Radiation dose response of normal brain

Dose response relationships were determined after hemibrain x-irradiation of normal beagle dogs. Radiation doses of 11.5, 13.5, 14.3, and 17 Gy were delivered in a single dose and results were compared to previous studies using doses of 15 and 30 Gy. Brain injury was quantified using computed tomography (CT), with serial studies obtained monthly up to 1 year following irradiation. Quantitative endpoints included low density volume and contrast enhancement. Doses above 14.3 Gy resulted in high lethality 5-8 months following irradiation, and an LD₅₀ of 14.9 Gy was calculated. At these lethal doses, low density volume representing edema, demyelination, and necrosis had a similar response with an ED₅₀ of 14.6 Gy. Radiation-induced decreases in white matter density appeared 5-6 months after sublethal doses (<14.3 Gy) and the volume of tissue characterized by this low density increased with time and dose. This sublethal low density change had an ED50 of 12.8 Gy, and may reflect a loss or generalized atrophy of glial cells and/or myelin. These results show that: (a) the dose response curves obtained after hemibrain x-irradiation are extremely steep; and (b) at least two processes may be involved in the development of late radiation damage, one that is rapid upon onset (a "delayed acute" reaction) and the other which is a slower and more degenerative process.