Effects of heavy ions on rabbit tissues: damage to the forebrain.

As part of a study of progressive radiation effects in normal tissues, the forebrains of New Zealand white rabbits (Oryctolagus cuniculus) (about 6 weeks old) were irradiated locally with single acute doses of 60Co gamma-photons (LET infinity = 0.3 keV/micron), Ne ions (LET infinity = 35 +/- 3 keV/microns) or Ar ions (LET infinity = 90 +/- 5 keV/microns). Other rabbits received fractionated doses of 60Co gamma-photons according to a standard radiotherapeutic protocol. Irradiated rabbits and appropriately aged controls were sacrificed at selected intervals, and whole sagittal sections of their brains were examined for pathological changes. Forebrain damage was scored with subjective indices based on histological differences between the anterior (irradiated) and posterior (unirradiated) regions of the brain. Those indices ranged from zero (no apparent damage) to five (severe infarctions, etc.). At intermediate levels of forebrain damage, the relative biological effectiveness (r.b.e.) of each heavy ion was similar to that found for alopecia and cataractogenesis, and the early expression of the damage was also accelerated as the LET infinity increased. Late deterioration of the forebrain appeared also to be accelerated by increasing LET infinity, although its accurate quantification was not possible because other priorities in the overall experimental design limited systematic sacrifice of the animals.