Minimising the impact of stable ²⁰⁸Pb on recovery of ²¹²Pb from a generator

Stable ²⁰⁸Pb will accumulate over time in all ²¹²Pb generators, forming an increasing proportion of the Pb atoms and potentially impacting the efficiency of chelation of ²¹²Pb. This paper models the accumulation of ²⁰⁸Pb in ²¹²Pb-parent generators with time, calculates the optimal elution time with respect to effective specific activity (ESA) of the ²¹²Pb produced, and considers whether the presence of ²⁰⁸Pb could prevent the required molar activities (A_m) being achieved. A matrix calculation was utilised to model the decay of ²²⁴Ra and ²²⁸Th parents in generators and the associated accumulation of ²⁰⁸Pb with time. This model was used to calculate the optimal elution times for each generator with respect to ESA and A_m.

The optimal elution time with respect to A_m of ²¹²Pb is 16.5 h for a ²²⁴Ra generator (61.0% of ²²⁴Ra starting activity, A_m = 6997 MBq ²¹²Pb/nmol total Pb) and 19.2 h for a ²²⁸Th generator (71.4% of ²²⁸Th starting activity, A_m = 6566 MBq ²¹²Pb/nmol total Pb).

The limited data available in literature shows the level of ²⁰⁸Pb present in these studies does not prevent attainment of the required molar activities for pre-clinical or clinical work. However, with other chelators and under other conditions this may not be the case. The authors would encourage others to measure and report the level of ²⁰⁸Pb in ²¹²Pb generator eluates alongside radiolabelling results and the max A_m of the radiopharmaceutical ²¹²Pb (vector molecule (VM)) achieved.