What Is Radioembolization?
It is a minimally invasive procedure that combines radiation and embolization to treat liver cancer by blocking the blood vessels supplying the tumor. Radiation therapy uses high-energy particles like radium-226, cesium-137, yttrium-90, etc., or waves like X-rays and gamma rays to destroy cancer cells. The high-energy particles or waves are delivered into the body through different techniques; the most popular ones are external beam therapy (EBT) and radioembolization.
EBT uses ionizing radiation to kill cancer cells; it delivers machine-generated high-energy X-rays to the tumor from outside the body. The complication with this is that the normal healthy cells surrounding the tumor also get destroyed during treatment. On the other hand, radioembolization is a more target-oriented therapy that places the radioactive material directly inside the body in tiny resin beads.
What Are the Uses of This Procedure?
Radioembolization is a palliative treatment, meaning it does not cure cancer; rather, it slows down the growth of the tumor. It is primarily used to treat tumors in the liver, which are formed either in the liver or have metastasized to the liver from a different organ.
The different types of cancers that respond well to radioembolization are hepatocellular carcinoma, intrahepatic cholangiocarcinoma, colorectal carcinoma, and nonendocrine tumors.
In a few selected patients, it is also used to treat pancreatic cancers caused due to liver metastasis. It is an alternative for patients who are not eligible for surgery and transplantation.
How Does the Procedure Work?
Radioembolization can be divided into two parts for convenience, they are:
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Radiation Part- Radioactive materials are metals with excess energy. Their molecular structure is unstable, making them highly energetic, and they radiate excess energy into the atmosphere to attain stability. This excess energy can damage our bodies' DNA (deoxyribonucleic acid). The right type and dose of this radioactive material can also damage the DNA of the cancer cells because the cancer cells are mutated versions of human cells. Like a healthy human cell, cancer cells also undergo cell division but more rapidly. During radioembolization, the DNA of these rapidly dividing cancer cells is targeted and destroyed, preventing them from further dividing.
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Embolization Part- Cancer cells derive nutrition from the blood vessels supplying them;for example- tumors in the liver absorb nutrition from the hepatic artery and hepatic vein that supply the liver, and they use this nutrition to divide and grow. During radioembolization, the interventional radiologist injects radioactive glass or resin beads (also called a radionuclide) directly into the blood vessels, thereby cutting off the blood supply to the tumors. This deprives them of the nutrients needed for growth.
The efficiency of radioembolization depends on the tumor's composition and the radionuclide's half-life. The cells in a tumor are not uniform; some will be in the dividing phase, and some in the resting phase. Radiation works best on cancer cells in the dividing phase; it does not work quickly on the cells in their resting phase. For example, skin and bone marrow tumors mimic their healthy counterparts and constantly divide. As a result, radiation works fast in these tumors. In contrast, nerve and breast tumors show slow results from radiation due to their prolonged resting phase.
The half-life of a radionuclide is the time during which it radiates excess energy. Once the radioactive material gets stabilized, it loses its ability to produce and destroy cancer cells. Of the various radiopharmaceuticals, yttrium-90, also known as y-90, has a half-life of 62 hours and is always preferred for radioembolization.
What Are the Tests Done Before Radioembolization?
All patients undergo complete evaluation, including past medical history, laboratory, and imaging work before the treatment. The test done specifically for radioembolization are mentioned below:
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Pretreatment Angiography- An angiogram is an X-ray of the blood vessels. Liver cancers exhibit arteriovenous shunting (an abnormal connection between arteries and veins), which makes it hard to locate the hepatic artery and vein to inject the radionuclide. An angiogram is necessary to map out this abnormal pattern of the tumor.
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Tc 99m Macroaggregated Albumin (MAA) Scan- It is an imaging test that evaluates the capillary patency. The radionuclide injected should not pass through the blood vessels and reach the liver. It should get lodged in the blood vessel; for this to happen, the vessels supplying the tumor should have less diameter at the location where they are connected to the tumor. In the MAA scan, the Tc 99m particles, which mimic y-90 and are larger than the capillary size, are injected into the hepatic artery and distributed according to the blood flow. If they get trapped on the first pass in the arteriolar-capillary bed of the liver, then it is deemed that the same will happen with the y-90 microspheres.
How Is Radioembolization Done?
The entire procedure is image-guided and performed by an interventional radiologist. The procedure is done under anesthesia and on an outpatient basis. First, a small incision is made in the groin area through which the catheter is inserted into the right femoral artery.
Using image guidance, the radiologist maneuvers the catheter to the treatment site. Once the catheter is positioned into the blood vessels (branches of the hepatic artery) supplying the tumor, the radionuclides/microspheres are injected through the catheter. Next, the stomach and the duodenum arteries are embolized to prevent the flow of the microspheres into these organs.
Once the procedure is done, the radiologist will remove the catheter and seal the incision, followed by dressing. The patient is monitored until their vitals are stable and then sent home. Over the next few days, the microspheres lodged in the blood vessels will release radiation.
What Are the Complications of Radioembolization?
Any procedure that penetrates the skin carries the risks of bleeding and infection, along with these complications specific to radioembolization are:
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Hepatobiliary Dysfunction- Radiation-induced liver diseases (RILD) are seen frequently after eight weeks post radioembolization. They can cause ascites, jaundice, and other life-threatening conditions.
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Radiation Pneumonitis- Inflammation of lung tissue in patients with predisposing high lung shunt factor (LSF) is occasionally seen post-treatment.
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Gastrointestinal Complications- Ulcers are commonly seen in the gastrointestinal tract after radioembolization and are caused by the escaped microspheres; they can cause severe epigastric pain.
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Vascular Injury- The procedure includes injuring the blood vessel to inject the microsphere; if there are no predisposing factors, the injury will heal like any other normal wound. But, if the patient has undergone the previous chemoembolization, the associated blood vessels will become fragile due to the deposition of the anti-cancer drugs and, when topped with radioembolization, will cause irreversible vascular injury.
What Is the Difference Between Chemoembolization And Radioembolization?
Chemoembolization is a procedure where anti-cancer drugs are directly delivered near the tumor through a catheter. Small resin particles (also called embolic particles) are coated with anti-cancer drugs and injected into the artery that supplies the tumor. These particles get stuck in the blood vessels, block their blood supply, and slowly kill the cancer cells with their cytotoxic properties.
Radioembolization is similar to chemoembolization, but the embolic particles (also called microspheres) in the former procedure are coated with radioactive materials (like yttrium-90), which also have cytotoxic properties to kill the cancer cells.
Conclusion:
Radioembolization is a nuance cancer treatment that needs to be explored for other organs in the body. Unlike conventional radiation therapy, it is a highly sophisticated treatment that targets only the mutated cells. Complications can be reduced by rigorous pre-assessment and careful patient selection. With new technological advances, it is possible to move radioembolization as a mainstream cancer treatment with improved patient outcomes.
