The effects of dose and field size on late radiation damage to the rat spinal cord.

In groups of young adult rats, 1, 2, 4, 8 or 16 mm lengths of the cervical spinal cord were irradiated with 2000 or 4000 R of 60Co gamma-rays. All groups showed significant life-shortening, all but five animals (which died from intercurrent disease) showing signs attributable to radiation. The clinical signs, latent period, and histological changes observed were dependent on the dose and field size. The animals irradiated with the higher dose to 8 or 16 mm of the cord were the first to die, all had developed paralysis and were found to have extensive areas of necrosis in the white matter. When the field size was decreased using this dose (4000 R), or the dose reduced (all field sizes), no extensive necrosis was found and only a proportion of the animals developed paralysis; the others died suddenly. The interpretation of these results is discussed.