A method to compute dose-to-water for Monte Carlo transport simulation in realistic materials

Purpose: In radiotherapy, “dose” has been referred to the dose-to-water Dw. On the other hand, Monte Carlo (MC) simulation allows the material properties to be modeled using explicit material properties, and consequently provides the dose-to-medium Dm. Since the clinical experience has been based on Dw and dose description in practice is given by Dw, it is necessary to compute Dw when using MC simulation to calculate dose distribution for clinical usage or retrospective analysis. To compute Dw in MC simulation, we proposed a method to compute Dw in therapeutic carbon beam and verified the method using a heterogeneous layer phantom.Material and methods: We computed Dw based on Bragg-Gray theory. The energy deposited by a particle in water is approximately given by that in realistic materials multiplied by the stopping power ratio of water to the material. Thus, as a first-order approximation, Dw can be deduced by summing up the energy obtained by applying the factor, which differs in each particle and energy, to deposited energy by all the particles including primary and secondary particles. To verify the proposed method, we compared depth dose distribution of a 290 MeV/u carbon beam with 6 cm SOBP in a heterogeneous layer phantom. The material of heterogeneous layer was set as: (i) Lung and (ii) bone. Results: The difference of 80 %-dose depth d80 in the heterogeneous layer phantom between MC and TPS are within 1.5 mm and the difference of relative dose between TPS and Dw by MC were within ±1 % around the heterogeneous region.Conclusion: In this work, a method to compute Dw for Monte Carlo transport simulation in realistic materials has been proposed and the effectiveness has been verified.第118回 日本医学物理学術大