LATE EFFECTS OF RADIATION ON THE LUMBAR SPINAL-CORD OF GUINEA-PIGS - RETREATMENT TOLERANCE

Purpose: Using a guinea pig model of lumbar myelopathy, various factor s affecting the tolerance of spinal cord to irradiation were assessed: (a) extent of initial injury (b) time interval between priming and te st doses (c) animal age at the time of initial radiation treatment. Me thods and Materials: A 3 cm section of lumbar spinal cord of guinea pi gs was irradiated with fractionated doses of 4.5 Gy gamma rays given a s 9 fractions per week. Guinea pigs were primed with 9 x 4.5 Gy in 7 d ays which is 60% of the ED50 for a continuous course of treatment. Aft er 28 or 40 weeks, animals were retreated with 6-14 fractions of 4.5 G y. Animals were observed for 2 years following the priming dose and bo th the incidence and latency of myelopathy recorded. Results: Young ad ult 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 a fter 9 x 4.5 Gy, only about 8% of the initial injury was remembered in young adult guinea pigs. Conclusion: The amount of residual injury wa s dependent on the initial damage as a proportion of the tolerance dos e. The spinal cord shows a greater capacity for long-term recovery tha n generally appreciated and re-treatment doses clinically prescribed m ay be lower than necessary.