Selective internal radiation therapy (SIRT)

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Selective internal radiation therapy (SIRT), also known as hepatic radioembolization, is a relatively new and developing modality for treating non-resectable liver tumors. The procedure consists of a transcatheter injection of radioactive particles via the hepatic artery.

It is generally considered efficacious in patients with hepatocellular cancer, neuroendocrine and colorectal liver metastases. It generally involves a single delivery of yttrium-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 microsphere device), treatment with the glass microsphere device has been successful in patients with this condition with superselective delivery
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

Pathophysiology

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 arterial supply. This allows the use of higher doses or internal radiation or chemotherapy than the normal liver can tolerate.

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

It is achieved by the intra-arterial injection of yttrium-90 (beta emitter) -labeled glass or resin microspheres as an interventional radiology procedure. An alternative method is the used of I-131-labeled Lipiodol.

Procedure

preprocedure evaluation
lab work to confirm bilirubin, coagulation profiles and platelets are adequate
injection of 99mTc-MAA (macroaggregated albumin) into hepatic artery proper with follow up scintigraphy to determine degree of shunting to lungs and bowel
confirmation of portal vein patency
calculation of dose of Y-90 based on body surface area, % tumor volume in liver, liver function and percentage shunting of spheres to lung

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 liver usually done on 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 actual infusion of Y-90 spheres, extrahepatic branches are embolized
meticulous procedure is used to minimize 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.