Caren van Roekel

19 Radioembolization 1.2.1.2. SIR-Spheres® 90 Y is permanently embedded in an acrylic polymer resin (13). Each resin microsphere has a diameter of 20-60 µm. The microspheres are labelled with 90 Y via cation-exchange with sufficiently strong binding. 90 Y is a decay product of 90 Sr obtained from a generator. SIR-Spheres ® have a low specific gravity and density (1.4 g/cm 3 ) and a specific activity of 50 Bq/sphere. A typical injected activity ranges from 40–80 million microspheres. The standard activity of SIR- Spheres ® is 3 GBq per patient in 5 ml water for injection. 1.2.2. 166 Ho microspheres (QuiremSpheres®) 166 Ho is a high-energy beta-emitting isotope for therapeutic use and emits primary gamma photons. The maximum energy of the beta particles is 1.85 MeV (50.0%) and 1.77 MeV (48.7%) with a half-life of 26.8 hours. The maximum range of emissions of the beta particles in tissue is 9 mm (mean 3.2 mm). They were developed by the University Medical Center Utrecht (the Netherlands) and are commercialized under the name QuiremSpheres® (Quirem Medical, Diepenveen, the Netherlands). Just like 90 Y microspheres, 166 Ho emits an electron to reach a stable state (beta radiation). Furthermore, it emits gamma photons (81 keV, 6%), which are useful for SPECT/CT imaging (14). 166 Ho is part of the metallic chemical elements known as lanthanides, which have paramagnetic properties, so magnetic resonance imaging (MRI) can also be used to image the distribution in the liver and quantify the dose in the tumors (15, 16). 166 Ho microspheres decay is faster due to a shorter half-life (26.8 vs 64.2 hour), but deposit less energy than 90 Y microspheres per decay (16 vs 49 J/GBq). This results in a higher administered activity to reach the same dose deposition. As a result, a higher dose rate is achieved. Microspheres loaded with Ho are irradiated per patient-dose in one of the pre-determined neutron reactors with a suitable irradiation profile: the reactor must have a high neutron flux without being accompanied by extensive gamma heating, as this detriments the structural integrity of 166 Ho microspheres (17). In the current commercially available product, the specific activity is 450 Bq/ sphere with a density of 1.4 g/mL. 2