Dosimetric Impact of a New Computational Voxel Phantom Series for the Japanese Atomic Bomb Survivors: Methodological Improvements and Organ Dose Response Functions

Owing to recent advances in computational dosimetry tools, an update is warranted for the dosimetry system for atomic bomb survivors that was established by the Joint U.S.Japan Working Group on the Reassessment of Atomic Bomb Dosimetry in 2002 (DS02). The DS02 system, and its predecessor, DS86, at the Radiation Effects Research Foundation (RERF), are based on adjoint Monte Carlo particle transport simulations coupled with stylized computational human phantoms. In our previous studies, we developed the J45 series of computational voxel phantoms representative of 1945 Japanese adults, children and pregnant females. The dosimetric impact of replacing the DS02/DS86 stylized phantoms by the J45 phantom series was also discussed through computation of organ doses for several idealized exposure scenarios. In the current study, we investigated the possible impact of introducing not only the J45 phantom series but also various methodological upgrades to the DS02 dosimetry system. For this purpose, we calculated organ doses in adults for 12 representative exposure scenarios having realistic particle energy and angular fluence, using different combinations of phantoms and dose calculation methods. Those doses were compared with survivor organ doses given by the DS02 system. It was found that the anatomical improvement in the J45 phantom series is the most important factor leading to potential changes in survivor organ doses. However, methodological upgrades, such as replacement of the adjoint Monte Carlo simulation with kerma approximation by the forward Monte Carlo simulation with secondary electron transport, can also improve the accuracy of organ doses by up to several percent.In addition, this study established a series of response functions, which allows for the rapid conversion of the unidirectional quasi-monoenergetic photon and neutron fluences from the existing DS02 system to organ doses within the J45 adult phantoms. The overall impact of introducing the response functions in the dosimetry system is not so significant, less than 10% in most cases, except for organs in which the calculation method or definition was changed, e.g., colon and bone marrow. This system of response functions can be implemented within a revision to the DS02 dosimetry system and used for future updates to organ doses within the Life Span Study of the atomic bomb survivors.