A 1-cm segment of rat cervical spinal cord was irradiated (16-150 Gy) and the animals were followed for 16 months to determine the dose-response relationship for the latent period for radiation myelopathy. The latent period was found to be dependent on dose in the${\rm ED}_{0}^{\bullet}$ to${\rm ED}_{100}$ dose range, independent of dose in the range${\rm ED}_{100}$ to 1.5 times the${\rm ED}_{100}$ dose and dependent on dose for doses more than 1.5 times the${\rm ED}_{100}$ dose. Histologically, some vascular injury and demyelinization were observed in the irradiated cord segment of all paretic animals. In the first dose-dependent region there was a bimodal distribution of latent periods, suggesting two mechanisms for paralysis in this dose region. A hypothesis based on glial and vascular injury is presented which explains the triphasic dose-response relationship for the latent period. The 95% confidence limits for the dose location of both of the dose-dependent regions were less than 10% of the administered dose. Therefore, if used correctly, the latent period as an end point should be able to be used to detect dose-modifying parameters (i.e. RBE and OER) that are greater than 20%.

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