Survival of mice and hematopoietic stem cells in bone marrow after intermittent total body irradiation.

As a preparative procedure for bone marrow transplantation, intermittent total body irradiation (TBI) has been used in our hospital. The biological significance of this method, in which the instantaneous dose rate is high but the average dose rate is low, has not been evaluated to date. The hematopoietic responses caused by both intermittent and continuous TBI were compared. In the intermittent irradiation, mice in a moving irradiation chamber were exposed under a small field (2 X 35 cm2), and the instantaneous and average dose rates were 1 Gy/min and 0.25-0.12 Gy/min, respectively. The average dose rate was adjusted to the same level in both irradiation methods. LD50/30 and survival of colony-forming units (CFU) in culture and survival of endogenous CFU in the spleen from female BDF1 mice were the same with the two methods. These results show that the response of hematopoietic stem cells depends on the average dose rate, not on the instantaneous dose rate. Our findings suggest that intermittent irradiation, as well as the continuous method, would be useful for preparing patients before bone marrow transplantation.