Radiation myelopathy of the mouse spinal cord--isoeffect correlations after fractionated radiation.
A one cm long segment of the lower thoracic upper lumbar spinal cord of mice was irradiated using one, two, three, five, seven, ten, 15, 20 and 25 daily fractions. The animals were checked for paraparesis of the hind legs over their entire life span up to two years. The ED 50-values for radiation myelopathy for intervals of 250 up to 550 days were calculated. The resulting isoeffect curves follow an upward bending course being flat for one to five fractions and then getting steeper up to 25 fractions with a slope of 0.27 to 0.30. For intervals between 250 and 550 days the isoeffect curves run a parallel course. Only ED 50-values for intervals of 550 days post radiation omitting data above 6 Gy/fraction fit the linear quadratic cell survival (LQ) model with an alpha/beta value of 6.5 Gy which is relatively high compared to alpha/beta-values for late damage of normal tissues. Incomplete repair between fractions or overlapping effects of the cell systems responsible for radiation myelopathy (oligodendrocytes and endothelial cells) might account for failure of the other isoeffect data to fit the LQ-model. There are important differences of the isoeffect correlations of rat and mouse radiation myelopathy probably due to different repair characteristics and kinetics between the involved cells. On the other hand similarities of isoeffect correlations of man, rat and mouse are observed. Therefore the model of mouse radiation myelopathy presented in this paper might be used to test new treatment strategies.