Radiation tolerance and fractionation sensitivity of the developing rat cervical spinal cord.

To investigate the influence of age at irradiation on single dose radiation tolerance and fractionation sensitivity, the cervical spinal cord of rats was irradiated at the age of 1 week and at 15-18 weeks (adult). While the main histological lesions seem to be comparable after irradiation at the two ages, differences were found in single dose tolerance, latency to paresis due to white matter lesions, and fractionation sensitivity. The 50% effect dose (ED50) for single dose irradiation at one week was 19.5 Gy, which is only 10%, but significantly (p < 0.05), lower than the ED50 of about 21.5 Gy at 3 weeks and above. The latency to paresis was clearly influenced by the age at irradiation. The latency in the rats irradiated at 1 week was about 2 weeks, while for adult rats a latency of about 8 months was observed. The fractionation sensitivity for irradiation at 1 week was lower than the fractionation sensitivity of the adult rats; the alpha/beta value at 1 week was estimated to be 4.5 Gy, while for the adult rats an alpha/beta value of 1.8 Gy was found. As a consequence, the observed small difference in tolerance to single doses between 1 week-old and adult rats is further enhanced after fractionated irradiation. During prolonged irradiation treatments this decreased tolerance may be compensated by a higher proliferation rate in the immature central nervous system. The results of the present experiments indicate that, for a single tissue and endpoint, paresis due to white matter lesions in the rat cervical spinal cord, the latency to expression of damage and the fractionation sensitivity clearly change with age at irradiation.