Treatments and Technology

Selective Internal Radiation Therapy


Overview


SIRT (Selective Internal Radiation Therapy):

Selective Internal Radiation Therapy (SIRT), also known as Radioembolization, is a liver-directed therapy for inoperable liver tumors.  A micro catheter is used to deliver millions of radioactive microspheres into the hepatic artery, where they are carried into the arterioles and selectively lodge in the tumor microvasculature.  The treatment is team approach between an interventional radiologist and a radiation oncologist.

SIRT1

The microspheres contain yttrium-90 which is a high-energy beta-emitting isotope with no primary gamma emission. This means that radiation only travels a very short distance.  The maximum range of emissions in tissue is 11mm with a mean of 2.5mm. The half-life is 64.1 hours so following administration, 94% of the radiation is delivered in 11 days.

The distribution of blood flow is 3 to 7 times greater within the tumor than the surrounding noncancerous tissue. Together with the properties of the microspheres mean that the microspheres lodge preferentially in the microvasculature of the tumor, maximizing tumoricidal effects and minimizing the effects on healthy liver parenchyma and adjacent tissues.

Toxicity is usually mild and temporary, including fatigue, anorexia, nausea, abdominal discomfort similar to a mild case of the flu.  In addition, there may be slight, temporary elevations of liver function blood tests.

Appropriate patients:

Appropriate patient selection is critical to maximizing benefit and minimizing side-effects of the SIRT procedure. SIR-Spheres microspheres should be used in unresectable liver cancer patients with:

  • Liver-dominant or liver-only disease;
  • Good performance status (ECOG/WHO PS 0-2)
  • Life expectancy > 3 months
  • Adequate liver function (bilirubin < 2.0 mg/dL)

Mapping procedure:

The treatment work-up includes a thorough angiographic evaluation of the liver vasculature to detect and occlude any vessels that could carry microspheres away from the liver to the stomach, duodenum, or gallbladder and to plan for the subsequent administration of the radioactive microspheres.

During this arteriography, the tip of the catheter is placed in the same position where the microspheres will be delivered and the Tc-99m MAA is administered into the hepatic artery as a tracer to determine the extent of arteriovenous shunting to the lungs and to confirm the absence of gastric and duodenal flow.  Some patients may have vasculature that will preclude accurate and reliable placement of the catheter and therefore prohibit the safe delivery of the microspheres. In addition, the amount of lung shunting may alter the activity that can be safely implanted into the liver and in such cases, it may be decided that treatment is not in the patient’s best interest.

Once the extent of extrahepatic shunting has been evaluated and the patient deemed acceptable for treatment, The radioacitve microspheres will be administered at the subsequent visit.

Treatment:

SIRT2

Once the micro catheter has been correctly placed in the hepatic artery by the radiologist, the proximal end of the micro catheter is connected to the microspheres delivery system. The radioactive microspheres are then slowly delivered into the trans-femoral catheter by the radiation oncologist while the radiologist periodically checks the position of the micro catheter to ensure it remains in the right spot during the delivery procedure and confirms that blood continues to flow forward.  At the conclusion of the procedure, the micro catheter is removed and the patient is taken for a post-treatment verification scan.  The SPECT scan will detect the Bremsstrahlung radiation from the yttrium-90 to confirm placement of the microspheres in the liver.  The patient is then discharged home after a short period of observation.   The treatment of the whole liver is often divided into two separate procedures, treating the lobe with the greatest burden of disease first and following this with treatment to opposite lobe about 3-4 weeks later.

 

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