Development and Implementation of a New Set of Anthropomorphic Phantoms for Dosimetry of the Japanese Atomic Bomb Survivors.

For many years, the dose estimates calculated at the Radiation Effects Research Foundation for the Japanese atomic bomb survivors in Hiroshima and Nagasaki have relied on models of the human body (phantoms) developed in the 1980s. These phantoms are used to calculate doses that specific organs of a survivor received from the gamma rays and neutrons emanating from the bombs. The phantoms from the 1980s are based on primitive geometrical shapes such as rods and cones and offer only three models to represent all age ranges from newborn to adult. These models included male and female sex organs (hermaphroditic in design) with no body differences by sex. Furthermore, no specific model of the female with a fetus was available to use for survivors who were exposed during pregnancy at the time of a bombing. A binational working group has recently developed a modern set of phantoms with more realistic shapes, six age ranges, and phantoms for pregnant women at four gestational ages, denoted the "J45" phantoms to indicate their bodily dimensions being based on the Japanese population of 1945. We discuss here the changes in the phantoms and the corresponding differences in body transmission factors which quantify body self-shielding for gamma rays and neutrons from the bombs for some important example organs. Results are presented as ratios of the body transmission factors for new vs. old phantoms. We discuss how the use of the new phantoms may change the dose estimates for the survivor population that are used in radiation epidemiology, including population average changes by distance, given survivor distributions by age and sex. Dose estimates for the new phantoms are not much more than 10% different from the old ones for gamma rays, the main source of dose, but are substantially larger for neutrons, a lesser source of dose. We also discuss additional implementation work, including issues related to the dependence of body transmission factors on survivor orientation (direction facing vis-à-vis the direction to the bomb) and on bodily position (standing, kneeling, lying down) and the related need to calculate averaged body transmission factors for survivors without those data. We also discuss remaining issues such as the use of the same phantom, i.e., the same body size and shape, for each combination of sex and age group, although body size varied among survivors and individual body size data are available for a large part of the cohort.