Dosimetric Impact of a New Computational Voxel Phantom Series for the Japanese Atomic Bomb Survivors: Pregnant Females

An important cohort of the atomic bomb survivors are women who were pregnant when exposed to the photon and neutron fields at both Hiroshima and Nagasaki, as well as their children who were exposed in utero. Estimates of organ dose to the developing fetus allow for the development of dose-dependent and gestational age-dependent models of deterministic (e.g., organ malformation) and stochastic (e.g., leukemia) risk of in utero exposure. To date, both the 1986 and 2002 dosimetry systems at the Radiation Effects Research Foundation have utilized the uterine wall in the non-pregnant adult female as a dose surrogate for individual fetal organs and tissues. Here we present a new J45 (Japanese 1945) series of high-resolution phantoms of the adult pregnant female at 8-, 15-, 25- and 38-weeks post-conception. These models, which were derived from the University of Florida (UF) series of ICRP Publication 89 compliant reference phantoms, have been rescaled to approximate the pregnant mother using 1945 Japanese morphometry data. Fetal and maternal organ doses were estimated by computationally exposing the pregnant female phantom series to DS02 free-in-air photon and neutron fluences at three distances from the hypocenter at both Hiroshima and Nagasaki under frontal (AP) and isotropic (ISO) particle incidence. As for the fetal organ doses, our results indicate that the uterine wall of the non-pregnant female generally underestimates fetal organ dose within the pregnant female. The magnitude of these differences varies with both radiation type and irradiation geometry, with the smallest differences (5–7%) seen for ISO photon fields and the largest differences (20–30%) seen for AP neutron fields. Significant discrepancies were seen in fetal brain dose and its uterine wall surrogate, particularly for photon AP fields (ratio of uterine wall to brain dose varied from 0.9 to 1.3) and neutron AP fields (dose ratios from 0.75 to 2.0). As for the maternal organ doses, the use of organ doses in a non-pregnant female was shown, in general, to overestimate the corresponding organ doses in the pregnant female, with greater deviations seen at later stages of pregnancy (12–16% for AP photons and 44–53% for AP neutrons). The one exception was the uterine wall dose in pregnancy which was seen to be underestimated by that in the non-pregnant female phantom, particularly for ISO and AP neutron fields. These results demonstrate that the J45 pregnant female phantom series offers the opportunity for significant improvements in both fetal and maternal organ dose assessment within this unique cohort of the atomic bomb survivors.