Calculation of photon path changes due to scatter in Monte Carlo simulations

Computation using Monte Carlo simulations is widely used for modelling the light-tissue interaction. Despite this, many of the methods used for building such simulations are poorly described in the literature. In particular, a scheme for translating the scatter angles produced from a phase function into updated photon direction vectors is not explicitly reported. To address this, a method for calculating the change in photon direction following a scattering event is described, thus illuminating one of the fundamental 'building blocks' for researchers developing their own Monte Carlo models. The equations derived in this paper may be readily incorporated into applicable Monte Carlo program code.