The prediction of the relative toxicities of radium 224 and of radium 226 in the bones of mice using Monte Carlo techniques.

Toxicity studies using rodents have shown that the long-lived radium isotope radium 226 is about six times less toxic per unit of absorbed radiation dose to the skeleton than the short-lived isotope radium 224 with respect to the production of osteosarcoma. This difference in toxicity has been attributed to differences in the distribution of these isotopes. It has been suggested that 224Ra is more toxic than 226Ra because it decays on bone surfaces rather than within the volume of the bone mineral. However, in rodents many bone structures are small compared with the track length of the alpha particles and this explanation may be inadequate to explain the magnitude of the observed effect. Consequently, Monte Carlo calculations have been made to test the validity of the distribution-difference hypothesis. The results indicate that, for bone structures the size of those in mice, less than half of the observed difference in toxicity can be explained by considerations of the distribution of these radionuclides with respect to bone surfaces. Instead, it is suggested that the greater irradiation of trabecular than of cortical bone that is a characteristic of 224Ra is responsible for its enhanced toxicity.