Late effects of whole brain irradiation within the therapeutic range

Whole brain exposure with supervoltage irradiation was carried out on three sets of Macaca mulatta. Two sets of 12 monkeys each, at puberty, received single and fractionated exposures respectively. One set of 21 monkeys in adulthood received a fractionated exposure. Exposure to 1000 rads in a single dose, at puberty, caused no late effects. Exposure to 1500 rads caused small areas of necrosis in the forebrain white matter at 26 weeks, but a much more extensive involvementat and beyond 52 weeks that included confluent areas of necrosis in gray and white matter. Brain loss resulted in ventricular dilatation. Gliomas appeared in two out of three monkeys at or beyond 52 weeks. Exposure to 2000 rads caused such a wide scatter of focal areas of necrosis, including those in the brain stem, that survival beyond 20-26 weeks was not possible. All showed enlarged ventricular systems. Whole brain exposure, 200 rads a day, five days a week, for a course of 4000 rads, at puberty, resulted in no delayed effects. An exposure to 6000 rads, in a six weeks course, caused small, less than 1 mm, widely scattered necrotic lesions with a predilection for the forebrain white matter but not excluding the central gray matter and brain stem, at 26 weeks. At 52 weeks, there was considerable mineralization of the lesions and widespread telangiectasia. In the developing lesions, multiple minute breaks in the blood brain barrier caused diffuse brain swelling, reflected by papilloedema. Whole brain exposure to 6000 rads in a six weeks course, in the adult, produced less effects than the same dose at puberty. The onset of the scattered necrotic lesions was later than expected, appearing in one out of three animals at 33 weeks, two out of three animals at 52 weeks, and two out of three at 104 weeks. The lesions at 104 weeks were predominantly mineralized, but were accompanied by a greater extent of telangiectasia than seen in the pubescent monkeys