Radiation myelopathy in primates treated with conventional fractionation.

A rhesus monkey model was used to assay the radiation tolerance of the spinal cord to 60Co radiation given in 2.2 Gy fractions. The D50 was found to be 76.1 Gy (SEM = 1.9 Gy); the estimated doses for 1% and 0.1% myelopathy were 59.1 +/- 5.5 Gy and 52.1 +/- 7.1 Gy, respectively. The latent period ranged from 5 to 20 months and decreased with increasing dose. All symptomatic animals had white-matter parenchymal lesions involving major motor tracts, and most also had vascular lesions in the white matter. Spinal cords of asymptomatic animals had either no histopathologic changes or small white-matter lesions that did not involve motor tracts.