Reirradiation tolerance of the immature rat spinal cord.

The dose dependence and time course of long-term recovery in the cervical spinal cord of 3-week-old rats was investigated, and compared with the recovery in adults rats. At intervals of 1 to 168 days after initial irradiation of the cervical spinal cord at the age of 3 weeks, reirradiation experiments were performed to test the pattern of long-term recovery in immature spinal cord. The single dose ED50 for white matter mediated paresis was about 21 Gy for 3-week-old as well as adult rats, although the latency to paresis development increased from about 90 days in 3-week-old rats to about 250 days in adult rats. The main long-term recovery was seen during the first month after the initial radiation treatment at 3 weeks. This is in contrast to long-term recovery in adult rats, in which the main recovery took place between 2 and 6 months after the first irradiation. Calculations according to the LQ model showed that the extra dose that can be given to the cervical spinal cord after a 1-6 months interval in the 3-week-old rats reaches a maximum of about 20% of the total biological effect resulting in paresis. In adult rats the extra dose that can be given to the cervical spinal cord after a 6 months interval represents about 40% of the total biological effect. These studies show that time course as well as extent of long-term recovery from radiation treatment not only depends on tissue and species, but also on age.