A l-cm segment of rat cervical spinal cord was irradiated (16-150 Gy)
and the animals were followed for 16 months to determine the dose-resp
onse relationship for the latent period for radiation myelopathy. The
latent period was found to be dependent on dose in the ED(0) to ED(100
) dose range, independent of dose in the range ED(100) to 1.5 times th
e ED(100) dose and dependent on dose for doses more than 1.5 times the
ED(100) dose. Histologically, some vascular injury and demyelinizatio
n were observed in the irradiated cord segment of all paretic animals.
In the first dose-dependent region there was a bimodal distribution o
f 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 th
e 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%.