Tumor Ablation in Interventional Radiology

Radiofrequency, Microwave, and Laser Ablation

Radiofrequency ablation (RFA) is the oldest and possibly still the most commonly used percutaneous thermal ablative technology. Single or multipronged devices are available, including straight and umbrella-shaped needles. Some of these needles circulate cool fluid internally to maintain a sufficiently high temperature without charring the surrounding tissue, which can impede electrical and thermal conduction about the needle tip and thereby prevent complete treatment of target tumor with a sufficient margin. Most commercial devices are capable of creating ablation zones of approximately 3 cm diameter, and several overlapping ablations or multiple needles activated synchronously may be capable of creating even larger treatment zones. Deposition of the energy may be limited by tissue types that have poor conductivity, including air-filled lung, dense bone, or charred soft tissue. Moreover, RFA may be limited by the “heat sink” effect, also called perfusion-mediated tissue cooling, whereby flowing blood within peritumoral vessels of sufficient size prevents adjacent target tissue from achieving lethal high temperatures, resulting in inadequate treatment and residual viable tumor. More recent RFA systems have been developed that are bipolar rather than monopolar. With bipolar systems, current passed from the activated device returns to the electrode in a closed loop, avoiding the need for ground pads to be placed on the skin. These bipolar systems are much less stimulating than the monopolar systems, allowing treatment of patients under minimal conscious sedation.

Microwave ablative (MWA) devices also use electromagnetic energy, albeit of higher frequency, to create a field about the needle tip or tips. Within this field, continuous oscillation of water molecules results in rapid heating to more consistent and even higher temperatures than RFA. The needles for MWA devices are termed antennas and are linear in shape. Theoretic advantages for MWA include faster heating of a larger volume of tissue to more consistent temperatures. These devices can create ablation zones capable of treating tumors over 4 cm to 5 cm in diameter. MWA is less limited by the heat sink effect and does not rely on electrical or thermal conduction, unlike RFA. It is possible that these devices will lead to improved oncologic outcomes, particularly in the treatment of liver tumors.

Laser ablation utilizes small caliber, flexible laser fibers to create thermal ablation zones using infrared photons rather than radiofrequency or microwave energy. Each burn with these devices is quite small and numerous overlapping ablations are required to produce large treatment areas like the other heat-based modalities. An advantage of laser systems is its compatibility with MRI guidance and monitoring.

Heat-based technologies produce the fastest ablation zones, although they cannot be monitored accurately without advanced thermometry MRI techniques. They generate gas within the ablation zone, which is visible as a hypodense, hyperechoic area on CT and ultrasound, respectively, but this immediately visible change does not reliably correlate with the volume of necrotic tissue. These devices also cauterize the tissues, so the risk of significant bleeding from these procedures is low.

Cryoablation

Cryotherapy using liquid nitrogen has been used for several decades, but smaller caliber segmentally insulated probes using the Joule-Thomson effect have vastly increased the application of tumor freezing by allowing percutaneous application of these devices. Modern cryoablation technology utilizes highly pressurized argon gas that expands within the sealed chamber of each needle probe to cause focal freezing of the surrounding tissue via a marked, rapid endothermic reaction (Joule-Thomson effect). Cell death occurs within the ablation zone from a combination of mechanical disruption of cell membranes by ice crystals, cellular dehydration, and delayed vascular thrombosis and ischemia. Helium gas flow or an electrical heater within the needles is subsequently used for tissue thawing to remove the needles. Several needles may be used simultaneously to produce large ablation zones and can be placed to create zones that match the morphology of tumors. Most importantly, the ice created by cryoprobes is readily visible within soft tissue on conventional CT, ultrasound, and MRI. Cryoablation has another advantage in that tissue freezing is generally less painful than heat-based ablation. This technique requires the use of tanks of argon and helium with associated gas regulators. Tissue freezing can cause platelet dysfunction with a slight increased risk of significant bleeding complications compared with heat-based therapies.

Irreversible Electroporation

Irreversible electroporation (IRE) is a newer, nonthermal ablative technology that uses multiple linear needle probes to deliver high voltage pulses into the target tissue. These pulses create irreversible pores in cell membranes, leading to loss of cellular homeostasis and cell death by apoptosis with preservation of noncellular stroma. This mechanism may allow for treatment of tumors abutting critical structures, such as bowel, central bile ducts, major nerves, or spinal cord. As the newest commercial percutaneous ablative technique, the clinical literature concerning percutaneous IRE consists of small to moderate-sized single-center series, primarily in the liver and pancreas.

Focused Ultrasound

Focused ultrasound (FUS) therapy is a noninvasive method of tumor ablation that utilizes specialized ultrasound equipment, usually embedded within the MRI scanner table, to produce areas of coagulative necrosis through concentrated focal heating. Most of the published literature addresses FUS treatment of uterine fibroids, although it is also used commonly for superficial bone tumors in some centers. Lung tumors are not amenable to this technique owing to the lack of accessibility to ultrasound energy across aerated lung, and liver tumors are not frequently treated with this technique owing to motion.