TOPAS simulation of photoneutrons in radiotherapy: accuracy and speed with variance reduction.

We provide optimal particle split numbers for speeding up TOPAS Monte Carlo simulations of linear accelerator (linac) treatment heads while maintaining accuracy. In addition, we provide a new TOPAS physics module for simulating photoneutron production and transport. 
TOPAS simulation of a Siemens Oncor linac was used to determine the optimal number of splits for directional bremsstrahlung splitting as a function of the field size for 6MV and 18MV x-ray beams. The linac simulation was validated against published data of lateral dose profiles and percentage depth-dose curves (PDD) for the largest square field (40cm side). In separate simulations, neutron particle split and the custom TOPAS physics module was used to generate and transport photoneutrons, called "TsPhotoNeutron". Verification of accuracy was performed by comparing simulations with published measurements of: 1) neutron yields as a function of beam energy for thick targets of Al, Cu, Ta, W, Pb and concrete; and 2) photoneutron energy spectrum at 40cm laterally from the isocenter of the linac from an 18MV beam with closed jaws and MLC. 
The optimal number of splits obtained for directional bremsstrahlung splitting enhanced the computational efficiency by two orders of magnitude. The efficiency decreased with increasing beam energy and field size. Calculated lateral profiles in the central region agreed within 1mm/2% from measured data, PDD curves within 1 mm/1%. For the TOPAS physics module, at a split number of 146, the efficiency of computing photoneutron yields was enhanced by a 27.6 factor, whereas it improved the accuracy over existing Geant4 physics modules. 
This work provides simulation parameters and a new TOPAS physics module to improve the efficiency and accuracy of TOPAS simulations that involve photonuclear processes occurring in high-Z materials found in linac components, patient devices, and treatment rooms, as well as to explore new therapeutic modalities such as very-high-energy electron therapy.