TheraSphere is a medical device containing yttrium-90 (Y-90), a radioactive material that has been used previously in the treatment of liver tumors. When Y-90 is incorporated into very tiny glass beads (TheraSphere), it can be injected to the liver through blood vessels supplying the liver. This allows a large 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. The radiation from TheraSphere is contained within the body and becomes minimally active within 7 days after treatment due to physical decay. The glass beads remain in the body, but do not cause any health problems. TheraSphere has been approved for use in the treatment of liver cancer in the United States by the Food and Drug Administration on a limited basis.