Backscatter for EPID dosimetry Tuesday Seminar Wonmo Sung.

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


The displacement of the EPID central pixel was measured during a 360◦ gantry rotation with and without lead which was 1 pixel different.


• 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.


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


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)


2)



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

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


Using Golden beam data means…

Golden EPID output factor & Golden kernel

<My opinion - limitation>

1) Machines specific characteristics

Ex. Same EPID? Same Linac?


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


BESTRESULTS

??


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?

Backscatter for EPID dosimetry Tuesday Seminar Wonmo Sung.

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Transcript of Backscatter for EPID dosimetry Tuesday Seminar Wonmo Sung.