International Comparison of Lu-177

Lutetium-177 decays with three primary branches (E_βmax = 176 keV, 385 keV, and 498 keV) and has two reasonably strong γ-rays at 113 keV and 208 keV, making it suitable for analysis using a variety of techniques, including coincidence counting.

The only previous comparison of ¹⁷⁷Lu that has been carried out was a bilateral comparison (BIPM-R(II)-K1.¹⁷⁷Lu) conducted between the NIST and the Physikalisch-Technische Bundesanstalt (PTB) in 2000. In that case, both laboratories were able to submit ampoules to the International Reference System (SIR) and report activity values based on liquid scintillation counting using the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT)/NIST efficiency tracing method. The results indicated a difference of about 1.4 % in the SIR equivalent activity. The short half-life of the ¹⁷⁷Lu did not allow for follow-up studies to be performed.

Since that time, several more National Metrology Institutes (NMIs) have standardized this radionuclide. In order to establish a link between primary standards of b-emitters in the NMIs and the SIR, as well as to provide a means for laboratories to substantiate Calibration and Measurement Capabilities (CMC) claims for β-γ nuclides, a CCRI(II) Key Comparison of ¹⁷⁷Lu was carried out in the Spring of 2009.

A total of 12 laboratories took part in the comparison, giving a total of 11 values to go into the calculation of the Key Comparison value. A plot of the reported values from each laboratory anonymized and multiplied by a normalization factor so as to not disclose the actual reported values.  Although none of the values can strictly be considered an outlier, tests performed on the data indicate that there are inconsistencies in the data set. Analysis is underway to determine the best method to calculate the Key Comparison Reference Value (KCRV) while maintaining the integrity of the data set. The Draft A Report, including the calculation of the KCRV and Degrees of Equivalence, should be completed by early 2011.