20 Ablation of Malignant Liver Tumors

10.1055/b-0038-162891

20 Ablation of Malignant Liver Tumors

Ronald S. Arellano

20.1 Introduction

Over the past 15 years, image-guided tumor ablation has evolved to become a well-established therapeutic option for the treatment of liver tumors. ¹ ^, ² ^, ³ ^, ⁴ ^, ⁵ ^, ⁶ Thermal ablation offers several distinct advantages over surgical resection including decreased morbidity and mortality, the ability to perform ablations as an outpatient procedure with lower costs, and the potential to offer treatment to patients who might otherwise be poor surgical candidates. Currently available ablation techniques include radiofrequency ablation (RFA), microwave ablation (MWA), cryoablation, and irreversible electroporation (IRE; ▶ Table 20.1). Although each modality has its own unique properties, they all share the common goal of generating necrotic tissue through the use of minimally invasive techniques. ⁷ ^, ⁸ ^, ⁹ This chapter will describe the currently available ablation techniques and their role in the treatment of liver tumors.

**Table 20.1** Summary of ablation types
Ablation type	Pros	Cons
Radiofrequency ablation (RFA)	Well studied Highly effective for lesions ≤ 3 cm	Limited efficacy as tumor size increases Most tumors require multiple overlapping ablations
Microwave ablation	Offers ability to generate large volumes of ablation in less time than RFA Less influenced by adjacent blood vessels Does not require grounding pads or antenna cooling	Less studied than RFA Limited clinical efficacy data
Cryoablation	Offers ability to visualize iceball during treatment Can be used to treat multiple lesions simultaneously	Associated with risk of cryoshock Limited ability to treat large lesions Multiple probes required for larger lesions
Irreversible electroporation	Nonthermal May have a role for tumors near critical biliary or vascular structures	Limited safety and clinical efficacy data Requires general anesthesia

20.2 Ablation Types

20.2.1 Radiofrequency Ablation

RFA is probably the most studied of the thermal ablation techniques. RFA induces tissue coagulative necrosis via delivery of electromagnetic energy to the patient. This is achieved by placing the patient within a closed-loop electrical circuit that incorporates an RF generator, an electrode applicator, and dispersive electrodes in the form of grounding pads that are placed on the patient. Within this closed-loop circuit, the RF generator creates a high-frequency alternating electrical field within the patient, and high electrical resistance within the tissues leads to ionic agitation as the ions within tissues attempt to align in the direction of the alternating electrical current. This ionic agitation results in friction, which in turn generates heat around the RF electrodes. In most instances, the heat that is generated exceeds 100 °C, which leads to coagulative necrosis (▶ Fig. 20.1). ¹⁰ ^, ¹¹ RF electrode designs range from straight, nondeployable internally cooled electrodes to expandable electrodes. Similarly, treatment algorithms vary among manufacturers, but all have the common feature of achieving tissue necrosis through the application of electromagnetic energy.

**Fig. 20.1** Computed tomography (CT) imaging in patient with hepatocellular carcinoma (HCC). **(a)** Axial contrast-enhanced CT scan of the liver obtained during the arterial phase of imaging demonstrates an enhancing mass in the left hepatic lobe consistent with HCC (*white arrow*). **(b)** Axial unenhanced CT scan of the liver at the time of radiofrequency ablation (RFA). *White arrows* indicate RFA electrodes within tumor. **(c)** Axial contrast-enhanced CT scan of the liver obtained during the arterial phase of imaging 1 month after CT-guided RFA. *Asterisk* indicates treated tumor. *White arrows* indicate the ablative margin extending beyond the tumor margins.

20.2.2 Microwave Ablation

MWA (▶ Fig. 20.2; ▶ Fig. 20.3) also uses electromagnetic energy to achieve tissue destruction. With MWA, needles that are inserted into the patient act as antennas that receive electromagnetic energy in the range of 900 to 2,450 MHz. ¹² In contrast to RF-induced agitation to induce heat, microwave energy in tissues results in rotation of water molecules within tissues. The rotational movement of water molecules generates frictional heat that then leads to coagulative necrosis. ¹³ Because an electrical circuit is not created within the patient as with RFA, MWA does not require the use of grounding pads. Currently available microwave devices do not employ internal cooling of antenna or perfusion options.

**Fig. 20.2** Microwave ablation. **(a)** Coronal gadolinium-enhanced magnetic resonance imaging (MRI) of the liver demonstrating cholangiocarcinoma in segment III (*arrow*). **(b)** Axial unenhanced computed tomography (CT) scan at time of microwave ablation (MWA) that demonstrates a 19-gauge Ultrathin CHIBA needle (*white arrow*) placed between liver edge (*) and transverse colon (#) for attempted for balloon displacement. **(c)** Axial unenhanced CT scan at the time of MWA demonstrating a 0.035-inch guidewire (*white arrow*) advanced through the 19-gauge Ultrathin CHIBA needle placed between liver edge (asterisk) and transverse colon (*asterisk*). **(d)** Axial unenhanced CT at the time of MWA that demonstrates a 12-mm angiographic balloon (*white arrow*) that was advanced over the 0.035-inch guidewire, separating the liver edge (*) and transverse colon (#). **(e)** Axial unenhanced CT scan of the liver demonstrating microwave antenna in tumor (*arrow*). **(f)** Coronal contrast-enhanced MRI of the liver obtained 1 month after microwave ablation that demonstrates zone of ablation that encompasses tumor (*arrow*).

**Fig. 20.3** Microwave ablation. **(a)** Axial gadolinium-enhanced magnetic resonance imaging (MRI) of the liver demonstrating hepatocellular carcinoma (HCC) in segment VIII (*white arrows*). **(b)** Axial computed tomography (CT) image of the liver at the time of microwave ablation. White arrow indicates 20-gauge needle used to instill 0.9% normal saline into the abdomen to displace the liver dome away from the diaphragm and lung. (c) Coronal and sagittal images of the liver at the time of microwave ablation demonstrating the microwave antennae in the lesion (*arrows*). Asterisk indicates fluid used to displace the liver away from the diaphragm and lung.

20.2.3 Cryoablation

Although not commonly used for liver tumor ablation, cryoablation is another thermal ablation option. In contrast to RFA and MWA, cryoablation achieves tissue necrosis by subjecting tissues to temperatures as low as −160 °C. This is accomplished by the circulation of high-pressure argon gas through the lumen of a cryoprobe. Through a thermodynamic process referred to as the Joule–Thomson effect, expansion of the argon gas within the cryoprobe results in tissue cooling. ¹⁴ When tissues are subjected to alternating freeze–thaw–freeze cycles, tissue destruction is achieved through different mechanisms. During freeze cycles, ice crystals form within the tissue; this increases osmotic pressure within tumor cells, which leads to cellular dehydration and disruption of cellular membranes, including those of intracellular organelles. ¹⁵ ^, ¹⁶ When tissue is subjected to the thaw cycle, the coalescence of ice crystals leads to cellular swelling and further disruption of cellular membranes. The ultimate result is tissue necrosis through cellular apoptosis. One of the limitations of cryoablation is a potentially devastating endotoxin-mediated post-treatment inflammatory response referred to as cryoshock, which is observed in approximately 1% of patients undergoing liver cryoablation and manifests as multi-organ failure and severe coagulopathy. ¹⁷ ^, ¹⁸ The cryoshock phenomenon may be associated with mortality rates as high as 18%. ¹⁸

20.2.4 Irreversible Electroporation

IRE (▶ Fig. 20.4; ▶ Fig. 20.5), one of the newest ablation devices available for liver ablation, is generally considered to be a non-thermal form of ablation that achieves tissue necrosis through the repetitive delivery of short-duration, high-voltage electrical pulses to tissues. ¹⁹ The exposure of tissues to high electrical fields leads to irreversible disruption of cellular membranes, causing tissue necrosis through apoptosis. Because of the strong electrical pulses required, general anesthesia is necessary to achieve complete neuromuscular blockage. Additionally, synchronization between the delivery of electrical energy and the cardiac cycle is necessary to minimize the risk of cardiac dysrhythmias. A potential benefit of the nonthermal nature of IRE is that it is believed to preserve blood vessels and bile ducts, thus enabling treatment near vital structures. ¹⁹ ^, ²⁰ ^, ²¹