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

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Abstract
The thoraco-lumbar (T12-L1) region of the spinal cord of rats was exposed to either single or fractionated (4 daily exposures) doses of X-rays (230 kVp) or heavy ions. The heavy ions used were C and Ne, and the relative biological effectiveness (RBE) of 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 C and Ne 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 C and Ne 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). In groups of rats receiving 4 daily exposures, the RBE values for C and Ne 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 C and Ne 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 nycleotide phosphohydrolase and .gamma.-glutamyl transpeptidase). For X-irradiation, the fractional amount of dose repaired (at the ED50 dose) was 0.64 .+-. 0.10 (95% confidence limits). For C and Ne 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 C and Ne 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 shows a high capacity for subeffective damage repair, and even at the highest LET investigated (Ne ions in the spread peak region) there is indication of a significant amount of repair of radiation damage.

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