SIR-Spheres microspheres are a medical device used in Selective Internal Radiation Therapy (SIRT) for liver tumors. SIR-Spheres microspheres are a permanent implant and for single use only. The biocompatible resin microspheres containing yttrium-90 have a median diameter of 32.5 microns (range between 20 and 60 microns).
Yttrium-90 is a high-energy beta-emitting isotope with no primary gamma emission. The maximum energy of the beta particles is 2.27MeV with a mean of 0.93MeV. The maximum range of emissions in tissue is 11mm with a mean of 2.5mm. The half-life is 64.1 hours. Following administration, 94% of the radiation is delivered in 11 days.
These properties of the SIR-Spheres microspheres mean that the microspheres lodge preferentially in the microvasculature surrounding the tumor, maximizing tumorcidal effects and minimizing the effects on healthy liver parenchyma (Also see Mode of Action and Radiation Safety Aspects).