Yttrium-90 (⁹⁰Y) microspheres are tiny spheres loaded with ⁹⁰Y, a radioisotope that emits pure beta radiation. ⁹⁰Y has a ” half-life” of about 64 hours. The radiation from ⁹⁰Y is largely confined to a tissue depth of 2 – 3 mm. After injection into the artery supplying blood to the tumors, the spheres are trapped in the tumor’s vascular bed, where they destroy the tumor cells by delivering the beta radiation. The majority of the radiation emitted from the tumor is contained within the patient’s body, and external radiation is so low that it does not present a significant risk to others.

Because the spheres may have trace amounts of free ⁹⁰Y on their surface, only very small amounts of ⁹⁰Y can be excreted in the urine.