Late effects of 50 MeV d leads to Be neutron and cobalt-60 irradiation of rhesus monkey cervical spinal cord.

The cervical spinal cords of 30 rhesus monkeys were irradiated with 50 MeV d leads to Be neutrons or 60Co gamma rays to evaluate the dose-response relationships for radiation myelopathy. Three groups were treated with 50 MeV d leads to Be neutrons using dosage schedules of 1300 rad n gamma (Group I), 1425 rad n gamma (Group II), or 1550 rad n gamma (Group III) in nine fractions over 29 days. Three groups were irradiated with 60Co gamma rays using dosage schedules of 4620 rad (Group IV), 5390 rad (Group V), or 5940 rad (Group VI) in 22 fractions over 29 days. A significant dose-response relationship was observed for the groups treated with neutrons. Whereas none of the monkeys in Group I showed clinical evidence of neurologic dysfunction, all five animals in Group III became paralyzed. One animal in Group II developed transient neck stiffness and mild unilateral leg paresis. No definitive signs of neurologic injury were seen in any of the animals irradiated with 60Co. The histopathologic changes correlated well with the clinical observations. All of the animals in Group III exhibited moderate to severe malacia and demyelination of the white matter of the cervical spinal cord. The histologic data indicated that the RBE for five times weekly fractionation (approximately 270 rad 60Co fractions) was in the range of 4.2 to 4.6, since the malacia and demyelination in the spinal cords irradiated with 5940 rad of 60Co gamma-rays were greater than that observed in the spinal cords irradiated with 1300 rad n gamma of neutrons but less than the changes in those irradiated with 1425 rad n gamma of neutrons.