Radioembolization also known as selective internal radiation therapy (SIRT), is a form of radiation therapy used to selected patients who are not candidates for surgery due to the location of their tumors or their performance (health) status. Radioembolization  is currently used to treat primary liver cancer (HCC) and metastatic colorectal cancer (liver metastases).

The treatment involves injecting tiny microspheres with low levels of radioactive material into the arteries that supply the tumor. The radioactivity destroys the liver tumor without affecting other parts of the body, thus minimizing exposure to healthy tissue. In the procedure, a radiologist inserts a catheter which deposits radioactive particles to the area of the tumor.

The microspheres themselves are made of glass or resin. The resin material embolizes (creates a barrier or obstruction) more readily and thus can be used to cut off the blood supply to the tumors. Microspheres target liver tumors by taking advantage of the liver's hypervascularity. For example, metastatic liver tumors greater than 3 mm receive in excess of 80% of their blood supply from the hepatic artery whereas normal liver tissue is predominantly fed by the portal vein.

The microspheres are delivered via fluoroscopic embolization. Millions of 30-micron beads are infused through a catheter into the hepatic artery, and become embedded in the liver. Over a period of two weeks, the therapeutic dose is delivered. The beads are bonded with yttrium-90 (Y-90). Y90 is a beta-emitting radionuclide that acts locally at the tumor site. This is the beta particles travel at most 11 mm in the liver due to their relatively low energy. This allows the beads to embed into and irradiate the tumor while healthy liver tissue is spared.

Currently there are 2 types of microspheres: TheraSphere and SIR-Spheres, which differ in their radioactivity per sphere and embolic effect. One is TheraSphere, FDA approved treatment for primary liver cancer (HCC) and the second is SIR-Spheres, FDA approved for the treatment of metastatic colorectal cancer (liver metastases).