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Mode of Action

The SIRT procedure enables high doses of radiation to be targeted to liver tumors by using the tumor's blood supply.

Healthy liver tissue derives up to 90% of its blood supply from the portal vein with only a small amount of the blood supply being derived from the hepatic artery. In contrast, liver tumors derive up to 90% of their blood supply from the hepatic artery since they need a profuse supply of highly oxygenated blood. The hepatic artery therefore provides an ideal channel to deliver targeted treatment to the tumor.

SIR-Spheres Y-90 resin microspheres consist of biocompatible polymer resin microspheres of a median diameter of 32.5 microns (range between 20 and 60 microns) loaded with yttrium-90 (beta radiation penetrating an average of 2.5 mm in tissue to destroy tumor cells). The resin microspheres are small enough to become lodged in the arterioles within the growing rim of the tumor but are too large to pass through the capillaries and into the venous system. Since yttrium-90 has a half-life of 64.1 hours, most of the radiation (94%) is delivered to the tumor over 11 days.

As SIR-Spheres Y-90 resin microspheres are targeted directly at the liver tumors via the hepatic artery, exposure to the remaining healthy liver tissue is minimized.

Due to their relatively small specific gravity of 1.1 g/ml, SIR-Spheres Y-90 resin microspheres travel with the blood stream and are taken deep into the tumor microvasculature where they become lodged.

SIR-Spheres Y-90 resin microspheres can therefore be delivered in a slow deliberate manner to achieve an even distribution, maximizing the number of microspheres delivered, and optimizing tumor coverage - even in patients with extensive disease.