Gamma counting protocols for the accurate quantification of 225Ac and 213Bi without the need for a secular equilibrium between parent and gamma-emitting daughter

EJNMMI Radiopharmacy and Chemistry

Table 3 Bound fraction of ²²⁵Ac (activity upper part iTLC/activity iTLC) to [²²⁵Ac]Ac-DEPA/DOTA complexes for samples 5 and 6 (see Table 1) after 15 min and 24 h incubation in human serum

EW	CPM	[²²⁵Ac]Ac-DOTA		[²²⁵Ac]Ac-DEPA
EW	CPM	15 min	24 h	15 min	24 h
\({\text{EW}}_{{213_{{{\text{Bi}}}} }}\)(at 5 h)	\(CPM_{{{\text{ EW}}_{{213_{{{\text{Bi}}}} }} }}\)	88%	91%	96%	96%
\({\text{EW}}_{{221_{{{\text{Fr}}}} }}\)(at 30 min, without scatter correction)	\(CPM_{{{\text{ EW}}_{{221_{{{\text{Fr}}}} }} }}\)	84%	78%	88%	83%
\({\text{EW}}_{{221_{{{\text{Fr}}}} }}\), \({\text{EW}}_{{213_{{{\text{Bi}}}} }}\)(at 30 min, with scatter correction)	\({\text{CPM}}_{{{\text{ SC,EW}}_{{221_{{{\text{Fr}}}} }} }}\), \(CPM_{{{\text{ EW}}_{{213_{{{\text{Bi}}}} }} }}\),	89%	90%	95%	93%

²²⁵Ac activities were determined with a single time-point GC measurement using \({\text{EW}}_{{213_{{{\text{Bi}}}} }}\) at 5 h, to ensure SEq between ²¹³Bi and ²²⁵Ac, and using \({\text{EW}}_{{221_{{{\text{Fr}}}} }}\) with and without scatter correction at 30 min, to ensure SEq between ²²¹Fr and ²²⁵Ac