Dosimetry for 223Ra and its daughters

Objectives ²²³Ra decays to stable lead via a relatively complicated decay chain (²²³Ra→²¹⁹Rn→²¹⁵Po→²¹¹Pb→²¹¹Bi→²⁰⁷Tl→²⁰⁷Pb). All radionuclides in the chain except ²¹⁵Po and ²⁰⁷Tl emit photons which therefore presents the possibility of measuring the pharmacokinetics for each daughter separately. The aim of this study was to investigate the dosimetry for ²²³Ra with these separate measurements.

Methods Six patients with bone metastases from prostate cancer received two ²²³Ra Cl₂ administrations (100 kBq/kg, twice the normal clinical amount) delivered by intravenous injection six weeks apart. Pharmacokinetics were followed for one week following administration using whole body measurements, scintillation camera imaging and blood sampling. Complete faecal and urine collection was performed up to 48 h after administration. The activity of ²²³Ra, ²²³Ra+²¹⁹Rn, ²¹¹Pb and ²¹¹Bi was measured for the whole body, urine and faecal samples and ²²³Ra and ²¹¹Pb were measured for the blood samples. Imaging was performed with energy windows enabling separate quantification of ²²³Ra and ²²³Ra+²¹⁹Rn.

Results The absorbed whole-body dose was 0.190 Gy (range: 0.142 Gy – 0.541 Gy) when the daughters were assumed to follow ²²³Ra and 0.188 Gy (range: 0.140 Gy – 0.516 Gy) when the pharmacokinetics for the daughters were considered separately.

Conclusions It is possible to measure whole-body pharmacokinetics for the daughters of ²²³Ra. The whole body absorbed dose in this case is similar to the absorbed dose calculated assuming that the daughters have the same pharmacokinetics as ²²³Ra.