A Monte Carlo method using octree structure in photon and electron transport

Object description is important in performing the photon and/or electron transport using a Monte Carlo method efficiently. We propose a new description method using an octree representation of an object. Thus, even if the boundaries of each medium are represented accurately, high-speed calculation of photon transport can be accomplished because the number of voxels is much fewer than that of the voxel-based approach which represents an object by a union of the voxels of the same size. Our Monte Carlo code using the octree representation of an object first establishes the simulation geometry by reading "octree string", which is produced by forming an octree structure from a set of serial sections for the object before the simulation; then it transports photons in the geometry. Using the code, if the user just prepares a set of serial sections for the object in which he or she wants to simulate photon trajectories, he or she can perform the simulation automatically using the suboptimal geometry simplified by the octree representation without forming the optimal geometry by handwriting.