Technical note: Assessment of radiation measurement devices for the detection of [177 Lu]Lu-labeled radiopharmaceuticals containing [177m Lu]Lu-impurities

Abstract

Background: Nuclear medicine therapies using [¹⁷⁷ Lu]Lu-labeled radiopharmaceuticals have increased significantly in recent years. Some of these radiopharmaceuticals contain long-lived impurities of [^177m Lu]Lu. Identification of this specific contamination and its quantification is important in different scenarios.

Purpose: In this study, standard measurement hardware used for handling radioisotopes was evaluated for the measurement of [^177m Lu]Lu and [¹⁷⁷ Lu]Lu at equilibrium. Device-specific detection limits (LODs) for [^177m Lu]Lu were determined according to international standards and then validated.

Methods: The LODs were determined according to the international standard ISO 11929-1 for [^177m Lu]Lu for five identical portable contamination monitors (PCMs), a wastewater counter (WWC), and a release counter system (RCS) at different measuring times. Subsequent activity measurements of the defined samples were used to validate the linearity of the measurement instruments down to the LOD for each system.

Results: The average LOD across all PCMs was 0.249 ± 0.009 and 0.129 ± 0.005 Bq/cm² for 10 and 30 s measurements, respectively. The LODs of WWC varied between 3.3 and 4.7 Bq/L for measurement times of 300 s and 0.8-1.3 Bq/L for 3600 s depending on the energy window studied. The LOD of the RCS depended on the container volume and was 0.08 Bq/g for the 50 L container at 60 s measurement. The measurements for all examined devices were linear down to the LOD (correlation coefficient R ≥ 0.96; coefficient of determination R² ≥ 0.92).

Conclusions: All investigated measuring instruments (PCM, WWC, RCS) were suitable for the determination of [^177m Lu]Lu at equilibrium, and their specific LODs were determined. Based on the measurements performed, activity is overestimated; however, this is tolerable because assumptions and measurements in the context of radiation protection should be conservative.

Keywords: [177Lu]Lu; [177mLu]Lu; detection limit; portable contamination monitor; release counter system; wastewater counter.