What is Selective Internal Radiation Therapy (SIRT)?
Selective Internal Radiation Therapy or SIRT, also known as radioembolisation, is an innovative therapy that has been developed for the treatment of unresectable primary and secondary liver cancer. The technique involves infusing millions of radioactive beads (Yttrium-90 resin microspheres or SIR-Spheres® Y-90 resin microspheres) into the arterial blood supply of the liver.
What are SIR-Spheres Y-90 resin microspheres?
SIR-Spheres Y-90 resin microspheres are radioactive microspheres used in SIRT. SIR-Spheres Y-90 resin microspheres deliver targeted internal radiation therapy directly to the tumour(s) with a dose of internal radiation up to 40 times higher than conventional radiotherapy, while sparing healthy tissue.
Direct delivery of the microspheres via the hepatic arteries helps to achieve maximum disease control through optimal tumour coverage. Randomised controlled studies in patients with liver metastases from colorectal cancer have demonstrated that SIRT using SIR-Spheres Y-90 resin microspheres significantly increases the tumour response or disease control rates, as well as significantly extending the time to progression and overall survival. SIR-Spheres Y-90 resin microspheres may also reduce the size of previously inoperable liver tumours to the extent that they are amenable to potentially curative resection or ablation.
How do SIR-Spheres Y-90 resin microspheres work?
The SIRT procedure enables radiation to be targeted directly into the liver tumours by using the tumour's own blood supply. Healthy liver tissue derives up to 90% of its blood supply from the portal vein (the vein that delivers nutrients to the liver from the gut), with only a small amount of the blood supply being derived from the hepatic artery. In contrast, liver tumours 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 route to deliver targeted treatment to the tumour.
SIR-Spheres Y-90 resin microspheres are between 20-60 microns in diameter which means that following infusion, they are small enough to become lodged in the arterioles within the growing rim of the tumour(s) where they emit a high dose of radiation, but are too large to pass through the capillaries and into the venous system. As SIR-Spheres Y-90 resin microspheres are targeted directly at the liver tumours via the hepatic artery, exposure to the remaining healthy liver tissue is minimised. The resin microspheres contain the radioactive element Yttrium-90, which delivers beta radiation over a relatively short distance: an average of 2.5 mm in human tissue and a maximum of 11 mm. Yttrium-90 has a half-life of approximately two-and-a-half days (64.1 hours), therefore most of the radiation (over 97%) is delivered to the tumour in the first two weeks after treatment.
How are SIR-Spheres Y-90 resin microspheres different from conventional radiotherapy?
Radiation is an effective agent for destroying tumours and is widely used in cancer treatment. However, the use of external beam radiation to treat liver tumours is limited by the low radiation doses that can be applied to the liver without the risk of radiation damage to the normal liver tissue.
Unlike conventional external beam radiation, SIR-Spheres Y-90 resin microspheres selectively irradiate liver tumours and therefore have the ability to deliver more potent doses of radiation directly to the cancer cells over a longer period of time. The therapeutic ratio with SIRT, compared to external beam radiotherapy, is significantly improved and the tumour-absorbed doses from SIRT are typically 4 to 6 times higher than those to the healthy liver tissue.
How is SIRT administered?
After a local anaesthetic is administered to the patient, the SIRT-trained interventional radiologist makes a small incision, usually into the femoral artery near the groin. A catheter is then guided through the artery into the liver. SIR-Spheres Y-90 resin microspheres are administered through this catheter. The whole procedure may take around 60-90 minutes. After the procedure is completed, patients may be sent to have a scan to check the level of radioactivity of the SIR-Spheres Y-90 resin microspheres in the liver. Patients will be monitored for a few hours after the procedure and most patients are discharged within 24 hours.
What are the side effects of SIRT?
Almost all treatments and drugs produce unwanted side effects. Most side effects following a SIRT procedure are minor, but a small number can be serious. Many patients experience abdominal pain or tightness in their abdomen, nausea and loss of appetite which normally subsides within a week. Patients may also develop a mild fever that may last for up to a week and fatigue which may last for several weeks.
What are the potential complications of SIRT?
In rare instances, a small number of microspheres may inadvertently reach other organs in the body, such as the gallbladder, stomach, intestine, or pancreas. If microspheres reach these organs, they can cause inflammation or ulceration. These complications are rare, but if they do occur they will require additional medical treatment.
Do patients have to take special precautions?
There are some simple precautions that patients need to take during the first 24 hours following the SIRT procedure. These precautions include: thorough hand washing after using the toilet, and cleaning up any spills of body fluids such as blood, urine or stools and disposing of them in the toilet. Otherwise, patients can resume normal contact with family members.
Patients must not receive SIRT treatment if they are pregnant, and must not become pregnant within two months of receiving the treatment as this may cause harm to the unborn baby.
What is the regulatory status of SIR-Spheres Y-90 resin microspheres?
SIR-Spheres Y-90 resin microspheres are classified as a Medical Device and approved for use in for the treatment of unresectable liver tumours in Australia and New Zealand, the European Union (CE Mark), Switzerland, Turkey, and many countries in Asia such as India, Korea, Singapore, Malaysia, Thailand, Vietnam, Philippines and Hong Kong.
SIR-Spheres Y-90 resin microspheres are also full PMA-approved by the FDA and are indicated in the U.S. for the treatment of non-resectable metastatic liver tumours from primary colorectal cancer in combination with intra-hepatic artery chemotherapy using floxuridine. They are also approved in Taiwan in chemotherapy refractory metastatic colorectal cancer patients.