Spinal Nerve Tolerance To Single-Session Stereotactic Ablative Radiation Therapy (Sabr)

Abstract Purpose This study was performed to determine the dose-related incidence of neuropathy from single-session irradiation of the C6-C8 spinal nerves using a pig model and to test the hypothesis that the spinal nerves and spinal cord have the same tolerance to full cross-sectional irradiation. Methods and Materials Twenty-five Yucatan minipigs received study treatment. Each animal underwent CT and MR imaging for treatment planning followed by single-session SAbR. A 1.5cm length of the left-sided C6, C7 and C8 spinal nerves was targeted. Pigs were distributed into five groups with a prescription dose of 16 (n=7), 18 (5), 20 (5), 22 (5) or 24 (3) Gy with corresponding maximum nerve doses of 17.3, 19.5, 21.6, 24.1, and 26.2 Gy. The neurologic status of all animals was followed for approximately 52 weeks by serial electrodiagnostic (EDX) exam and daily observation of gait. Histopathologic examination of paraffin-embedded sections with Luxol fast blue/periodic acid-Schiff’s staining was performed on bilateral spinal nerves and the spinal cord. Results Marked gait change was observed in 15 of the 25 irradiated pigs. Affected animals presented with a limp in their left front limb and electromyography demonstrated evidence of denervation in C6 and C7 innervated muscles. Probit analysis showed the ED 50 for gait change following irradiation of the spinal nerves to be 19.7 Gy (95% confidence interval, 18.5–21.1). The latency for all responding pigs was 9-15 weeks following irradiation. All symptomatic pigs had demyelination and fibrosis in their irradiated nerves while contralateral nerves and spinal cord were normal. Conclusions The ED 50 for symptomatic neuropathy following full cross-sectional irradiation of the spinal nerves was found to be 19.7 Gy. The dose-response of the C6-C8 spinal nerves is not significantly different from that of full cross-sectional irradiation of the spinal cord as observed in companion studies.