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B ackscatter for EPID dosimetry

B ackscatter for EPID dosimetry. Tuesday Seminar Wonmo Sung. Contents. Introduction Literature Review SNUH clinic implementation Conclusion. Introduction. Electronic Portal Imaging Devices (EPID) : Amorphous silicon photodiode. 1) For patient positioning. 2) For DOSIMETRY!

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B ackscatter for EPID dosimetry

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  1. Backscatterfor EPID dosimetry Tuesday Seminar Wonmo Sung

  2. Contents • Introduction • Literature Review • SNUH clinic implementation • Conclusion

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

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

  5. 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)

  6. 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

  7. Physical Approach - lead sheet- BSS-EPID

  8. Physical Approach - lead sheet(Rowshanfarzad et al, PMB, 2010) Optimal lead sheet: 2 mm (Backscatter reduction vs. weight) For 30x30 cm2 field, at CAX

  9. 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

  10. 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.

  11. 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 • For >99% pixels, 0.5% change • At CAX, maximum 0.2% effect on the central axis

  12. 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

  13. Software Approach - backscatter kernel1) field size independent2) field size dependent3) dose conversion- just correct measurement(Varian updated)

  14. Software approach – backscatter kernel (Rowshanfarzad et al, MP, 2010) Rowshanfarzad developed simple backscatter kernel using measurements & Monte Carlo. EPID+ARM EPID Deconvolution +Gaussian fit

  15. Software approach – backscatter kernel(Rowshanfarzad et al, MP, 2010) BEFORE AFTER Gamma evaluation (2%/2 mm) for 20 × 20 cm2 field size

  16. 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

  17. 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.

  18. So far, we talked about…Energy fluence-convolution PDIP (Peter B. Greer, MP 36 (2009) 547-555)~ similar to CCC1) energy fluence (open beam+MLCleakage+head scatter) from Pinnacle2) several kernels TERMA However, we use… Varian simple convolution PDIP (Ann Van Esch, Green 71 (2004) 223-234) 1) No energy spcetracondisderation 2) Single kernelto 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,…

  19. Varian PDIP algorithm (Ann Van Esch)

  20. 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.

  21. Software approach – Just correct measurement!(Vinall et al, BJR, 2010) R-arm, 40 × 30 cm2field, 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

  22. Software approach – Just correct measurement!(Vinall et al, BJR, 2010) R-arm, 10 × 10 cm2field, 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

  23. Bunker backscatter

  24. 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)

  25. 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)

  26. 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)

  27. SNUH clinic implementation

  28. 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

  29. Varian PDIP commissioning procedure EPID calibration Imager Dark & Flood field calibration Dosimetry calibration Profile Correction Dmax1D diagonal profile (w2CAD) Beam data import Output Factor measurement Actual Fluencemeasurement Optimal & Actual fluence Intensity profile Copy from PBC model

  30. 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 Dmax1D diagonal profile (w2CAD) Beam data import (a) Golden Beam Data Output Factor measurement Actual Fluencemeasurement Optimal & Actual fluence Intensity profile Copy from PBC model

  31. Varian Update (CTB-PV-887-A, Oct. 15th, 2012 released) Using Golden beam data means… Golden EPID output factor & Golden kernel <My opinion - limitation> • Machines specific characteristics Ex. Same EPID? Same Linac?

  32. 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

  33. Possible choices Prediction Measurement Varian Previous Approach 1D-corrected Measured Beam Data 1D-corrected Measurement 1D-corrected Measurement P1 M1 2D-corrected Measured Beam Data P2 2D-corrected Measurement M2 BEST RESULTS ?? Golden Beam PG Varian Updated Approach

  34. 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) So far, “Varian previous approach” was the worstand “Varian updated approach” was the best. However, NOT ALWAYS.

  35. 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

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

  37. Thanks for your listening!Any questions?

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