(Saturday, 4/7/2018)
Room: Foyer
Purpose: With the implementation of InCise™ 2 Multileaf Collimator (iMLC) in CyberKnife® M6™ system (M6CK), dose calculations in a heterogeneous media becomes more challenging with a serious of discrete highly intensity-modulated MLC beamlets and non-isocentric FFF beam projections. An independent dose prediction model is essential for validating its treatment planning system and dose measurement methodologies. This research is to investigate the efficacy of using BEAMnrc and DOSXYZnrc programs in EGSnrc system for M6CK Monte Carlo (MC) modeling.
Methods: The M6CK Linac head was modeled using BEAMnrc program for millions of particles and stored as phase space files before and after iMLC. These files were then fed into the DOSXYZnrc program for dose calculation. We have already modeled the linac head before MLC using a monoenergetic electron beam as a primary source with Gaussian profile, with energy and FWHM optimized by comparing with measurements, and calculated the dose at 800 mm SAD in a water phantom to verify the model. During MC simulations, various variance reduction techniques were applied as a trade off between computation time/simulation histories and statistical uncertainties.
Results: The early results at 10 millions histories showed that the dose distribution for 53.8mm by 53.9mm iMLC segment were within an acceptable 2%/2mm statistical uncertainty. A preliminary model for iMLC module was easily added to linac head module and soon be evaluated.
Conclusion: The current MC model of M6CK using BEAMnrc and DOSXYZnrc programs suggest a promising module-based approach to establish a full system model (including iMLC), with more flexibility to modify the components/parameters without need of programing expertise, for accurate dose predictions in heterogeneous phantom and multiple measurement detectors.
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