Novel radiation technique to treat liver cancer
Killing Liver Tumor
Physicians at Mayo Clinic are now using tiny glass bubbles filled with radioactive material to deliver high doses of tumor-killing radiation directly to liver tumors. They say the procedure is better tolerated than other forms of intra-arterial liver cancer treatments, and may be the best option for some patients who aren't candidates for other treatments, including surgery or liver transplantation.
The technique, called either radioembolization or intra-arterial brachytherapy, uses the blood supply to send the little spheres, smaller in diameter than a human hair, into the newly formed, microscopic vessels that feed cancer. They eventually become lodged at the tumor sites where they deliver a high dose of radiation.
And because these liver tumors use a supply of blood that is largely separate from the blood that nourishes normal liver tissue, few of the microspheres end up in the healthy liver, Mayo clinicians say.
"The technique is a clever way of exploiting the differences in blood supply between the liver tumor and normal liver tissue," says Mayo Clinic interventional radiologist Ricardo Paz-Fumagalli, M.D. He, along with Mayo Clinic radiation oncologists, deliver the therapy to patients.
There are two primary blood vessels that bring blood to the liver. Normal liver tissue receives about three-fourths of its blood supply from the portal vein and only about one-fourth from the hepatic artery and its branches, explains Paz-Fumagalli. Liver tumors, on the other hand, get most of their life-sustaining blood supply from the hepatic artery and absorb a greater proportion of the radioactive microspheres. "So if you give a treatment through the arteries, it more specifically hits the tumor, and the normal liver is relatively spared," he says.
The outpatient procedure only takes about an hour to complete. Patients are given a local anesthetic around the femoral artery in the leg and mild intravenous sedation. Physicians insert a catheter into the femoral artery and under x-ray guidance, advance it to a targeted artery that branches off from the hepatic artery. Then the physicians inject tiny glass spheres into the artery.
As the microspheres release radiation over 10-14 days, the tumors receive a higher dose than is generally tolerated if it were given by the external beam method used to treat many types of cancers such as breast and prostate, Paz-Fumagalli says.
However, the radiation given off by the microspheres doesn't penetrate too deeply or stay potent for too long. "The type of radiation used penetrates a very thin layer of tissue, about two to three millimeters in average, so that once you have enough thickness of tissue over the area that received the treatment, very little radiation will escape and expose other tissues or other people to it," he says.
In addition, the half-life of the Yttrim-90 radiation used is about 64 hours. That means in 64 hours, one-half of the radioactivity decays into a non-radioactive substance. The half that remains will decay by one half in another 64 hours and so on. "By the time two weeks go by, you have very little radiation left," Paz-Fumagalli says. "So it's very safe. It's very highly concentrated to the tumor, yet doesn't go very far," he says. "For that reason the body can tolerate these doses of radiation quite well."
Mayo doctors perform detailed imaging tests prior to treatment to make sure that excess blood flow through the hepatic artery is not going to other organs. That's because the high doses of radiation used in this new treatment would cause severe damage to the lungs, stomach and bowel if transported beyond the liver.
Patients may be candidates for radioembolization if they are not a candidate for liver transplantation, surgery or another procedure called percutaneous ablation, Mayo clinicians say. Patients may be rejected for one of these better treatment options if their tumor is too large, they have too many tumors, not enough good liver reserve or have metastatic cancer.
If radioembolization is not possible or safe, other intra-arterial treatment methods are available, they say. One alternative is chemoembolization, in which physicians inject a chemotherapy cocktail and small occlusive particles into the liver arteries. This procedure, however, can be very painful, require hospitalization and tends to be more toxic to healthy liver tissue.
The prognosis for primary liver cancer (hepatocellular carcinoma) is usually poor when surgery is not an option. For these patients, radioembolization is used as a palliative therapy to improve quality of life, to improve length of survival, and in some cases it may be chosen as a therapy to shrink a tumor in order to prepare a patient for a curative liver transplant.