TheraSphere is a medical device containing yttrium-90 (Y-90) a radioactive material that has been used to treat liver tumors. When Y-90 is put into very tiny glass beads (TheraSphere), it can be injected into the liver through a blood vessel. This allows a large local dose of radiation to be delivered to the tumor with less risk of toxic effects from radiation to other parts of the body or to healthy liver tissue.