Percutaneous microwave ablation reaches where RFA can’t

Once Dr. Andrew Hines-Peralta resolved the problem of how to percutaneously cook large liver tumors, he undertook a more pressing challenge: remodeling his condo.

The 28-year-old son of a U.S.-born father and a Mexican-born mother, Hines-Peralta picked up a 2004 RSNA Resident Research Trainee Prize for his work on tumor ablation using microwave technology. The award delivers prestige and a $1000 check.

Over his girlfriend's objections, the young researcher took his check to the home supply story and bought all the necessary accoutrements to give his crib a much-needed makeover.

"I should have used the money to pay back my college loans, but I don't regret what I did," he said.

Nor does he regret experimenting with a new microwave ablation system in Dr. S. Nahum Goldberg's lab at Beth Israel Deaconess Medical Center in Boston. The challenge for interventional radiologists is to ablate sizable tumor volumes quickly and safely, in a way that can be easily replicated. Many patients seen in clinical practice have cancers too large to be treated with radiofrequency ablation. Microwave ablation offers an alternative.

Hines-Peralta and his colleagues performed multiple ablations on 128 dissected cow livers and in the livers of 45 live pigs using a 5.7-mm diameter microwave ablation applicator. The crayonlike probe delivered a 2.45-GHz frequency microwave from a generator with an output power range of 50 W to 150 W. This is the same frequency microwave ovens use and exceeds that of conventional systems, which border 900 MHz.

The ablation injuries to the ex vivo livers were uniform and small. Researchers had expected even smaller in vivo burns in the live animals because blood perfusion in and around tumors takes away the heat. To their surprise, they were able to produce large ablation volumes in a relatively short time.

"Ablating a tumor surrounded by a lot of blood flow is like trying to shish kebab a piece of meat under the faucet," Hines-Peralta said. "When we applied the technique in the living pigs, instead of seeing the ablation size get smaller, the exact opposite happened."

RF ablation has achieved prominent status as a safe, efficient means to treat solid tumors. It faces several shortcomings, however, including slow and uneven heating of tissue that may lead to charring. Because RFA relies on tissue's ability to conduct and resist a tiny amount of electrical current to induce heat, it can be less effective in tumors larger than 4 cm. It can also be subject to the "heat-sink" effect from large blood vessels that are too close to target lesions.

Microwave ablation has been proposed as an alternative to cook tumors that aren't amenable to RFA. Microwave energy rotates water molecules at ultrahigh speeds, producing the type of heat required for ablation. MWA thus tends to heat tissue evenly and quickly, and it may be less vulnerable than RFA to the heat-sink effect. The technique is not yet as readily available or clinically tested as RFA, however.

Many scientists contend that microwave energy should work more efficiently in a water-rich environment. Others speculate that blood perfusion enables the quick spread of heat. Nobody really knows. They do know that microwave heats up tissue much more efficiently than does RFA, Hines-Peralta said.

"Perhaps the most clinically relevant part of the experiment is seeing that those lesions that couldn't be treated with RFA could be treated with microwave," he said.

Young pioneers like Hines-Peralta see interventional oncology as the new frontier, where medicine and technology combine forces and come up with revolutionary treatments. The discipline requires problem solving and pursuit of a vast amount of knowledge.

"So little is known about interventional oncology that the only way to extend its use in clinical practice is through research," he said.

While Hines-Peralta is conducting research, he has also set his sights on becoming an excellent radiologist. Can the two ideals coexist? He thinks so, but his parents may have different ideas. Both of them urged him not to go into medicine. They said he wouldn't have a life.

"They were totally right," he said with a laugh.