Brain atrophy after foetal 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.