Late effects of radiation on the lumbar spinal cord of guinea pigs: Re-treatment tolerance

Purpose: Using a guinea pig model of lumbar myelopathy, various factors affecting the tolerance of spinal cord to irradiation were assessed: (a) extent of initial injury (b) time interval between priming and test doses (c) animal age at the time of initial radiation treatment. Methods and Materials: A 3 cm section of lumbar spinal cord of guinea pigs was irradiated with fractionated doses of 4.5 Gy gamma rays given as 9 fractions per week. Guinea pigs were primed with 9 × 4.5 Gy in 7 days which is 60% of the ED₅₀ for a continuous course of treatment. After 28 or 40 weeks, animals were retreated with 6-14 fractions of 4.5 Gy. Animals were observed for 2 years following the priming dose and both the incidence and latency of myelopathy recorded. Results: Young adult guinea pigs (8 wk old) showed both a decreased radiation tolerance and latency compared to old individuals (40 wk old). At 28 or 40 wk after 9 × 4.5 Gy, only about 8% of the initial injury was remembered in young adult guinea pigs. Conclusion: The amount of residual injury was dependent on the initial damage as a proportion of the tolerance dose. The spinal cord shows a greater capacity for long-term recovery than generally appreciated and re-treatment doses clinically prescribed may be lower than necessary.