The Effect of High-Energy Proton Irradiation on the Cardiovascular System of the Rhesus Monkey

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Abstract
During the first 24 hours after total-body irradiation of the rhesus monkey (Macaca mulatta) with 3680 rads of 138-Mev or 400-Mev protons, there is a decrease in mean arterial pressure similar to that observed in monkeys irradiated with an equivalent dose of Co60 gamma-radiation. Irradiation of monkeys with the same dose of 55-Mev protons also causes a decrease in mean arterial pressure, but of much lesser magnitude than at the higher energies. After irradiation with 32-Mev protons, although there is an immediate decrease of mean arterial pressure, recovery occurs by 6 hours postirradiation. The response of the mean arterial pressure to small intravenous doses of norepinephrine is diminished immediately after irradiation with 55-Mev, 138 Mev, or 400-Mev protons. The animals in the 138-Mev and 400-Mev proton-irradiated groups showed recovery by 24 hours, a pattern similar to that seen in the Co60 gamma-irradiated animals. The abnormality persisted in the 55-Mev proton-irradiated animals. The 32-Mev proton-irradiated animals showed only a minimal decrease in responsiveness to norepinephrine immediately after irradiation. The differences in response of the rhesus monkeys to the same surface dose of different proton energies is attributed to differences in depth-dose distribution of the protons. At energies of 138 Mev and 400 Mev, where deposition of energy is relatively uniform, the effects are similar to those of Co60 gamma-irradiation, and it is suggested that the RBE [Relative Biological Efficiency] for these effects approximates 1. At lower energies, where only the outer portion of the volume of the body is irradiated, the effect on the blood pressure is not as great. Because of difference in depth-dose distribution, and RBE cannot be validly calculated.

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