Estimation of relative biological effectiveness for boron neutron capture therapy using the PHITS code coupled with a microdosimetric kinetic model

The absorbed doses deposited by boron neutron capture therapy (BNCT) can be categorized into four components: alpha and Li-7 particles from the B-10(n, alpha) Li-7 reaction, 0.54-MeV protons from the N-14(n, p) C-14 reaction, the recoiled protons from the H-1(n, n)H-1 reaction, and photons from the neutron beam and H-1(n, gamma)H-2 reaction. For evaluating the irradiation effect in tumors and the surrounding normal tissues in BNCT, it is of great importance to estimate the relative biological effectiveness (RBE) for each dose component in the same framework. We have, therefore, established a new method for estimating the RBE of all BNCT dose components on the basis of the microdosimetric kinetic model. This method employs the probability density of lineal energy, y, in a subcellular structure as the index for expressing RBE, which can be calculated using the microdosimetric function implemented in the particle transport simulation code (PHITS). The accuracy of this method was tested by comparing the calculated RBE values with corresponding measured data in a water phantom irradiated with an epithermal neutron beam. The calculation technique developed in this study will be useful for biological dose estimation in treatment planning for BNCT.