Paralysis following stereotactic spinal irradiation in pigs suggests a tolerance constraint for single-session irradiation of the spinal nerve

Background and purpose Paralysis observed during a study of vertebral bone tolerance to single-session irradiation led to further study of the dose-related incidence of motor peripheral neuropathy. Materials and methods During a bone tolerance study, cervical spinal nerves of 15 minipigs received bilateral irradiation to levels C5-C8 distributed into three dose groups with mean maximum spinal nerve doses of 16.9 ± 0.3 Gy (n = 5), 18.7 ± 0.5 Gy (n = 5), and 24.3 ± 0.8 Gy (n = 5). Changes developing in the gait of the group of pigs receiving a mean maximum dose of 24.3 Gy after 10-15 weeks led to the irradiation of two additional animals. They received mean maximum dose of 24.9 ± 0.2 Gy (n = 2), targeted to the left spinal nerves of C5-C8. The followup period was one year. Histologic sections from spinal cords and available spinal nerves were evaluated. MR imaging was performed on pigs in the 24.9 Gy group. Results No pig that received a maximum spinal nerve point dose ≤19.0 Gy experienced a change in gait while all pigs that received ≥24.1 Gy experienced paralysis. Extensive degeneration and fibrosis were observed in irradiated spinal nerves of the 24.9 Gy animals. All spinal cord sections were normal. Irradiated spinal nerve regions showed increased thickness and hypointensity on MR imaging. Conclusion The single-session tolerance dose of the cervical spinal nerves lies between 19.0 and 24.1 Gy for this model.