Cellular dose conversion factors for alpha-particle-emitting radionuclidesof interest in radionuclide therapy

alpha -Particle-emitting radionuclides are of increasing interest in radion uclide therapy. The decay scheme of alpha -emitting radionuclides typically includes a chain of unstable progeny. It is generally assumed that alpha - particle emission by the parent radionuclide will break the chemical bond w ith its carrier molecule and that the resulting daughter atom will no longe r be associated with the carrier molecule. If the daughter is very short li ved, it will not have enough time to be carried any significant distance fr om the site of parent decay and a cellular, absorbed dose estimate must con sider the energy deposited by the daughter as well as the parent. Depending on the site of parent decay and the expected removal rate of daughter atom s from this site, the contribution of emissions from longer-lived daughters may also be warranted. In this study, dose conversion factors (DCFs) for c ellular dimensions that incorporate the fate of daughter radionuclides were derived for Ac-225, Bi-213, At-211, and Ra-223, the alpha -particle-emitti ng radionuclides of interest in radionuclide therapy. Methods: The dose con tribution of daughter radionuclides at the site of parent decay was made de pendent on a cutoff time parameter, which was used to estimate the fraction of daughter decays expected at the site of parent decay. Previously tabula ted S values (cell-surface to nucleus and cell-surface to cell) for each da ughter in the decay scheme were scaled by this fraction and a sum over all daughters was performed to yield a cutoff time-dependent set of correspondi ng DCF values for each radionuclide. Results: DCF values for the absorbed d ose to the nuclear or cellular volume from cell-surface decays are presente d as a function of the cutoff time for 4 different cellular and nuclear dim ensions. Conclusion: In contrast to the cellular S values that account only for parent decay, the DCF values provided in this study make it possible t o easily include the contribution of daughter decays in cellular a-particle emitter dose calculations.