Biological Dose Estimation for Charged-Particle Therapy Using an Improved PHITS Code Coupled with a Microdosimetric Kinetic Model

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  • マイクロドジメトリに基づく高エネルギー粒子線RBE計算手法の構築

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Abstract

The microdosimetric quantities such as lineal energy y are better indexes for expressing the RBE of HZE particles in comparison to LET. However, use of the microdosimetric quantities in the computational dosimetry was severely limited because of the difficulty in calculating their probability densities (PDs) in macroscopic matter. We therefore improved the particle transport simulation code PHITS to be capable of estimating the microdosimetric PDs in macroscopic area, by means of incorporating a mathematical function that can instantaneously calculate the PDs around the trajectory of HZE particles with precision equivalent to a microscopic track-structure simulation. A new method for estimating biological dose, a product of physical dose and RBE, from charged-particle therapy was established, using the improved PHITS coupled with a microdosimetric kinetic model. The accuracy of the biological dose estimated by this method was reliably verified by comparing the calculated physical doses and RBE values with the corresponding data measured in a slab phantom irradiated with several kinds of HZE particles. The simulation technique established in this study will help to optimize the treatment planning of charged-particle therapy for maximizing the therapeutic effect on tumor while minimizing unintended harmful effects on surrounding normal tissues.

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