Radiopharmacokinetic modelling and radiation dose assessment of 223 Ra used for treatment of metastatic castration-resistant prostate cancer

Purpose Ra-223 dichloride ( 223 Ra, Xofigo®) is used for treatment of patients suffering from castration-resistant metastatic prostate cancer. The objective of this work was to apply the most recent biokinetic model for radium and its progeny to show their radiopharmacokinetic behaviour. Organ absorbed doses after intravenous injection of 223 Ra were estimated and compared to clinical data and data of an earlier modelling study. Methods The most recent systemic biokinetic model of 223 Ra and its progeny, developed by the International Commission on Radiological Protection (ICRP), as well as the ICRP human alimentary tract model were applied for the radiopharmacokinetic modelling of Xofigo® biodistribution in patients after bolus administration. Independent kinetics were assumed for the progeny of 223 Ra. The time activity curves for 223 Ra were modelled and the time integrated activity coefficients, ∼a(r_S,T_D), in the source regions for each progeny were determined. For estimating the organ absorbed doses, the Specific Absorbed Fractions (SAF) and dosimetric framework of ICRP were used together with the aforementioned ∼a(r_S,T_D) values. Results The distribution of 223 Ra after injection showed a rapid plasma clearance and a low urinary excretion. Main elimination was via faeces. Bone retention was found to be about 30