cause
“Assignable” Variation
Systematic Error
Special Cause
Control Charts
Control Charts Help Distinguish Between the Two Types of Error
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Different Types of Control Charts
Attribute Data
Discrete
Y/N
Good/Bad
Variable Data
Continuous Scale
Control Charts
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Variable Data Charts (Actually Pairs of Charts)
X and Moving Range Chart
Sample Size (n) = 1
X-Bar and Range Chart
n = 2-9
X-Bar and S Chart
n > 10
Control Charts
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Control Charts
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Control Charts
How do I apply control charts to Ep,0 measurements?
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Control Charts
Steps
Collect Initial Data
Ensure meets specification
Establish Control Limits Using Collected Data
Create Charts
Record Data in Charts
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Control Charts
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Control Charts
Control Limits Represent 3
Standard Errors from the Mean
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Individual Control Chart
8.2
8.4
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2011 2012 2013
9 MeV
Cc
LCL
UCL
USL
LSL
Baseline
Ep
,0 (
Me
V)
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Moving Range Chart
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
May
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2011 2012 2013
9 MeV
Rc
Rl
Ru
Ran
ge
(M
eV
)
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All control limits well within specification limits
Shows that the process is in control
All data points fall within control limits
Data follows a random pattern
Process stability should allow for distinction between random and systematic errors
Noise small so signal should be able to be detected
Control Charts
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Individual Control Chart
8.2
8.4
8.6
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9.4
Ap
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2011 2012 2013
9 MeV
Cc
LCL
UCL
USL
LSL
Baseline
Ep
,0 (
Me
V)
Deliberately
Introduced Data Point
with Energy Difference
(~0.4 MeV)
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Moving Range Chart
0
0.05
0.1
0.15
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2011 2012 2013
9 MeV
Rc
Rl
Ru
Ran
ge
(M
eV
)
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Most Applicable
Control Limits & Control Charts
Process Capability
Process Acceptability
Statistical Process Control Techniques
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Process Capability and Acceptability
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Process Capability and Acceptability
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Process Capability and Acceptability
NIST/SEMATECH e-Handbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook
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Process Capability and Acceptability
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Process Capability and Acceptability
NIST/SEMATECH e-Handbook of Statistical Methods, http://www.itl.nist.gov/div898/handbook/
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Process Capability and Acceptability
Sanghangthum et al, JACMP, Vol 14 (1), 2013, Pg 151, Used with Permission
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Data Requirements
Normal Distribution
“Large Enough” data sample
Typically ≥ 30 data points
Need to assess measured data to determine if requirements met
Process Capability and Acceptability
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Process Capability and Acceptability
Normal Distribution Assessment Anderson-Darling Test
P ≥ 0.05, data considered normal
P values range 0.14 – 1.0
All data considered normally distributed
Data Size ~ 40 -4
-3
-2
-1
0
1
2
3
4
8.7 8.75 8.8 8.85 8.9z
Ep,o (MeV)
Normal Probability Plot 9 MeV Data
P = 0.78
Conclusion: Data Meets Requirements for Analysis Using Cp and Cpk
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Cp Results
All values > 1
2.7 – 10
Process for all energies capable of meeting specifications
Process Capability and Acceptability
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Cpk Results
All values > 1
1.8 – 9.6
Center of the process for all energies within specification limits
Process Capability and Acceptability
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Comparison of Cp and Cpk
Cpk < Cp in all but 1 case
Indicated that process has some shift from baseline
Baseline value single measurement
Indicates that target value should be an average of in control values vs. single measurement
Process Capability and Acceptability
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Process Capability and Acceptability
0
2
4
6
8
10
12
8.2 8.4 8.6 8.8 9 9.2 9.4
9 MeV Ep,0
LAL
UAL
Baseline
Cp = 5.32Cpk = 5.18
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Measurement Method Established to Measure Ep,0
MapCheck2 and Aluminum Wedge
Specification Limits Established
2 mm PDD shift = +/- 0.5 MeV
Statistical Process Control Techniques
Control Limits
Process Capability
Process Acceptability
Summary
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Statistical Process Control Techniques Utilized Control Limits
Process in Control
Control Limits well within Specification Limits
Process Capability
Cp >1 in all Cases
Process Capable of Meeting Specs
Process Acceptability
Cpk > 1 in all Cases
Process Centered within Specification Limits
Summary
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Electron energy verification method
Efficient
Effective
Good option for centers with limited equipment
Small Centers
Satellite Centers
Statistical Process Control
Tools useful for analyzing QA processes
Conclusion
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La Fin
Merci
Me at the Marie Curie Museum in Paris
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Questions?
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U. F.Rosenow, M. K. Islam, H. Gaballa, and H. Rashid, “Energy constancy checking for electron beams using a wedge-shaped solid phantom combined with a beam profile scanner,” Med. Phys. 18, 19-25, (1991).
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References