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Accurate, reproducible measurement of radioactivity in nuclear medicine applications is vital to ensure the safety and effectiveness of disease diagnosis and treatment using unsealed radioactive sources. The need to maintain a high degree of confidence in those measurements requires that they be carried out so as to be traceable to national and international standards. In addition, measurement traceability for radioactivity in medicine helps ensure international consistency in measurement at all levels of practice (national measurement laboratories, research institutions, isotope producers, radiopharmaceutical manufacturers and clinics). This paper explores the importance of radioactivity measurement in nuclear medicine and demonstrates how traceability can be extended from international standards to the quantity of the drug administered to the patient.

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The international reference system for radioactivity relies on stable instrumentation for comparing primary standards of radioactivity; these instruments enable national metrology institutes to demonstrate the equivalence of their primary standards. The international bureau of weights and measures (BIPM) holds two such instruments for gamma-ray emitting radionuclides, and is working with the POLATOM, LNE/LNHB, NIM, NPL and PTB to develop a new system called the 'extended international reference system' (ESIR). The ESIR will address pure beta-particle emitting radionuclides as well as other radionuclides (such as 55 Fe) which cannot be measured in the established international systems. The ESIR will be a liquid scintillation system based on three photon-counting channels operating in coincidence. This article reports the results from validation studies carried out using solutions of 3 H, 55 Fe, 63 Ni and 14 C to assess the reproducibility of the results. Several key comparison indicators have been developed and tested to find the best way to obtain a robust and reproducible international reference value. An important conclusion is that the new ESIR can deliver accurate comparison values, immune from changes in detection efficiency or asymmetry of the counting channels. A relative uncertainty better than 0.2% can be expected for radionuclides emitting beta particles with an end-point energy above 150 keV while using commercial liquid scintillation cocktails.