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Validation work is continuing for the International Reference System for α- and pure β-emitting radionuclides (ESIR) in anticipation of a significant expansion of the therapeutic use of radiopharmaceuticals.

The CCRI 2018-2028 Strategy and underlying stakeholder consultation predicted a significant expansion of the therapeutic use of radiopharmaceuticals. This motivated an extensive analysis of the status of primary radioactivity standards underpinning metrological traceability for nuclear medicine. Recently published as a review in Metrologia, the study conducted by experts from the BIPM, NPL, NIST and the International Atomic Energy Agency (IAEA) reached important conclusions regarding:

• the implementation by the radionuclide metrology community of a robust system to cross-check primary standards of radionuclides used world-wide in nuclear medicine.
• the realization accuracies of existing primary standards and their conformance probability with different tolerances required by medical imaging or therapy applications.
• the absence of comparisons for some radionuclides used for single photon emission computerized tomography (SPECT-CT), for positron emission tomography (PET) and for in vitro diagnostics.
• in certain cases, the lack of formal evidence that primary standards have been realized.

This situation also applies to candidate species for next-generation radiopharmaceutical products. In this context, the opportunity is now with the radionuclide metrology community to collaboratively work with concerned parties to prioritize the study of these materials. In the same context, CCRI-Section II initiated the development of a new international reference system for α- and pure β-emitting radionuclides (ESIR). The ESIR enables metrology institutes to evaluate the international equivalence of targeted particles used in medical diagnosis and treatments. Based on liquid scintillation counting, the ESIR shall:

• for α-emitting standards (actinium, terbium, astatine) enable bilateral comparisons of low activity sources therefore reducing the amount of work required by the strict conditions for shipment and manipulation of radioactive solutions.
• for pure β-emitting standards (yttrium, strontium, scandium, erbium) enable bilateral comparisons hence reduce the drive for complex large-scale exercises that overburden the piloting laboratories.
• provide traceability services for standards employed in Auger-therapy (erbium, lanthanum).

Validation work on the ESIR is currently in progress, based on a CCRI(II) pilot study completed in 2023. A list of radionuclides is being established and will be discussed at the upcoming meeting of CCRI(II) and by its Key Comparisons Working Group to progressively open the new service.