A "HOWFARLESS" option to increase efficiency of homogeneous phantom calculations with DOSXYZnrc

This paper describes a "HOWFARLESS" transport option, which has been added to DOSXYZnrc to increase the efficiency of beam commissioning calculations in homogeneous phantoms. The algorithm speeds up charged particle transport by only considering the distance to the extreme outer boundaries of the phantom, thus eliminating the need to stop at voxel boundaries. Dose is deposited by approximating the total curved charged particle steps by two straight-line steps joined at a hinge point. Good agreement with normal simulations is achieved at all beam energies and for all practical maximum step lengths with a 1:1 mixture of approximations based on the initial position/ direction of the particle and on its final position/direction. Use of the "HOWFARLESS" option in phantom calculations for 6 and 18 MV photon beams (10 x 10 cm2 and 40 x 40 cm2 fields) from BEAMnrc-simulated accelerators increases the efficiency at the optimum photon splitting number by a factor of 2.9-5.4 when the exact EGSnrc boundary crossing algorithm (BCA) is used and by 51%-89% when the faster PRESTA-I BCA is employed. The efficiency gain due to the "HOWFARLESS" transport option increases with increasing beam energy and decreases with increasing field/dose voxel size. Efficiency improvement is greater when the efficiency of the particle source itself is not a factor, and in such cases the "HOWFARLESS" option improves the DOSXYZnrc efficiency by up to a factor of 13.1 (exact BCA) or 3.5 (PRESTA-I BCA) for photon beams, and up to a factor of 17.2 (exact BCA) or 5.2 (PRESTA-I BCA) for electron beams.