Patient-dependent beam-modifier physics in Monte Carlo photon dose calculations

Model pencil-beam on slab calculations are used as well as a series of deta iled calculations of photon and electron output from commercial accelerator s to quantify level(s) of physics required for the Monte Carlo transport of photons and electrons in treatment-dependent beam modifiers, such as jaws, wedges, blocks, and multileaf collimators, in photon teletherapy dose calc ulations. The physics approximations investigated comprise (1) not tracking particles below a given kinetic energy, (2) continuing to track particles, but performing simplified collision physics, particularly in handling seco ndary particle production, and (3) not tracking particles in specific spati al regions. Figures-of-merit needed to estimate the effects of these approx imations are developed, and these estimates are compared with full-physics Monte Carlo calculations of the contribution of the collimating jaws to the on-axis depth-dose curve in a water phantom. These figures of merit are ne xt used to evaluate various approximations used in coupled photon/electron physics in beam modifiers. Approximations for tracking electrons in air are then evaluated. It is found that knowledge of the materials used for beam modifiers, of the energies of the photon beams used, as well as of the leng th scales typically found in photon teletherapy plans, allows a number of s implifying approximations to be made in the Monte Carlo transport of second ary particles from the accelerator head and beam modifiers to the isocenter plane.