Lei Xing , Ph.D. & Jacob Haimson Professor Department of Radiation Oncology & MIPS Stanford University School of Medicine Ben Fahimian Ben Fahimian Ben Fahimian 3D & 4D End-to-End Test of TrueBeam SBRT QA 3D modeling • Systematic QA (end-to-end test). Treatment planning Pt setup and treatment delivery Imaging • Imaging QA. • Patient specific QA. • CT simulation, planning, image guided setup, and dose delivery (IMRT and ARC). • System test: to determine both dosimetric and targeting accuracy achievable with TrueBeam system. End-to-End Test of TrueBeam A B End-to-End Test of TrueBeam C L. Wang, k. Kielar, et al 2010 Gamma map (3%/1mm) for film measurements. L. Wang, et al, PMB, 2011 Patient specific QA: A Few Questions • Do I really need to do QA for each SBRT patient? • If I use an independent MU program do I still need QA for each patient? • Will I still need to SBRT QA after we’ve treated 500 patients? • If I expand my monthly machine QA can I eliminate SBRT QA for each patient? ACR Practice Guidelines • For IMRT/VMAT, it is recommended that an independent dose calculation method as an alternative to physical measurements once dosimetric accuracy of the planning / delivery system has been demonstrated. • We use ImSure from Standard Imaging Inc. for SBRT independent MU verification. Medicare Billing Guidelines • The accuracy of dose delivery must be documented for each course of treatment by irradiating a phantom that contains either calibrated film to sample the dose distribution or an equivalent measurement system to verify that the dose delivered is the dose planned • The dosimetry should be verified using an ionization chamber Patient specific QA for SBRT… IMSure MU check QA Devices Patient Specific QA A “phantom plan” or “hybrid plan” • Delta4/Film/Chamber • Ion chamber/Diode Arrays (PTW729, MapCheck etc) • EPID Dosimetry • Software independent check (IMSURE) Gantry Angles are all the same Gantry Angles = Tx Angles IMSure software independent check • At Stanford we use IMSURE What do I do when Gamma index from QA measurements fail? Small fields SBRT – output scaling…. 5%...? Dose Reconstruction Technique Actual Parameters at Control Points System’s Logfiles MUi-1 Varian TrueBeam System Delivery Parameters: MUi • gantry angle • leaf positions • delivered MUs … MUi+1 3D modeling Treatment planning Pt setup and treatment delivery Imaging Treatment Phase of 4D pCT or Pre-treatment 4D CBCT in-house program Treatment Planning System Dosimetric Evaluation Gated-VMAT Dose Reconstruction Re-constituted DICOM RP file TrueBeam Gated RapidArc – original plan, reconstructed dose distribution and PTW Measurement Planned and Reconstructed Dose Profile Comparison A Gamma passing rate = 99.4% Qian J, Lee L, Liu W, Fu K, Luxton, G, Le Q, and Xing L, Cone beam CT-based dose reconstruction for modulated arc therapy, Physics in Medicine and Biology, 2010. R L A-P profile R-L profile P Gated VMAT delivery with 3 sec respiration period Non-gated VMAT delivery (Reference) J. Qian, G. Luxton, L. Xing, 2010 Dose Distribution Comparison Reconstructio n PTV DVH Comparison Plan Positioning Errors and Dose Delivered to PTV Positioning errors intentionally introduced reconstruction plan reconstruction plan Position #1: same as the plan Position #2: L R: 0 mm; A P: -2 mm; S I: 2 mm Position #3: L R: 3 mm; A P: 5mm; S I: 5 mm EXAMPLE: VMAT therapy of Head and Neck Tumors Reconstructed and Planned Dose in QUASAR Phantom Planned (AAA) Target Volume: PTV_66 Prescribed Dose: 6600.0 cGy (220.0 cGy / fraction) Number of Fractions: 30 Delivered using RapidArc (Varian) Clinac Xin Chen, Gary Luxton, Lei Xing, Karl Bush, 2012 Comparison between Reconstructed and Planned Dose Reconstructed (MC) Planned dose in phantom: Eclipse AAA algorithm Reconstructed dose in phantom: Monte Carlos simulation using actual delivery parameters Xin Chen, Gary Luxton, Lei Xing, Karl Bush, 2012 Ion chamber point measurement in QUASAR phantom Computed 4 3 Dose profiles 1 2 3 4 5 6 7 8 1 2 Log-based reconstructed 4 Planned 99.90 80.00 198.10 --125.80 78.50 112.00 158.10 Measured 98.54 80.89 197.92 -123.55 80.65 113.80 157.41 Diff % -1.36% 1.11% -0.09% --1.79% 2.74% 1.61% -0.44% Note: 4th Reference point is an outlier, with Diff% = 10%, due to the high gradient region. 3 1 2 Xin Chen, Gary Luxton, Lei Xing, Karl Bush, 2012 Xin Chen, Gary Luxton, Lei Xing, Karl Bush, 2012 Case 1: DVHs Case 1: Dose Distribution pCT DVH comparison of the intended and delivered plans Relative volume (%) 100 80 CBCT1 CBCT2 pCT PT V 60 CBCT3 CBCT1 CBCT2 Brainstem 40 CBCT3 20 0 0 40 80 120 160 200 240 Dose (cGy) •Lee L, Le Q, Xing L: Retrospective IMRT dose reconstruction based on cone-beam CT and MLC logfile. International Journal of Radiation Oncology, Biology and Physics, 70: 634-644, 2008. Patient Specific QA – EPID EPID-based absolute dosimetry • EPID Dosimetry • unflattened beam • high dose rate • small sized fields in (SBRT) Routine SBRT QA – High efficiency – High dose resolution – Ease of use Bin Han, E. Mok, G. Luxton, L. Xing, ASTRO, 2013 EPID-based absolute dosimetry EPID Response Core 0.6 Dose (cGy) • Monte Carlo dose distribution kernel, Optical spread kernel, Total EPID response kernel Optical spread kernel 1.0E-01 Relative amplitude Relative dose 1.0E-02 1.0E-03 1.0E-04 1.0E-05 1.0E-06 1.0E-02 1.0E-06 1.0E-08 0 -20 -50 0 50 Off-axis position (mm) 100 EPID Response Kernel 1.0E-01 1.0E-05 1.0E-08 0.2 1.0E+00 1.0E-04 1.0E-07 1.0E-07 Exp(-45 x r) 1.0E-03 -10 0 10 Radial distance from center (cm) Relative Amplitude 1.0E-01 0.3 0 -100 Pencil beam dose distribution in EPID 6XFF F 10XF FF 0.4 0.1 1.0E+00 1.0E+00 TPS EPID PTW729 0.5 1.0E-02 6XFFF 1.0E-03 10XFFF 0.1 0.2 0.3 1.0E-04 20 from center (cm) Radial distance 0.4 1.0E-05 1.0E-06 1.0E-07 -20 -10 0 10 Radial distance from center (cm) Bin Han, E. Mok, G. Luxton, L. Xing, AAPM, 20102 20 Bin Han, E. Mok, G. Luxton, L. Xing, 20102 Result: EPID dosimetry for SBRT pt QA Profiles of Square Field 6XFFF EPID vs. TPS verification plan Isodose line LR profile TG profile • EPID profile vs water phantom scan 120 120 6XFFF Dose (cGy) 80 60 10XFFF 100 Dose (cGy) 100 80 2%, 2mm γ histogram 60 40 40 20 20 0 0 -10 -10 -5 0 Off-axis distance (cm) 5 10 -5 0 Off-axis distance (cm) 5 10 2%, 2mm γ distribution • Gamma results: 3mm/3%: 99.9% 2mm/3%: 98.5% 2mm/2%: 92.8% 1mm/3%: 63.0% SUMMARY • Features available in new generation of LINACs facilitate RT workflow and improve the efficiency & accuracy. • End-to-end test is important. • Beam level imaging. • SPORT. • New QA tools are urgently needed.
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