Accelerating photon transport in Monte Carlo simulation using octree structure

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The study was carried out to develop a high-speed calculation method in photon/electron transportation using a Monte Carlo simulation. Object description is important in performing the photon transport efficiently. In order to describe an object, three methods have been proposed : a shape-based (SV) approach, a voxel-based (VB) approach, and a solid geometry based approach. However, these three methods have some shortcoming (e.g., accuracy of boundaries, calculation time, amount of memory, manual instructions). To the problems we introduced an octree representation of an object to the photon transport using a Monte Carlo method. The representation of the object yields accurate boundaries of each medium and fast calculation of the photon transport with the same accuracy as the VB method using the finest voxels. Moreover, the octree structure for any object can be formed systematically.Our Monte Carlo code, 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 automatically, and then it transports photons in the geotry. Using the code, if the user just prepares a set of serial sections for the object in which he wants to simulate photon trajectories, he can perform the simulation automatically using the suboptimal geometry simplified by the octree representation without forming the optimal geometry by handwriting. The accuracy and ability of our code were confirmed by comparison with Electron Gamma Shower code system (EGS4) and simulations using a head phantom, respectively. Moreover, we examined our code using Zubal's digital phantoms (head and thorax) by comparing the results of the VB approach. In summary, the proposed method is efficient in the Monte Carlo simulation of any object.