Yittrium-90 microsphere brachytherapy procedures have increased in number due to their efficacy in treating some unresectable metastatic liver tumors. The discovery of long-lived impurities in two microsphere products, first reported between 2006 and 2007, has resulted in some radiation safety concerns. Since then, microsphere production processes have been refined, which reportedly lead to a reduction in detectable by-products. In this study unused vials of TheraSphere and SIR-Spheres, manufactured in early January 2011, were analyzed to identify and quantify the low-level radioactive impurities. Absorbed dose calculations were performed to assess the potential increased dose to the patient due to long-lived impurities. Results showed that while the SIR-Spheres vials contained no detectable impurities (contrary to other published results in the literature), the TheraSphere vials contained 17 radionuclides in one sample and 15 in the other. The dominant impurities were Y and Y, with specific activities ranging from 0.99 ± 3.40 × 10 kBq mg to 6.30 ± 0.40 kBq mg at vendor assay date. Other impurities were on the order of Bq mg. Based on Medical Internal Radiation Dose (MIRD) liver and lung dose estimates, the long-lived impurities would be expected to increase an administered dose by less than 0.1% from the prescribed dose.

Three different 90Y internally administered radionuclide therapies are currently used in both standard-of-care and clinical trial procedures atMD Anderson Cancer Center. TheraSphere and SIR-Spheres therapies utilize 90Y-labeled microspheres, while Zevalin is an 90Y-labeled radioimmunotherapeutic agent. Several publications have indicated radionuclidic impurities resulting from 90Y production methods. The 90Y in SIR-Spheres and Zevalin are produced from a 90Sr/90Y generator, which leaves measurable quantities of 90Sr in the final product. TheraSphere 90Y is produced in a nuclear reactor which results in a large number of impurities, most notably 88Y and 91Y. Product information sheets reference these impurities with specific limits given. These limits represent a tiny fraction of the total product activity, and in the case of TheraSphere and SIR-Spheres gamma-emitting impurities, this has been verified in the literature. An analysis of 90Sr impurities in SIR-Spheres and Zevalin is presented in this paper. Impurity quantities were found to be within the vendors’ documented limits.

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