Purpose: To develop a GPU-accelerated Monte Carlo (MC) engine incorporating the Varian TrueBeam linac head geometry for rapid calculation of electron fields collimated using the photon multi-leaf collimator (MLC).
Methods: A CUDA framework was created for the following: (1) transport of electrons and photons through the linac head geometry, taking into account multiple scattering, Bremsstrahlung, Moller, Compton, and pair production interactions; (2) electron and photon propagation through the CT geometry, taking into account all aforementioned interactions plus the photoelectric effect; and (3) secondary particle cascades through the linac head and within the CT geometry. The linac head collimating geometry was modelled according the specifications provided by the vendor, who also provided phase space files above the collimator jaws. The MC was benchmarked against EGSnrc by simulating individual interactions with simple geometries and pencil beam dose calculations in water phantoms. Lastly, MC-calculated dose distributions for MLC-collimated electron fields were compared to measurements in a water phantom.
Results: Pencil beam dose distributions within a water phantom are in good agreement with DOSXYZnrc. Angular and spatial distributions for multiple scattering and secondary particle production are in good agreement with EGSnrc. Dose profiles for MLC-collimated 6-20 MeV electron beams had differences of <5% for 5x5 cm² and 10x10 cm² field sizes as compared to measurement. The computation time on a single NVIDIA GTX TITAN X card is 15 minutes for 10⁸ electron histories, which is approximately 100 times faster than published results in a similar geometry using a single-CPU core.
Conclusion: The GPU-based MC can quickly calculate dose for electron fields collimated using the conventional photon MLC. Dose profiles in homogeneous medium agree well with DOSXYZnrc and measured data. The fast calculation times will allow for rapid calculation of electron fields for mixed photon and electron particle therapy.
Funding Support, Disclosures, and Conflict of Interest: Dr. Eric Brost was funded for his work by the Mayo Clinic School of Graduate Medical Education. Dr. John Antolak is funded, in-part, by Varian Medical Systems, CA, USA.