UI researchers investigate the use of magnetic rods to treat prostate cancer

February 11, 2001

Iowa City, IA -- University of Iowa researchers are developing a new approach to treat prostate cancer. The treatment uses heat generated by implanted magnetic rods to destroy the cancer. The UI scientists hope the new technique will be as successful as surgery and radiation therapy in treating the disease, but will avoid the difficult and unpleasant side effects often associated with those standard treatments.

"Our results, and those of our international collaborators, suggest that these rods could be extremely effective in treating the cancer with potentially fewer side effects," said Robert D. Tucker, M.D., Ph.D., UI associate professor of pathology and adjunct associate professor of biomedical engineering. "And we think that this approach could also prove useful against other localized tumors."

Last year more than 180,000 men in the United States were diagnosed with prostate cancer. When the cancer is confined to the prostate gland, surgical removal of the prostate or radiation therapy are the two most commonly recommended treatment options. Although both these methods offer good odds for success, they each entail risks of damage to the tissue around the prostate, which in turn can cause incontinence and impotence.

The treatment under development at the UI will involve implanting small magnetic alloy rods into the prostate using methods similar to those employed to place radioactive brachytherapy seeds. Each cylindrical rod is 1.4 centimeters long and 1 millimeter in diameter. When the patient with implanted rods is placed in an external alternating magnetic field, the rods heat up and transfer the heat to the surrounding tissue. The heat from the rods does two things: it causes proteins to denature or unravel, which kills cells, and it coagulates the blood supply, which starves the cells and causes them to die.

Scientists have known for decades that certain alloys (mixtures of metals) heat up in a magnetic field to a specific temperature, determined by the composition of the alloy, and maintain that temperature while they remain in the magnetic field.

"Different alloys have different Curie temperatures, which is the temperature at which the alloy goes from being magnetic to nonmagnetic," Tucker explained. "When the rod is magnetic, it heats up in a magnetic field. At the Curie temperature, the rod becomes nonmagnetic and ceases to heat up and it simply maintains the Curie temperature as long as it remains in the magnetic field."

The rods used by Tucker's team are made of cobalt and palladium and are biocompatible, which has not been true of other materials tested for this purpose. This technology is being developed by Ablation Technologies of San Diego, Calif. Tucker serves on the company's board of directors.

The UI research team has conducted a series of experiments to test the properties of the rods. The investigators have confirmed that at temperatures necessary to destroy the tissue, the rods are capable of producing enough heat to achieve a uniform temperature increase throughout the targeted tissue. Each rod has a power output of half a watt, so an array of 60 rods, as might be used in the prostate, would generate as much heat as a 30-watt light bulb.

"Our experiments have shown that when the rods are arranged in arrays, the heat or power is concentrated between the rods. The heating only extends a few millimeters beyond the outside edge of the array," Tucker said. "This means you can place the rods close to the edge of the prostate and minimize the risk of damaging tissue beyond the gland."

The magnetic field used to activate the rods is low and not commonly found in everyday life, thus the risk of inadvertent heating of these permanent implants is very small. Also, the strength of the magnetic field used drops off sharply with increased distance from the coil generating the field. This means that magnetic objects in a patient's body that are more than about 8 inches away will not heat up. Unfortunately, a hip replacement device containing metal would present a problem for using this technique.

In addition to the promising laboratory studies conducted by Tucker and his colleagues at the UI, clinical trials conducted by Tucker's international collaborators at the Charité Hospital in Berlin, Germany, and the University of Chile in Santiago have started to yield exciting results.

The results of these trials were so compelling that the U.S. Food and Drug Administration approved a clinical study now being conducted at University of California San Francisco to treat patients who have undergone radiation treatment, but whose cancers have returned.

Implanting the rods using a long hollow needle takes about 45 minutes. The patient receives only a spinal anesthetic. The patient undergoes a single treatment in the magnetic field and is able to go home on the same day.

"In patients treated so far, the results have been encouraging," Tucker said. "Another advantage of these permanent rods is that, unlike radiation treatment, thermal therapy can be repeated non-invasively if the patient's serum PSA values start to rise again."

University of Iowa

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