Radiation tolerance of the rat spinal cord: time-dose relationships.

The cervical or lumbosacral region of the rat spinal cord was irradiated with 300-kV x rays. Syndromes of radiation myelopathy similar to those in man were observed in rats. Iso-effect curves for tolerance in relation to the number of fractions at daily intervals have a slope of 0.44 and 0.42 on a log-log scale for the cervical and lumbosacral regions, respectively. With intervals of up to 7 days, no significant further increase in tolerance was observed. Low dose rate treatments indicate a half-time of 1.5 hr. for repair of subeffective damage. These results suggest that tolerance of the spinal cord depends very strongly on the number of fractions given and only to a small extent on the overall time of treatment.