Caren van Roekel

18 Chapter 2 • The microspheres can be based on polymers, polymeric resins, albumin or inorganic materials. Currently, three different types of radioactive microspheres are commercially available. The type of microspheres can be divided based on the embedded radioactive isotope (yttrium-90 ( 90 Y) or holmium-166 ( 166 Ho)) or microsphere material (resin, glass or poly-L-lactic acid). 1.2.1. 90 Y microspheres 90 Y is a pure (99.99%) β - -emitter that decays to stable zirconium-90 ( 90 Zr) with an average β-energy of 0.9 MeV and a half-life of 64.2 hours (6, 7). 90 Y can either be produced by neutron bombardment of stable yttrium-89 ( 89 Y) in a commercial reactor or by chemical separation from the parent isotope strontium-90 ( 90 Sr), a fission product of uranium (7). The mean tissue penetration is approximately 2.5 mm and the maximum range in tissue is 1.1 cm (8, 9). Imaging of 90 Y is possible by using either the bremsstrahlung for bremsstrahlung-single photon emission computed tomography/computed tomography (SPECT/CT) (10, 11) or the emitted positrons (32 positrons per 1 million decays) for positron emission tomography/computed tomography (PET/CT) imaging (6). Two commercially available 90 Y-labelled microspheres are currently on the market: • Glass microspheres (TheraSphere ® , BTG international Medicine) • Resin microspheres (SIR-Spheres ® , SIRTEX Medical Limited) 1.2.1.1. TheraSphere® 90 Y is permanently embedded in the glass. Each glass sphere has a diameter of 20-30 µm. 89 Y yttriumoxide is stable incorporated. For labelling, the microspheres are irradiated with neutrons in a nuclear reactor (12). TheraSphere ® has a high specific gravity and density (3.3 g/cm 3 ). A typical injected activity ranges from 3–5 million microspheres with a high specific activity of 2500 Bq/sphere. An extended shelf-life activity ranges from 5-10 million microspheres with a high specific activity of approximately 1250 Bq/ sphere. The standard activity of TheraSphere ® in water for injection is 5 GBq per patient; activities of 3 GBq up to 20 GBq are available.