backscatter for epid dosimetry tuesday seminar wonmo sung
TRANSCRIPT
Backscatterfor EPID dosimetry
Tuesday Seminar
Wonmo Sung
Contents
• Introduction• Literature Review• SNUH clinic implementation• Conclusion
Electronic Portal Imaging Devices (EPID)
: Amorphous silicon photodiode
1) 2)
Introduction
1) For patient positioning
2) For DOSIMETRY!
-> PDIP (Portal Dose Image Prediction)
However…
non-uniform support arm backscatter ?
+ How about backscatter from bunker?
Literature ReviewArm 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)
Devices: aS500 & E-arm(same physical components as the aS1000)
For 40x30 cm2 field(EPID + arm) – (EPID) = Max 6%
EPID+ARM 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)
6 MV 18 MV
Lead 2 mm
0.1% 0.2%
w/o lead 2.3% 1.3%
For 30x30 cm2 field, at CAX
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 - lead sheet(Rowshanfarzad et al, PMB, 2010)
• 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
Physical Approach – Varian BSS-EPID(King et al, MP, 2012)
• 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 gammaBlack dots: failed
Software Approach
- backscatter kernel1) field size independent2) 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
Gamma evaluation (2%/2 mm) for 20 × 20 cm2 field size
BEFORE AFTER
Software approach – backscatter kernel(Rowshanfarzad et al, MP, 2010)
2%/2 mm Gamma evaluation, H&N IMRT Per FieldAvg. 2% improvement with backscatter kernel
Software approach – backscatter kernel (Rowshanfarzad et al, MP, 2010)
FS dependent kernel…(Berry et al, MP, 2010)Kernel for dose reconstruction…(Wang et al., MP, 2013)
Anyway, better kernel development so far.
Software approach – backscatter kernel
So far, we talked about…Energy fluence-convolution PDIP (Peter B. Greer, MP 36 (2009) 547-555)~ similar to CCC1) energy fluence (open beam+MLC leakage+head scatter) from Pinnacle2) several kernels
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,…
TERMA
Varian PDIP algorithm (Ann Van Esch)
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 correction3) No spectral consideration (for head scatter, MLC…)
Software approach – Just correct measurement!(Vinall et al, BJR, 2010)
She suggested 2D correction method to measurement dataPlease 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 cor-rection
2D with BS : Measurement + 2D-correction + no backscatter correc-tion
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 cor-rection
2D with BS : Measurement + 2D-correction + no backscatter correc-tion
Bunker backscatter
Bunker Backscatter(Rowshanfarzad et al, JACMP, 2012)
1) Portable brick wall experiment2) Response variation with various gantry angle (MatriXX: output variation)Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
1)
1) Portable brick wall experiment2) Response variation with various gantry angle (MatriXX: output variation)Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
2)
Bunker Backscatter(Rowshanfarzad et al, JACMP, 2012)
1) Portable brick wall experiment2) Response variation with various gantry angle (MatriXX: output variation)Distance btw isocenter – left wall:370, right wall:385, floor:130, ceiling 145 cm3) “Worst case scenario” : SDD=150+phantom, relative differences within 1.0%
3)
Bunker Backscatter(Rowshanfarzad et al, JACMP, 2012)
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
Beam data import
EPID calibra-tion
Imager Dark & Flood field calibration
Dosimetry calibration
Profile Correction
Dmax 1D diagonal profile (w2CAD)
Output Factor measurement
Actual Fluence measurement
Optimal & Actual fluence
Intensity profileCopy 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.
Beam data import
EPID calibra-tion
Imager Dark & Flood field calibration
Dosimetry calibration
Profile Correction
Dmax 1D diagonal profile (w2CAD)
Output Factor measurement
Actual Fluence measurement
Intensity profileCopy from PBC model
(b) 2D profile includ-ing backscatter cor-
rection
Optimal & Actual fluence(a) Golden Beam Data
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)
2D profile correction to measurement
including backscatter provided by Varian
G
<My opinion - limitation>
1) field-size dependency on backscatter
2) Imager at off-axis
Possible choices
Prediction
1D-corrected Measured Beam Data
Golden Beam
Measurement
2D-corrected Measured Beam Data
1D-corrected Measurement
2D-corrected Measurement
BESTRESULTS
??
1D-corrected Measurement
Varian Previous Approach
Varian Updated Approach
P1
P2
PG
M1
M2
Preliminary resultsMachine : 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 worstand “Varian updated approach” was the best. How-ever, 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 im-plementation.
Thanks for your listening!Any questions?