Molecular radiotherapy consists in the injection of a therapeutic agent with known activity in order to deliver a high-dose radiation directly to the tumors while sparing healthy tissues. The Euramet/EMRP project MetroMRT "Metrology for Molecular Radiotherapy" was intended to bring together national metrology laboratories and nuclear medicine services in order to give them metrological support in the field of molecular radiotherapy. In particular, LNE-LNHB was involved in the project for the standardization of 90Y-labelled resin microspheres (SIR-Spheres). This therapeutic agent produced by Sirtex (Sydney, Australia) for selective internal radiotherapy is dedicated to the treatment of unresectable hepatic tumors by radioembolization. The primary activity measurement of 90Y microspheres was carried out after their complete dissolution in the Sirtex vial. Two types of measurements using the TDCR method were used, one based on liquid scintillation and the other on the Cherenkov effect. An original method for the dissolution was developed at LNE-LNHB to optimize the homogeneity of the radioactive solution dedicated to primary measurements. A comprehensive description of the dissolution protocol implemented is reported in this article. The calibration of the ionization chambers at LNE-LNHB for the reference transfer of 90Y-microspheres to end-users is also addressed. The influence of the inhomogeneity of the vial geometry on the uncertainty associated with calibration factors in the case of pure β - emitters such as 90Y is presented. The standardization of SIR-Spheres was also aimed at lowering the 10% relative uncertainty given by Sirtex on the 90Y-microspheres activity (3 GBq).