Spinal Cord Tolerance to Reirradiation With Single-Fraction Radiosurgery: A Swine Model

Abstract

This study was performed to determine swine spinal cord tolerance to single-fraction, partial-volume irradiation 1 year after receiving uniform irradiation to 30 Gy in 10 fractions. A 10-cm length of spinal cord (C3-T1) was uniformly irradiated to 30 Gy in 10 consecutive fractions and reirradiated 1 year later with a single radiosurgery dose centered within the previously irradiated segment. Radiosurgery was delivered to a cylindrical volume approximately 5 cm in length and 2 cm in diameter, which was positioned laterally to the cervical spinal cord, resulting in a dose distribution with the 90%, 50%, and 10% isodose lines traversing the ipsilateral, central, and contralateral spinal cord, respectively. Twenty-three pigs were stratified into six dose groups with mean maximum spinal cord doses of 14.9 ± 0.1 Gy (n = 2), 17.1 ± 0.3 Gy (n = 3), 19.0 ± 0.1 Gy (n = 5), 21.2 ± 0.1 Gy (n = 5), 23.4 ± 0.2 Gy (n = 5), and 25.4 ± 0.4 Gy (n = 3). The mean percentage of spinal cord volumes receiving ≥10 Gy for the same groups were 34% ± 1%, 40% ± 1%, 46% ± 3%, 52% ± 1%, 56 ± 3%, and 57% ± 1%. The study endpoint was motor neurologic deficit as determined by a change in gait during a 1- year follow-up period. A steep dose-response curve was observed with a 50% incidence of paralysis (ED(50)) for the maximum point dose of 19.7 Gy (95% confidence interval, 17.4-21.4). With two exceptions, histology was unremarkable in animals with normal neurologic status, while all animals with motor deficits showed some degree of demyelination and focal white matter necrosis on the irradiated side, with relative sparing of gray matter. Histologic comparison with a companion study of de novo irradiated animals revealed that retreatment responders had more extensive tissue damage, including infarction of gray matter, only at prescription doses >20 Gy. Pigs receiving spinal radiosurgery 1 year after receiving 30 Gy in 10 fractions were not at significantly higher risk of developing motor deficits than pigs that received radiosurgery alone.