Severe mental retardation after large prenatal exposures to bomb radiation. Reduction in oxygen transport to fetal brain: a possible abscopal mechanism.

Reasonable expectations, based on the normal physiology of the human fetus, cast new light on the possibilities of extrapolation from the finding of severe mental retardation (SMR) in Japanese bomb survivors after maternal exposure to 1.8-5.5 Gy T65DR Gy. After such large exposures the fetal haematopoietic tissues (DS86 dose 0.9-2.2 Gy) cannot escape severe damage and a consequent reduction in erythropoiesis. Diminished fetal erythropoiesis will diminish oxygen transport from placenta to fetus. Impaired oxygen transport to the developing forebrain will augment the localized forebrain damage caused directly by large radiation doses. Linear extrapolation of an observed linear dose response for SMR after large radiation exposures is unlikely to be a valid method for predicting the frequency of SMR after much smaller exposures causing minimal damage to the fetal haematopoietic tissues.