Response of Rat Spinal Cord to Single and Fractionated Doses of Accelerated Heavy Ions

The thoraco-lumbar (T12-L1) region of the spinal cord of rats was exposed to either single or fractionated (four daily exposures) doses of X rays (230 kVp) or heavy ions. The heavy ions used were carbon and neon, and the relative biological effectiveness (RBE) of both the plateau ionization region and the midpeak region of 4-cm spread-out Bragg peaks of each heavy ion were investigated. For single doses of carbon and neon ions in the plateau ionization region, RBE values of 1.45 ± 0.25 (propagated 95% confidence limits) and 1.46 ± 0.33, respectively, were obtained. In the spread peak regions for carbon and neon ions, the RBE values were 1.48 ± 0.18 and 1.86 ± 0.42, respectively. These values were obtained using the dose needed to produce 50% paralysis in a group of irradiated rats as the isoeffect comparison dose ( ED50 dose). Similarly, in groups of rats receiving four daily exposures, the RBE values for carbon and neon ions in the plateau ionization region were 1.31 ± 0.27 and 1.80 ± 0.24, respectively. In the spread peak regions of ionization for carbon and neon ions, the RBE values were 1.95 ± 0.19 and 2.18 ± 0.23, respectively. Similar values for RBE were obtained using changes in the activity of enzymes in spinal cord tissue (cyclic nucleotide phosphohydrolase and γ-glutamyl transpeptidase). Also, it was estimated that, for X irradiation, the fractional amount of dose repaired (at the ED50 dose) was 0.64 ± 0.10 (95% confidence limits). For carbon and neon ions in the plateau ionization region, the values for the fractional amount of dose repaired were 0.70 ± 0.27 and 0.48 ± 0.20, and for carbon and neon ions in the spread peak region of ionization, the fractional repair values were 0.40 ± 0.10 and 0.52 ± 0.17. Spinal cord tissue therefore shows a high capacity for subeffective damage repair, and even at the highest LET investigated (neon ions in the spread peak region) there is still indication of a significant amount of repair of radiation damage.