Backscatter
for EPID dosimetry
Tuesday Seminar
Wonmo Sung
Contents
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Introduction
Literature Review
SNUH clinic implementation
Conclusion
Introduction
Electronic Portal Imaging Devices (EPID)
: Amorphous silicon photodiode
1) For patient positioning
2) For DOSIMETRY!
-> PDIP (Portal Dose Image Prediction)
1)
2)
However…
non-uniform support arm backscatter ?
+ How about backscatter from bunker?
Literature Review
Arm Backscatter
• Physical approach
lead sheet btw arm & EPID (2010)
→ Varian BSS-EPID development (2012)
• Software approach
backscatter kernel (2010) → (2013)
+ Bunker backscatter (2012)
All of above research were conducted in the same research group
(PI: Peter B. Greer, University of Newcastle, Australia)
Backscatter
(Rowshanfarzad et al, MP, 2010)
EPID+ARM
Devices: aS500 & E-arm
(same physical components as the aS1000)
For 40x30 cm2 field
(EPID + arm) – (EPID) = Max 6%
EPID
Physical Approach
- lead sheet
- BSS-EPID
Physical Approach - lead sheet
(Rowshanfarzad et al, PMB, 2010)
Optimal lead sheet : 2 mm
(Backscatter reduction vs. weight)
For 30x30 cm2 field, at CAX
6 MV
18 MV
Lead 2 mm 0.1%
0.2%
w/o lead 2.3%
1.3%
Physical Approach - lead sheet
(Rowshanfarzad et al, PMB, 2010)
Symmetry improvement of in-line profiles
Contrast-to-noise ratio & resolution are decreased.
But not significant reduction in image quality
Physical Approach - lead sheet
(Rowshanfarzad et al, PMB, 2010)
The displacement of the EPID central pixel was measured during a 360◦
gantry rotation with and without lead which was 1 pixel different.
Physical Approach – Varian BSS-EPID
(King et al, MP, 2012)
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•
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Varian developed
BackScatter-Shielded (BSS)-EPID.
BSS-EPID incorporates a layer of lead shielding beneath the active area of EPID
Compare BSS-EPID + arm vs. BSS-EPID
• For >99% pixels, 0.5% change
• At CAX, maximum 0.2% effect on the central axis
Physical Approach – Varian BSS-EPID
(King et al, MP, 2012)
• Use BSS-EPID to reconstruct dose in water for IMRT fields
• Compare with MapCheck
2 cm depth,
2%/2 mm gamma
Black dots: failed
Software Approach
- backscatter kernel
1) field size independent
2) field size dependent
3) dose conversion
- just correct measurement
(Varian updated)
Software approach – backscatter kernel
(Rowshanfarzad et al, MP, 2010)
Rowshanfarzad developed simple backscatter kernel using measurements & Monte Carlo.
EPID+ARM
EPID
Deconvolution
+Gaussian fit
Software approach – backscatter kernel
(Rowshanfarzad et al, MP, 2010)
BEFORE
AFTER
Gamma evaluation (2%/2 mm) for 20 × 20 cm2 field size
Software approach – backscatter kernel
(Rowshanfarzad et al, MP, 2010)
2%/2 mm Gamma evaluation, H&N IMRT Per Field
Avg. 2% improvement with backscatter kernel
Software approach – backscatter kernel
FS dependent kernel…(Berry et al, MP, 2010)
Kernel for dose reconstruction…(Wang et al., MP, 2013)
Anyway, better kernel development so far.
So far, we talked about…
Energy fluence-convolution PDIP (Peter B. Greer, MP 36 (2009) 547-555)
~ similar to CCC
1) energy fluence (open beam+MLC leakage+head scatter) from Pinnacle
2) several kernels
TERMA
However, we use…
Varian simple convolution PDIP (Ann Van Esch, Green 71 (2004) 223-234)
1) No energy spcetra condisderation
2) Single kernel to model 1) radiation source distribution 2) head scatter on the incident energy fluence,
3) effect of EPID scatter, 4) optical glare on the dose deposition in the EPID, 5) MLC leaf end
transmission,…
Varian PDIP algorithm (Ann Van Esch)
Software approach – Just correct measurement!
(Vinall et al, BJR, 2010)
Ann accepted three major drawbacks of her algorithm + 1D correction
1) 40x40cm2 diagonal profile does not describe 40 x 30 cm2 field edge.
2) No backscatter correction
3) No spectral consideration (for head scatter, MLC…)
She suggested 2D correction method to measurement data
Please note that this is correction to measurement!
Not to prediction as Peter did.
Therefore…this method still possess problem.
Software approach – Just correct measurement!
(Vinall et al, BJR, 2010)
R-arm, 40 × 30 cm2 field,
in-line profile,
Predicted : Calculation
Official : Measurement
+ 1D-correction
2D no BS : Measurement
+ 2D-correction + backscatter
correction
2D with BS : Measurement + 2Dcorrection + no backscatter
correction
Software approach – Just correct measurement!
(Vinall et al, BJR, 2010)
R-arm, 10 × 10 cm2 field,
in-line profile,
Predicted : Calculation
Official : Measurement
+ 1D-correction
2D no BS : Measurement
+ 2D-correction + backscatter
correction
2D with BS : Measurement + 2Dcorrection + no backscatter
correction
Bunker backscatter
Bunker Backscatter
(Rowshanfarzad et al, JACMP, 2012)
1) Portable brick wall experiment
2) Response variation with various gantry angle (MatriXX: output variation)
Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm
3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
1)
Bunker Backscatter
(Rowshanfarzad et al, JACMP, 2012)
1) Portable brick wall experiment
2) Response variation with various gantry angle (MatriXX: output variation)
Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm
3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
2)
Bunker Backscatter
(Rowshanfarzad et al, JACMP, 2012)
1) Portable brick wall experiment
2) Response variation with various gantry angle (MatriXX: output variation)
Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm
3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
3)
SNUH clinic implementation
• 방사선치료임상실습
(805.611 Practicum in Clinical Radiation Therapy)
Instructor : 예성준
• 10-11 Weeks : three clinic-related topics
Lecturer : 김정인
① OSLD sensitivity trends : 박소연, 박종인
② MatriXX commissioning : 이재기, 이현석
③ PDIP commissioning : 성원모, 허태민
Varian PDIP commissioning procedure
EPID calibration
Imager Dark & Flood field calibration
Dosimetry calibration
Profile Correction
Dmax 1D diagonal
profile (w2CAD)
Beam data
import
Output Factor measurement
Actual Fluence measurement
Optimal & Actual fluence
Intensity profile
Copy from PBC model
Varian Update
(CTB-PV-887-A, Oct. 15th, 2012 released)
(a) The pre-configured PDIP algorithm for import into ‘Beam Configuration’.
(b) A file for 2D profile correction including backscatter correction of the support
arm imported during ‘Dosimetry Calibration’ on the treatment console.
EPID calibration
Imager Dark & Flood field calibration
Dosimetry calibration
Profile Correction
Beam data
import
(b) Dmax
2D profile
including
1D diagonal
profile (w2CAD)
backscatter
correction
Output Factor measurement
Actual Fluence measurement
Optimal
Actual fluence
(a) Golden
Beam& Data
Intensity profile
Copy from PBC model
Varian Update
(CTB-PV-887-A, Oct. 15th, 2012 released)
Using Golden beam data means…
Golden EPID output factor & Golden kernel
<My opinion - limitation>
1) Machines specific characteristics
Ex. Same EPID? Same Linac?
Varian Update
(CTB-PV-887-A, Oct. 15th, 2012 released)
G
2D profile correction to measurement
including backscatter provided by Varian
<My opinion - limitation>
1) field-size dependency on backscatter
2) Imager at off-axis
Possible choices
Prediction
Measurement
Varian Previous Approach
P1
1D-corrected
Measured Beam Data
P2
2D-corrected
Measured Beam Data
PG
1D-corrected
1D-corrected Measurement
Measurement
M1
2D-corrected
Measurement
M2
Golden Beam
Varian Updated Approach
Preliminary results
Machine : Varian Trilogy 6X, aS1000, E-arm
Varian provided plan
Relative dose normalized at isocenter
ROI selection : Threshold of 5%
Gamma Pass rate (%, 3 mm / 3% criteria, Local)
P1 - M1 P1 - M2 PG - M1 PG - M2
IMRT1
84.1
82.2
91.5
93.5
IMRT2
91.5
94.3
91.7
94.2
VMAT1
72.2
76
65.6
69.5
VMAT2
95.3
91.7
98.7
97.9
Average
85.775
86.05
86.875
88.775
So far, “Varian previous approach” was the worst
and “Varian updated approach” was the best.
However, NOT ALWAYS.
Future plan
 Re-Commissioning 1D-corrected measured Beam Data
 Commissioning 2D-corrected measured Beam Data
 Select 10 H&N clinical cases
 Mapcheck measurements to verify Trilogy TPS parameters (?)
 EPID measurements for 1D-corrected & 2D-corrected 10 H&N
Conclusions
 Further improvement in the agreement between the
model prediction and EPID measurements is required.
 More investigation is necessary for SNUH clinic
implementation.
Thanks for your listening!
Any questions?