title: Late effects of radiation on the lumbar spinal cord of guinea pigs: re-treatment tolerance.
abstract: OBJECTIVE
  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
  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 x 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 x 4.5 Gy, only about 8% of the initial injury was remembered in young adult guinea pigs.
  
  
  CONCLUSIONS
  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.
authors.affiliation: University of Texas M.D. Anderson Cancer Center, Dept. of Experimental Radiotherapy, Houston 77030.
authors.forename: K A
authors.initials: KA
authors.lastname: Mason
authors.forename: H R
authors.initials: HR
authors.lastname: Withers
authors.forename: C S
authors.initials: CS
authors.lastname: Chiang
mesh_terms: D000367:Age Factors; D000818:Animals; D004307:Dose-Response Relationship, Radiation; D005260:Female; D006168:Guinea Pigs; D011836:Radiation Tolerance; D013116:Spinal Cord / Q000528:radiation effects*; D013997:Time Factors