BNCT dosimetry has often employed heavy Monte Carlo calculations for the beam characterization and the dose
determination. However, these calculations commonly ignored the scattering influence between the radiations and
the room structure materials in order to facilitate the calculation speed. The aim of this article attempts to explore
how the room scattering affects the physical quantities such as the capture reaction rate and the gamma‐ray dose rate
under in‐phantom and free‐air conditions in the THOR BNCT treatment room. The geometry and structure materials of
the treatment room were simulated in detail. The capture reaction rates per atom, as well as the gamma‐ray dose rate
were calculated in various sizes of phantoms and in the free‐air condition. Results of this study showed that the room
scattering has significant influence on the physical quantities, whether in small phantoms or in the free‐air condition.
This paper may be of importance in explaining the discrepancies between measurements and calculations in the BNCT
dosimetry using small phantoms, in addition to provide a useful consideration with a better understanding of how the
room scattering influence acts in a BNCT facility.