Bath and Shower Effect in Spinal Cord: The Effect of Time Interval

Purpose: To evaluate the time dependency of the sensitizing effect of a large low-dose field on a small high-dose field in the rat cervical spinal cord. Methods and materials: Irradiation experiments with a relatively low dose to a large volume (bath, 2 cm, 4 Gy) were combined with high doses to a small volume (shower, 4.7 mm, 26-43 Gy) at intervals of 8 minutes and 3, 12, and 24 hours. Both a functional score defined as motor impairment and a histologic score characterized as white matter necrosis were used as end points. Results: Application of the 4-Gy bath dose resulted in a significant decrease in 50% isoeffective dose (ED₅₀) from 48.7 Gy (small field) to 40.8 Gy. If the interval was extended, the ED₅₀ increased to 44.4 (3 hours) and 44.8 Gy (12 hours), whereas a 24-hour interval resulted in a significant increase to 51.9 Gy. If the histologic end point was considered, the ED₅₀ for all dose-response curves decreased slightly with 0.2 to 2.6 Gy without significantly changing the kinetics. Conclusions: The bath effect as applied in the bath-and-shower experiment lasted for at least 12 hours and disappeared in the 24-hour interval. This time scale clearly deviates from the repair kinetics in spinal cord derived from low-dose-rate and fractionated irradiations.