Brain Atrophy After Fetal Exposure To Very Low-doses of Ionizing-radiation

Acute, high dose-rate, exposure of the rat embryo on day 15 post-conception (PC) causes a reduction of brain weight in adult life that is proportional to the dose received. Doses as low as 10 mGy of 600 keV neutrons, from a Van de Graaff accelerator, or 100 mGy of 250 kV X-rays are capable of eliciting a significant effect. The relative biological effectiveness for acute neutron exposure compared with 250 kV X-rays was 3.5. A brain weight reduction was also observed after gamma-ray exposures protracted over 4 or 6 days, during cerebral corticogenesis. The dose-rate reduction factor was only 1.5 for exposure from days 12 to 16 PC and 3-3 for exposure from days 14 to 20 PC. In relation with the decrease in brain weight, the cingulum bundle, a myelinated structure associated with the corpus callosum, displayed a significant reduction in size. The implications of these observations for human exposures are discussed