Five-Year Follow-Up of Primates Exposed to 55 Mev Protons

Rhesus monkeys (Macaca mulatta) were observed over a 5-year period following irradiation with 55 MeV protons. The exposures were whole-body surface exposures which penetrated to a depth of 2.5 cm. Skin fibrosis was severe within 2 years after exposures above 800 rad, while a mild skin fibrosis was noted 5 years after exposure of 600 rads. Varying degrees of fibrosis had been observed postmortem in the heart, lungs, intestines, liver, thyroid, testicles, and skeletal muscle in animals exposed to 600 rads or more. One animal had two fibrosarcomas, and four animals had malignant brain tumors that were considered to be related to the radiation. Mortality 5 years after 55-MeV proton irradiation is similar, on a radfor rad basis, to the acute mortality of penetrating whole-body exposures. No long-term effects have been noted below 600 rads at this time.