Diagnostic Imaging
November 2002

Tech Watch

Starburst probe advances art of radio-frequency tumor ablation

Better techniques create new treatment options, eclipsing surgical resection for metastatic liver disease

Surgical resection, the gold standard for treating liver tumors, has an 85% success rate, but only a small percentage of patients presenting with a malignant hepatic tumor are candidates for surgery. Patients with untreated primary or secondary liver lesions have a mortality rate of 100% at five years (AJR 2001;176:3-16).

In the last few years, the FDA has approved several percutaneous radio-frequency devices for the treatment of unresectable liver lesions. Many patients who are not candidates for surgery because of lesion size or location, or other comorbid disease have undergone RF ablation with varying success. Elements that contribute to successful ablation include operator experience, patient selection, and a thorough understanding of tumor morphology.

At the Society of Interventional Radiology meeting in April, Rita Medical Systems of Mountain View, CA, kicked into high gear the marketing of its StarBurst XLi RF ablation system. This next-generation probe, which can ablate an area as large as 7 cm, relies on microinfusion technology. No other product ablates lesions as large as fast, said Mike Dominici, senior marketing manager at Rita.

HOW IT WORKS

Using CT or ultrasound guidance, the interventional radiologist places the 14-gauge needle through the skin to the liver and directly into the lesion.

An array of nine wires, guided through the needle, spread out into the lesion in a starburst form to infiltrate the lesion. Four passive electrodes containing thermocouples record the temperature at the most distal portion of the ablation. The five active electrodes slowly drip a saline solution onto the tissue to increase conductivity. The ablation grows quickly at the infusion sites, creating five ablation "balls" that eventually coalesce, grow out toward the passive thermocouples, and fill the targeted site on the liver. The saline solution also acts as a coolant, preventing charring of the ablated tissue.

The 150W 1500X RF generator (operated at 460 kHz) has software upgrades that increase power to 250W. An automated program starts the generator at approximately 25W when the electrodes are two thirds deployed. Power gradually increases to peak, where it is maintained until the temperature exceeds the preselected target temperature (typically between 95 degrees C and 105 degrees C). At target temperature, the curved electrode is slowly advanced by the interventional radiologist to full deployment. As the tissue begins to desiccate, the amount of power needed to maintain the target temperature decreases. Ablation is usually complete within 25 minutes, depending on size and location of the tumor.

An important factor affecting local tumor recurrence rate after hepatic resection is the presence and size of a tumor-free margin around the lesion, said Dr. Gerald D. Dodd III, radiology chair at the University of Texas Health Science Center at San Antonio. The surgical literature documents that tumor-free resection margins of less than 1 cm are directly related to increased local hepatic tumor recurrence rates and decreased overall patient survival.

"If interventionalists want to have success rates that approximate those of hepatic resection, they need to adhere to the same margin," Dodd said.

A 360 degrees 1-cm-thick tumor-free margin essentially adds 2 cm to the diameter of the ablation. For RF devices producing 3-, 5- and 7-cm ablation spheres, the largest tumors that can be treated adequately would be 1, 3, and 5 cm, respectively.

OVERLAPPING SPHERES

The StarBurst XLi is designed to provide one large, scalable, spherical ablation, thereby reducing or eliminating the need for overlapping ablations. A computer analysis by Dodd and colleagues at UTHSC showed that multiple overlapping spheres are poor building blocks that leave "pits" at the intersection of the spheres (AJR 2001;177:777-782).

The researchers created six overlapping spheres, each one 3 cm in diameter. The six-sphere model showed that the largest tumor that may be treated with a 3-cm ablation device (while providing an adequate tumor-free margin) is 1.75 cm, whereas 4- and 5-cm ablation spheres can be used to treat tumors measuring 3 and 4.25 cm, respectively.

"We had a device to produce 3-cm ablations, we put six spheres together, and didn't get even an additional centimeter out of it," Dodd said. "Our success rate in killing tumor is going to be better with a single ablation when possible. When you start overlapping ablations, it becomes much more complex and the chance of missing tumor goes up."

Dodd's computer model is based on the exact placement of perfect spheres, which doesn't reflect real-life variation. In addition, not all devices produce spherical ablations, nor do they produce ablations of equal size.

Much of the recent success of RF ablation can be attributed to newly developed techniques that include the use of hooked-array systems, adjuvant saline injection, and single or cluster arrays of internally cooled electrodes. It is imperative, Dodd said, that an ablation strategy be modified for the RF device that is used. A thorough knowledge of each system's strengths and weaknesses should translate into more appropriate stratification of the relative importance of ablation factors such as thermal lesion size, uniformity, and predictability. The interventional suite at UTHSC keeps several RF ablation devices on hand.

"Unless one company were to manufacture all the different configurations of the devices, interventional radiologists shouldn't be limited to a single device," Dodd said.