Megan A. Wood, M.S. Under the direction of Larry DeWerd, Ph.D. University of Wisconsin Medical Radiation Research Center (UWMRRC) NCCAAPM Spring Meeting May 3, 2013
Introduction FFF background Project motivation Materials and Methods Phase space files BEAMnrc linac model Commissioning data acquisition Results Measured vs modeled Conclusions and Future Direction 2
FFF studies began as early as 1991 1 The motivation for widespread FFF availability was the advent of IMRT Flattening + modulation throws away dose rate Faster treatments may be beneficial for some patients The recently released Varian TrueBeam TM is the first commercial C-arm linac with FFF capability 1. P. F. O Brien, B. A. Gillies, M. Schwartz, C. Young, and P. Davey, Radiosurgery with unflattened 6-MV photon beams, Med. Phys. 18(3), 519-521 (1991) 2. (Image source) www.variantruebeam.com/truebeam-varian/ 3
Changes that will affect dosimetry: Profile shape Softer spectrum Higher dose rate 1. (Image source) O. N. Vassiliev, U. Titt, F. Pӧnisch, S. F. Kry, R. Mohan, and M. T. Gillin, Dosimetric properties of photon beams from a flattening filter free clinical accelerator, Phys. Med. Biol. 51(7), 1907-1917 (2006) 4
1. (Image source) E. S. M. Ali and D. W. O. Rogers, Functional forms for photon spectra of clinical linacs, Phys Med Biol 57, 31-50 (2012) 5
From Georg et al. (2011) several aspects related to standardization, dosimetry and dose determination, treatment planning, and radiobiology need to be addressed for a widespread clinical implementation of unflattened beams A reliable, benchmarked linac model is a crucial component of a comprehensive dosimetric study 1. D. Georg, T. Knoos, and B. McClean, Current status and future perspective of flattening filter free photon beams, Med. Phys. 38 (3), 1280-1293 (2011) 6
Varian provides IAEA-compliant phase space files above the jaws Keeps upper linac components proprietary Standardizes simulations across researchers These files contain particle data from their GEANT4 simulations Position, direction, energy, particle type, etc. Matched to Golden Beam data, not individual machines Full phase space is split into 55 files, each 1 GB 1. Varian correspondence, Varian Monte Carlo data package 7
EGSnrc currently has a phase space size limitation of 2 GB Is it necessary to use every phase space file in the set to get accurate results? Combination tests were performed, comparing PDDs with each individual file to those resulting from all files combined 8
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Dose (10-16 Gy per starting particle) PDD Comparison: all phsp files 2.870 2.770 2.670 2.570 00 01 02 03 04 05 06 07 08 09 2.470 2.370 0.45 0.95 1.45 1.95 2.45 Depth (cm) 10
Conclusion of phase space combination tests: One or two phase space files can represent all, with enough histories for good statistics This solution saves time This allows a workaround for the EGSnrc 2 GB limit The NRCC is currently addressing this problem 11
BEAMnrc is an EGSnrc user code designed to model typical clinical linear accelerators An accelerator is built using several sections called component modules (CMs) A basic model of the TrueBeam TM was developed using the JAWS and CHAMBER modules 12
Jaw simplifications Solid blocks only, no bores or cutouts No arc translation Baseplate simplifications Missing many cutouts, some tungsten shielding Simplified shape to donut (no straight edges) Upper shield shortened (cannot overlap jaw CM) MLC was omitted for the preliminary comparison 13
A second phase space was scored after the baseplate CM in BEAMnrc This was used as the source in a DOSXYZnrc calculation of PDDs DOSXYZnrc is another user code designed for dose calculation in voxelized phantoms PDDs were scored using 0.5 x 0.5 x 0.5 cm 3 voxels along the central axis 14
A full set of commissioning data was acquired at the UW Carbone Cancer Center PDDs were acquired using Wellhӧfer CC13 compact scanning chambers (0.13 cm 3 ) for both reference and field IBA Blue Phantom and OmniPro Accept software were used for data collection Chamber was always shifted to the effective point of measurement 1. (Image source) Wellhofer compact chamber specification sheet, http://www.cnmcco.com 2. (Image source) Blue Phantom specification sheet, http://www.test.scanditronix-wellhoefer.com 15
Dose (relative to maximum) PDD Comparison: Open 40cm x 40cm Field Measured 40x40 field Simulated open field 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 40 Depth (cm) 16
Dose (relative to max) PDD Comparison: 10x10 Field Measured Simulated 1 0.8 0.6 0.4 0.2 0 0 5 10 15 20 25 30 35 Depth (cm) 17
Good agreement was seen between open field and 10x10 measured and modeled results Max discrepancies were at shallowest and deepest depths: -8.2% and -6.39%, respectively All other points along PDDs agreed within 4% Better agreement should be achievable Limitations of EGSnrc/BEAMnrc Limitations of current phase space capabilities Future direction GEANT4 will be used to develop a more detailed, fully benchmarked model Varian s upcoming cloud computing platform will be used to match the model to measured data This model will be used for a comprehensive assessment of current dosimetry methods for FFF linacs 18
Dr. Larry DeWerd John Micka UWMRRC students and staff UWADCL customers UW Carbone Cancer Clinic TrueBeam commissioning team Charlie Wallace Dr. Jeni Smilowitz 19
Questions are welcome