Beam Quality Effects in Nonstandard Fields of the Varian TrueBeam

Transcription

1 Beam Quality Effects in Nonstandard Fields of the Varian TrueBeam Megan A. Hyun, M.S. Under the supervision of Larry A. DeWerd, Ph.D. NCCAAPM Fall Meeting October 30, 2015 Medical Radiation Research Center

2 Introduction FFF motivation: FF unnecessary for advanced techniques Faster hypofractionated treatments FFF studies published from Varian TrueBeam TM 6MV and 10MV FFF modes 1. D. M. Galbraith, Med. Phys. 16, (1989) 2. (Image source) Varian Medical Systems, Palo Alto, CA, USA, 2

3 FFF Dosimetry Challenges Profile shape More peaked Less variation with depth Spectrum Softer Less off-axis variation Dose rate Up to 2400 MU/min (4x higher) Volume averaging Energy dependence Beam quality corrections Dose-rate dependence 1 1. (Image source) O. N. Vassiliev, U. Titt, F. Pӧnisch et al., Phys. Med. Biol. 51 (2006) 3

4 FFF Dosimetry TG-51 1,2 Beam quality specified by PDD(10) X FFF accounted for using Pb foil TRS Beam quality specified by TPR 20,10 FFF discrepancies up to 2% Need additional TPR to correct for beam quality Dose relative to 10x Output Factors 6MV 6FFF 10MV 10FFF Side of square field (cm) Alfonso et al. 4 (TG-155) 1. P. R. Almond, P. J. Biggs, B. M. Coursey et al., Med. Phys. 26 (1999) 2. M. McEwen, L. DeWerd, G. Ibbott et al., Med. Phys. 41 (2014) 3. IAEA TRS-398, International Atomic Energy Agency (2004) 4. R. Alfonso, P. Andreo, R. Capote et al., Med. Phys. 35 (2008) 4

5 Beam Quality Correction Factors Corrected for in TG-51:, For FFF 10x10 (MSR) field: For small fields:,,,,,,,,,,,, Ω,,,, Ω,,,, 1. P. R. Almond, P. J. Biggs, B. M. Coursey et al., Med. Phys. 26 (1999) 2. M. McEwen, L. DeWerd, G. Ibbott et al., Med. Phys. 41 (2014) 3. R. Alfonso, P. Andreo, R. Capote et al., Med. Phys. 35 (2008),,,,

6 Measurement Methods Reference detectors L-α-alanine pellets (10x10, 5x5, 2x2 cm 2 ) Exradin W1 PSD (10x10 to 0.6x0.6 cm 2 ) Corrected detectors Ionization chambers Exradin A12 Farmer-type chamber Exradin A1SL scanning chamber Exradin A26 microchamber Diodes Sun Nuclear Edge PTW E diode Measured: D w (alanine, W1) M (chambers, diodes) 6MV, 6FFF, 10MV, 10FFF, 60 Co,,,, (Image source) Far West Technology, Inc., Goleta, CA, USA, 2. Virtual Water TM paddles, manufactured by Med-Cal Inc., Verona, WI 3. (Image source) Standard Imaging, Inc., Middleton, WI 6

7 Monte Carlo Model The TrueBeam TM accelerator was modeled using BEAMnrc/EGSnrc Varian-provided phase spaces Simplified geometries PDDs and profiles were obtained using DOSXYZnrc Compared to commissioning data (measured with CC13 ionization chamber) JAWS: X and Y jaws JAWS: tungsten shield 35 x 35 x 35 cm 3 water phantom CHAMBER: steel baseplate 0.5 x 0.5 x 0.5 cm 3 voxels 7

8 Model Validation: PDDs 8

9 Model Validation: Profiles 9

10 Monte Carlo Methods Chambers modeled in egs_chamber/egsnrc D ch : dose to air in chamber volume D w : dose to water at measurement point,, Point sources at 100 cm SSD 1. B. R. Muir and D. W. O. Rogers, Med Phys 37 (2010) 10

11 Results: MSR Fields,,,, 11

12 Results: A1SL k Q 12

13 Results: A12 k Q 13

14 Results: 6MV Small-Field Corrections Scintillator-based,,,,,,,, 14

15 Results: 6FFF Small-Field Corrections Scintillator-based,,,,,,,, 15

16 Conclusions and Future Work Conclusions Measured,, effects up to 0.7% for the chambers in 10x10 cm 2 fields Agreed with TG-51-predicted values and Monte Carlo results within uncertainties Measured,, effects greater than 6% for chambers and diodes in fields down to 6x6 mm 2 1 Future Work Small-field Monte Carlo Treatment planning study 1. (Image source) Varian Medical Systems, Palo Alto, CA, USA, 16

17 Acknowledgements Dr. Larry DeWerd UW Carbone Cancer Center Dr. Zac Labby Dr. Wesley Culberson Dr. Jessica Miller UWMRRC students and staff Special thanks to: John Micka Mike Lawless Sam Simiele UW Accredited Dosimetry Calibration Lab customers Continued support helps fund student research National Physical Laboratory (UK) Claire Gouldstone Russ Thompson Simon Duane Thank you for your attention! Questions are welcome 17

18 FFF Spectral Effects 18

19 Risks vs. Benefits of FFF: Controversy Benefits Higher dose rate Faster treatment Reduced head scatter Less off-axis spectral variation Easier beam modeling Risks Higher dose rate Interplay effect Target motion errors (coughing) Potentially increased in-patient scatter and surface dose 2 1. C. Liu, M. G. Snyder, and C. G. Orton, Med. Phys. 40 (2013) 2. (Image source) Varian Medical Systems, Palo Alto, CA, USA, 19