Implementation of simplified stochastic microdosimetric kinetic models into PHITS for application to radiation treatment planning

Abstract

Purpose

The stochastic microdosimetric kinetic (SMK) model is one of the most sophisticated and precise models used in the estimation of the relative biological effectiveness of carbon-ion radiotherapy (CRT) and boron neutron capture therapy (BNCT). However, because of its complicated and time-consuming calculation procedures, it is nearly impractical to directly incorporate this model into a radiation treatment-planning system.

Materials and methods

Through the introduction of Taylor expansion (TE) or fast Fourier transform (FFT), we developed two simplified SMK models and implemented them into the Particle and Heavy Ion Transport code System (PHITS). To verify the implementation, we calculated the photon isoeffective doses in a cylindrical phantom placed in the radiation fields of passive CRT and accelerator-based BNCT.

Results and discussion

Our calculation suggested that both TE-based and FFT-based SMK models can reproduce the data obtained from the original SMK model very well for absorbed doses approximately below 5 Gy, whereas the TE-based SMK model overestimates the original data at higher doses. In terms of computational efficiency, the TE-based SMK model is much faster than the FFT-based SMK model.

Conclusion

This study enables the instantaneous calculation of the photo isoeffective dose for CRT and BNCT, considering their cellular-scale dose heterogeneities. Treatment-planning systems that use the improved PHITS as a dose-calculation engine are under development.

Keywords:

• Microdosimetry
• treatment planning
• Monte Carlo
• RBE
• PHITS

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work is partially supported by Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Number [JP18K19899].

Notes on contributors

Tatsuhiko Sato

Tatsuhiko Sato, Ph.D., is a researcher at Nuclear Science and Engineering Center, Japan Atomic Energy Agency and a specially appointed professor at Research Center for Nuclear Physics, Osaka University.

Shintaro Hashimoto

Shintaro Hashimoto, Ph.D., is a researcher at Nuclear Science and Engineering Center, Japan Atomic Energy Agency.

Taku Inaniwa

Taku Inaniwa, Ph.D., is a researcher at Department of Accelerator and Medical Physics, National Institute of Quantum and Radiological Science and Technology.

Kenta Takada

Kenta Takada, Ph.D., is an associate professor at Graduate School of Radiological Technology, Gunma Prefectural College of Health Sciences.

Hiroaki Kumada

Hiroaki Kumada, Ph.D., is an associated professor at Faculty of Medicine, University of Tsukuba.