Selective internal radiation therapy (SIRT), also known as transarterial radioembolization (TARE) or hepatic radioembolization, is a treatment for non-resectable liver tumors. The procedure consists of a transcatheter injection of radioactive particles via the hepatic artery. 

It is technically a form of brachytherapy, which involves an injectable form of the radiation source being delivered to a selected volume of tissue perfusion.

It is generally considered efficacious in patients with hepatocellular cancer, neuroendocrine and colorectal liver metastases ¹. It generally involves a single delivery of yttrium-90 (Y-90) microspheres into the hepatic artery. Preferential uptake is achieved into liver tumors, because of their predominant hepatic arterial blood supply. Average tumor doses of radiation in excess of 200 Gy are achieved.

Indications

It is a method of treating liver tumors (primary or secondary) in patients in whom surgery is not an option, such as:

• unresectable hepatic metastatic disease

• unresectable hepatocellular carcinoma (HCC)

• hepatic tumor progression despite treatment

• symptoms related to hepatic tumor bulk or hormonal excess (neuroendocrine tumors)

• "bridge to transplant": stop tumor progression while awaiting liver transplant

• life expectancy >90 days

• liver-dominant tumor burden

Contraindications

• extensive or progressive extrahepatic disease

• poor baseline liver function

• Eastern Cooperative Oncology Group (ECOG) performance status >3

• exaggerated hepatopulmonary shunting

• reflux into the arteries that supply the gastroduodenal region

• uncorrectable extrahepatic shunts

• portal venous thrombosis (while it is listed in the package insert as a contraindication to use of the resin microspheres), treatment with the glass microspheres has been successful in patients with this condition with superselective delivery ⁴

• life expectancy <90 days

• total bilirubin >2.0 mg/dL

• contraindications to angiography:

  • contrast allergy causing anaphylaxis

  • renal insufficiency

  • peripheral vascular disease

  • uncorrectable bleeding diathesis

Pathology

Primary and metastatic hepatic malignancies derive 80-100% of their blood supply from the hepatic artery unlike normal liver that receives only 20% from the hepatic artery ¹. This allows the use of higher doses of internal radiation or chemotherapy than the normal liver would otherwise be able to tolerate. 

The transcatheter hepatic artery infusion of radioisotope delivers high doses of intratumoral radiation, while sparing the surrounding healthy liver parenchyma (or at least only delivers a low, tolerable dose to normal liver). 

It is achieved by the intra-arterial injection of Y-90 (a beta emitter) labeled glass or resin microspheres as an interventional radiology procedure. Alternative isotopes used include I-131 labeled Lipiodol , Re-188, Ho-166.

Y90 is the only commercially available isotope that is widely in use across the world, with widely accepted dosimetry guidelines.

Ho-166 has the advantage of using a smaller scout dose of the therapeutic isotope itself for localization and lung shunt fraction estimation, provising a more reliable calculation of therapeutic dosimetry.

I-131 was an application which initially showed a lot of promise but did not find widespread acceptability due to its dissociation from the ligand and need for prolonged isolation time post administration.

Re-188 has found growing interest as a poor man's Y90 due to the long (6month) half life of its generator.

Procedure

• pre-procedure evaluation

• lab work to confirm bilirubin, coagulation profiles and platelets are adequate

• injection of 99mTc-MAA (macroaggregated albumin) into the hepatic artery proper with follow up scintigraphy to determine the degree of shunting to lungs (lung shunt fraction) and bowel

• confirmation of portal vein patency

• calculation of dose of Y-90 based on body surface area, percentage tumor volume in liver, liver function and lung shunt fraction

Procedure steps

• preliminary angiogram to determine vascular anatomy, confirm location of metastases, inject 99mTc-MAA for scan shortly after angiogram, determine extrahepatic arteries arising from the celiac axis that will need to be avoided or embolized including cystic artery if gallbladder is still present

• left and right lobes of the liver are usually treated with two separate procedures at least 4 weeks apart to ensure adequate liver function is maintained and any change in bilirubin, liver enzymes and platelets returns to baseline

• prior to infusion of Y-90 spheres, extrahepatic branches are embolized or occluded as needed

• meticulous care is used to minimize radiation exposure to staff including confining and preparing spheres in an acrylic shielded box that absorbs beta radiation and minimizes Bremsstrahlung radiation production

• after Y-90 infusion and catheter removal, gamma camera images are usually performed using the Bremsstrahlung radiation produced by the beta particles to produce images of where the spheres are located

Post-procedure evaluation

• Tc-99m-MAA scan after first angiogram to determine extrahepatic shunting

• lab work just before and after two therapeutic procedures

• follow up CT or MRI of the liver 3-6 months after treatment

Potential complications

• nausea, vomiting, fever, diarrhea and abdominal pain

• transient lab abnormalities including liver function, hemoglobin and platelet levels

• acute pancreatitis, radiation pneumonitis, radiation gastritis and hepatitis, acute cholecystitis

See also

• liver and biliary interventional procedures

• radioembolization

• post-TARE assessment of hepatocellular carcinoma