Radiation tolerance of the spinal cord. Dependence on fractionation and extended overall times

The cervical region of the rat spinal cord was irradiated with single and fractionated doses of 300kV X-rays. The isoeffect curve for 50% incidence of paralysis in relation to the number of fractions at daily intervals has a slope of 0.44 on a double logarithmic scale diagram. In split-dose experiments with intervals between successive courses of radiotherapy of up to 8 weeks, no further increase in tolerance was observed. However, with a longer interval of 16 weeks, significant additional recovery occurred. The results fit an isoeffect formula (Ellis type) DαNsup(0.44) for the first 8 weeks of treatment, and DαNsup(0.44)(T-56)sup(0.03) for periods between 8 and 16 weeks. The additional recovery starting between 8 and 16 weeks after irradiation is attributed to repopulation by cells with a slow turnover under normal conditions. (author)