NEUROLOGICAL OBSERVATIONS AFTER LOCAL IRRADIATION AND HYPERTHERMIA OFRAT LUMBOSACRAL SPINAL-CORD

Purpose: Investigation of the effects of hyperthermia on the radiation response of rat lumbosacral spinal cord with respect to: (a) incidenc e of paralysis, (b) latency, (c) histopathology, and (d) tumor inducti on. Methods and Materials: Rat lunbosacral spinal cord with the cauda equina was single-dose irradiated with 15 to 32 Gy of x-rays. Hyperthe rmia for 30 min at a spinal cord temperature of 41.1, 42.3, and 42.6 /- 0.4 degrees C was applied 5 to 10 min after irradiation by means of a 434 MHz microwave applicator. Animals were observed for 21 months w hile recording myelopathy and development of tumors. Results: The late nt period for hind leg paralysis decreased with increasing radiation d ose from 359 +/- 31 days (n = 9) after 20 Gy to 200 +/- 4 days (n = 5) after 32 Gy. Hyperthermia enhanced the radiation response of the lumb osacral spinal cord as evidenced by shortening of the latent period fo r paralysis and a decrease in the biological effective dose. After 20 Gy followed by 30 min 41.1 degrees C, latency was diminished to 214 +/ - 16 days (n = 7, p < 0.001 vs. 20 Gy alone). The ED(50) was 21.1 Gy, which was diminished to values between 16 and 17 Gy if radiation was f ollowed by hyperthermia, giving a thermal enhancement ratio between 1. 24 and 1.32. Histopathological examination of the spinal cord after co mbined treatment of x-rays and hyperthermia showed necrosis of nerve r oots. Irradiation with 16, 20, 24, and 28 Gy (n = 77) alone led to tum or induction in 17 +/- 8% of the animals (pooled data). If followed by hyperthermia (n = 96), it was increased to 33 +/- 12% (p < 0.01). Mos t tumors induced by radiation and hyperthermia were sarcomas. Conclusi on: First, the radiation response of rat lumbosacral spinal cord was e nhanced by heat. Second, latency for paralysis was shortened in the lo wer dose range. Third, no difference in pathology between x-rays alone or in combination with hyperthermia. Fourth, hyperthermia did increas e radiation carcinogenesis.