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**Backscatter for EPID dosimetry**

Tuesday Seminar Wonmo Sung

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**Contents Introduction Literature Review SNUH clinic implementation**

Conclusion

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Introduction Electronic Portal Imaging Devices (EPID) : Amorphous silicon photodiode 1) For patient positioning 2) For DOSIMETRY! -> PDIP (Portal Dose Image Prediction) 1) 2)

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**However… non-uniform support arm backscatter**

However… non-uniform support arm backscatter ? + How about backscatter from bunker?

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**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) He received PhD at University of Adelaide, Australia. Calvary Mater Newcastle 에서 research physicist

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**Backscatter (Rowshanfarzad et al, MP, 2010)**

EPID+ARM EPID Devices: aS500 & E-arm (same physical components as the aS1000) For 40x30 cm2 field (EPID + arm) – (EPID) = Max 6%

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**Physical Approach - lead sheet - BSS-EPID**

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**Physical Approach - lead sheet (Rowshanfarzad et al, PMB, 2010)**

Optimal lead sheet : 2 mm (Backscatter reduction vs. weight) Optimal lead sheet: 2 mm (차폐 vs 무게) The arm backscatter at the central axis with the 2 mm lead in place decreased to 0.1% and 0.2% for the largest field size of 30×30 cm2 using 6 and 18 MV beams, from 2.3% and 1.3% without lead. For 30x30 cm2 field, at CAX 6 MV 18 MV Lead 2 mm 0.1% 0.2% w/o lead 2.3% 1.3%

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**Physical Approach - lead sheet (Rowshanfarzad et al, PMB, 2010)**

Symmetry improvement of in-line profiles The symmetry of the in-plane profiles improved for all field sizes for both beam energies. The addition of lead decreased the contrast-to-noise ratio and resolution by 1.3% and 0.84% for images taken in 6 MV and by 0.5% and 0.38% for those in 18 MV beams. Contrast-to-noise ratio & resolution are decreased. But not significant reduction in image quality

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**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.

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**Physical Approach – Varian BSS-EPID (King et al, MP, 2012)**

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 이전 EPID와 7%에서 0.5%이하로 낮춤. For >99% pixels, 0.5% change At CAX, maximum 0.2% effect on the central axis

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**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

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Software Approach - backscatter kernel 1) field size independent 2) field size dependent 3) dose conversion - just correct measurement (Varian updated)

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**Software approach – backscatter kernel (Rowshanfarzad et al, MP, 2010)**

Rowshanfarzad developed simple backscatter kernel using measurements & Monte Carlo. EPID+ARM EPID TRG spec : E-arm, aS1000 밑의 그림에서 아래쪽이 gantry Varian알고리즘(AnnVanEsch)이 아니고 자체개발한 Greer알고리즘(TERMA도입)을 Dose prediction에 사용하였음. Deconvolution +Gaussian fit

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**Software approach – backscatter kernel (Rowshanfarzad et al, MP, 2010)**

BEFORE AFTER Gamma evaluation (2%/2 mm) for 20 × 20 cm2 field size

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**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

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**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.

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**1) No energy spcetra condisderation **

So far, we talked about… Energy fluence-convolution PDIP (Peter B. Greer, MP 36 (2009) ) ~ 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) ) 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,… • Tx, y is the TERMA, i.e., the primary dose deposited in the EPID, expressed in cGy per MU; • Kx, y is the EPID dose-deposition kernel; • OcF output factor • Px, y off-axis ratio. EPID가 energy에 따라 반응이 다르므로 open beam, MLC leakage, head scatter 다 분리해야 한다. 분리하면 non-uniformbackscatter의 양도 정확히 알 수 있다.

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**Varian PDIP algorithm (Ann Van Esch)**

PDIP: Portal Dose Image Prediction

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**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.

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**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 + 2D-correction + no backscatter correction 40x30에서는 backscatter correction 한 것은 잘 맞음. 하지만 backscatter correction 안 하면 backscatter가 드러남. 10x10에서는 backscatter correction 한 것이 잘 안 맞음. (왜냐하면 40x30에서는 backscatter가 있는 것으로 correction파일이 되어있는데 10x10에서는 backscatter가 없으니까 underestimation하게 됨.) Backscatter correction 안 하면 상관없으니까 잘 맞음.

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**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 + 2D-correction + no backscatter correction 40x30에서는 backscatter correction 한 것은 잘 맞음. 하지만 backscatter correction 안 하면 backscatter가 드러남. 10x10에서는 backscatter correction 한 것이 잘 안 맞음. (왜냐하면 40x30에서는 backscatter가 있는 것으로 correction파일이 되어있는데 10x10에서는 backscatter가 없으니까 underestimation하게 됨.) Backscatter correction 안 하면 상관없으니까 잘 맞음.

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Bunker backscatter

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**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) 2) Field size 18x18 1번: 그냥 bunker만 있을 때 기준 relative EPID response(%) 2번과 3번: Gantry 0도 기준 relative EPID response(%) Linac output variation with gantry angle은 MatriXX로 측정했으므로 유의해서 볼 ㄴ것.

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**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) 2) Field size 18x18 1번: 그냥 bunker만 있을 때 기준 relative EPID response(%) 2번과 3번: Gantry 0도 기준 relative EPID response(%) Linac output variation with gantry angle은 MatriXX로 측정했으므로 유의해서 볼 ㄴ것.

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**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) 2) Field size 18x18 1번: 그냥 bunker만 있을 때 기준 relative EPID response(%) 2번과 3번: Gantry 0도 기준 relative EPID response(%) Linac output variation with gantry angle은 MatriXX로 측정했으므로 유의해서 볼 ㄴ것.

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**SNUH clinic implementation**

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방사선치료임상실습 ( 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

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**Varian PDIP commissioning procedure**

EPID calibration Imager Dark & Flood field calibration Dosimetry calibration Profile Correction Dmax 1D diagonal profile (w2CAD) PDIP: Portal Dose Image Prediction Beam data import Output Factor measurement Actual Fluence measurement Optimal & Actual fluence Intensity profile Copy from PBC model

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**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 (b) 2D profile including backscatter correction Dmax 1D diagonal profile (w2CAD) Truebeam EPID용은 아직 release 안 됨. Beam data import (a) Golden Beam Data Output Factor measurement Actual Fluence measurement Optimal & Actual fluence Intensity profile Copy from PBC model

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**Varian Update (CTB-PV-887-A, Oct. 15th, 2012 released)**

Varian은 알고리즘에 추가하는 게 아니라 off axis correction에 쓰는 것에 아예 2D correction 파일을 넣음 Using Golden beam data means… Golden EPID output factor & Golden kernel <My opinion - limitation> Machines specific characteristics Ex. Same EPID? Same Linac?

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**Varian Update (CTB-PV-887-A, Oct. 15th, 2012 released)**

G 2D profile correction to measurement including backscatter provided by Varian Varian은 알고리즘에 추가하는 게 아니라 off axis correction에 쓰는 것에 아예 2D correction 파일을 measurement에 넣음 <My opinion - limitation> 1) field-size dependency on backscatter 2) Imager at off-axis

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**1D-corrected Measurement**

Possible choices Prediction Measurement Varian Previous Approach 1D-corrected Measured Beam Data 1D-corrected Measurement 1D-corrected Measurement P1 M1 BEST RESULTS ?? 2D-corrected Measured Beam Data P2 Varian Updated Approach 2D-corrected Measurement M2 그러나 C2-M2도 정확히 measurement based는 아닌 것이, 2d correction 파일은 golden beam data라 할 수 있음. PG Golden Beam

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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 94.3 91.7 94.2 VMAT1 72.2 76 65.6 69.5 VMAT2 95.3 98.7 97.9 Average 85.775 86.05 86.875 88.775 결과의 일부분만 보여드리는 것. So far, “Varian previous approach” was the worst and “Varian updated approach” was the best. However, NOT ALWAYS.

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**Future plan Re-Commissioning 1D-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

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Conclusions Further improvement in the agreement between the model prediction and EPID measurements is required. More investigation is necessary for SNUH clinic implementation.

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**Thanks for your listening! Any questions?**

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