HIGH-DOSE-RATE (HDR) AND LOW-DOSE RATE (LDR) INTERSTITIAL IRRADIATION(IRT) OF THE RAT SPINAL-CORD

Purpose: To describe a newly developed technique to study radiation to lerance of rat spinal cord to continuous interstitial irradiation (IRT ) at different dose rates. Material and methods: Two parallel catheter s are inserted just laterally on each side of the vertebral bodies fro m the level of Th-10 to L(4). These catheters are afterloaded with two Ir-192 wires of 4 cm length each (activity 1-2.3 mCi/cm) for the low dose rate (LDR) IRT or connected to the HDR micro-Selectron for the hi gh dose rate (HDR) IRT. Spinal cord target volume is located at the le vel of Th-12-L(2). Due to the rapid dose fall-off around the implanted sources, a dose inhomogeneity across the spinal cord thickness is obt ained in the dorso-ventral direction. Using the 100% reference dose (r ate) at the ventral side of the spinal cord to prescribe the dose, exp eriments have been carried out to obtain complete dose response curves at average dose rates of 0.49, 0.96 and 120 Gy/h. Paralysis of the hi nd-legs after 5-6 months and histopathological examination of the spin al cord of each irradiated rat are used as experimental endpoints. Res ults: The histopathological damage seen after irradiation is clearly r eflected the inhomogeneous dose distribution around the implanted cath eters, with the damage predominantly located in the dorsal tract of th e cord or dorsal roots, With each reduction in average dose rate, spin al cord radiation tolerance is significantly increased. When the dose is prescribed at the 100% reference dose rate, the ED(50) (induction o f paresis in 50% of the animals) for the HDR-IRT is 17.3 Gy. If the av erage dose rate is reduced from 120 Gy/h to 0.96 or 0.49 Gy/h, a 2.9- or 4.7-fold increase in the ED(50) values to 50.3 Gy and 80.9 Gy is ob served; for the dose prescribed at the 150% reference dose rate (dorsa l side of cord) ED(50) values are 26.0, 75.5 and 121.4 Gy, respectivel y, Using different types of analysis and in dependence of the dose pre scription and reference dose rare, the alpha/beta ratio varies between 1.46 (0.06-3.08 CL) and 2.17 Gy (0.08-4.61). The half time of repair during continuous irradiation is 1.76 h (1.33-2.64), while no indicati on is found for a biphasic pattern of the kinetics of repair. Conclusi on: The implantation technique in our study has shown to be a reliable model to compare the effectiveness of HDR- and LDR-interstitial conti nuous irradiation at different dose rates. (C) 1997, Elsevier Science Ireland Ltd.