${}^{224}{\rm Ra}$ was applied with one single injection or with repeated injections (twice weekly over various time periods, up to 36 weeks). In general the bone tumor incidence increased with dose but with the restriction that the maximum instantaneous dose rate be not too high. The 1080-rad total skeletal dose thus resulted in a bone tumor incidence between 15 and 90% depending on the length of the injection spans. These results were compared with the effect of singly injected long-lived226 Ra, using data by M. Finkel et al. (1). It was observed that the specific bone tumor production by226 Ra, expressed as incidence per unit of total accumulated skeletal dose, was steadily diminished with an increase of the dose. As a consequence, we found that mice after a single injection of the long-lived226 Ra within the tested dose range had in general a considerably lower bone tumor incidence than those mice having received comparable skeletal doses through repeated injections of the short-lived${}^{224}{\rm Ra}$ over a longer period.

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