Selective internal radiation therapy

Selective internal radiation therapy (SIRT) is a form of radiation therapy used to treat cancer. It is generally for selected patients with unresectable cancers, those which cannot be treated surgically, especially hepatic cell carcinoma or metastasis to the liver. The treatment involves injecting tiny microspheres of radioactive material into the arteries that supply the tumor.

The microspheres themselves are made of glass or resin. The resin material tends to embolize more easily and thus can be used to stop blood supply to the tumors. Inside the microsphere is the isotope yttrium-90 which has beta-emission with half-life of 2.6 days. Currently there are 2 types of microspheres: Therasphere and SIR-Spheres which differ in their radioactivity per sphere and their embolic effect. SIR-Spheres have more embolic effect and less activity per sphere. This makes them better for certain cancers such as neuroendocrine tumors, which are highly vascular.

Before a patient can have the SIRS treatment, the patient must have a hepatic angiogram, a technetium macroaggregated albumin scan (a radiopharmacological lung scan), and CT scan. The angiogram defines the arterial anatomy and helps the interventional radiologist embolize certain arteries that might carry the microspheres away from the target location to the gastrointestinal system or lungs. Technetium is used to measure lung shunting. Higher lung shunting means increased likelihood of the radioactive particles traveling to the lung and doing damage there. Patients exceeding 20% are usually excluded from this treatment because of the dangers of radiation pneumonitis. CT measures the tumor size which helps determine the correct dosage of microspheres.

Most patients will have lethargy, anorexia, and nausea after the surgery. Ten percent will have gastric ulcerations because the microspheres have moved into the arteries that supply the GI tract. The interventional radiologist tries to prevent this by blocking the common hepatic artery.

After microspheres are injected, they can be confirmed with a Bremsstrahlung image which uses a gamma camera to catch the gamma radiation from the yttrium-90. A PET scan is then done weeks later to monitor the tumor progression. Most patients will have some response to the treatment but about one third do not respond to this treatment.

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