Image-Guided Tumor Ablation: Basic Principles

Chemical Ablation

Chemical ablation is achieved with image-guided instillation of a chemical agent. The most common chemical agent used for tumor ablation is ethanol, although other agents such as acetic acid have been used. Percutaneous ethanol injection (PEI) has been shown to be a safe, inexpensive, and effective treatment for small (3-5 cm) hepatocellular carcinomas (HCCs). Ethanol works by protein denaturation, leading to coagulative necrosis, thrombosis of small vessels, and formation of fibrotic and granulomatous tissue. It is effective for encapsulated tumors, such as HCC, where surrounding tissue is made firm by underlying disease, the cirrhotic liver. Injected alcohol diffuses throughout the tumor but is prevented from diffusing into the liver by the tumor capsule and surrounding cirrhotic parenchyma. PEI is less effective for metastases because they are often firm tumors surrounded by normal tissue. PEI is performed by placing a small (19-gauge) needle or similar sized lateral side-hole needle into the center of the untreated portion of tumor. Absolute ethanol (96%) is injected during continuous sonographic monitoring. Alcohol droplets appear as a hyperechoic cloud. The volume injected is based on the tumor size, considered as a sphere, using the equation:

Generally, 10-20 mL per injection per lesion is given. Injections are repeated as needed on a weekly basis until the calculated volume is achieved. Multiple needle tracts are to be avoided to decrease the risk of alcohol leaking into the peritoneal cavity, which can be very painful. Although computed tomography (CT) can be used for imaging guidance, ultrasound is the preferred modality for performing PEI in the liver due to the ease of use. Ethanol injection for HCC results in complete tumor necrosis in 70%-80% of cases ( Fig. 25-1 ). Cure rates equal those of surgery in selected patients. Results for metastases are less favorable, with complete necrosis rates closer to 50%. PEI has not gained widespread popularity in the United States, probably because multiple treatments are needed and because it has decreased efficacy in treating colorectal metastasis. As a result, PEI is less effective than other treatments for colorectal metastasis.

Radiofrequency Ablation

RFA works by transforming RF energy into heat, which is deposited into a tumor. An RF generator in the range of 60-250 W is commonly used as the source. After grounding pads are placed on the patient and connected to the power unit, applicators (electrodes) are then placed into the tumor and connected to the RF generator. Alternating current applied to the electrode passes through the patient to the grounding pads. The alternating nature of the current causes ionic agitation of the molecules surrounding the uninsulated electrode tip, ultimately leading to the production of frictional heating. As tissue temperatures increase to the range of 60°C-100°C, irreversible cellular damage referred to as coagulation necrosis occurs instantly. Lower temperatures (50°C-60°C) may induce coagulation in minutes. Temperatures less than 50°C do not reliably induce necrosis. Temperatures greater than 100°C are generally avoided because, as tissues boil, gas is produced, which acts as an insulator and significantly impedes further diffusion of heat energy into the surrounding tumor.

In general, there are three types of RF applicators available: single straight needles, cluster straight needles, and multitined expandable electrodes. The applicator diameters are typically 14-17.5 gauge. The major difference between the different RF applicators is the size of ablation zone possible during a single treatment session. Maximum achievable uniform ablation zone with a single straight electrode is approximately 1.6 cm in vivo. A method has been developed to increase ablation zone size through the use of an internally cooled applicator where chilled perfusate flows through the applicator to decrease temperatures at the tip and allow for a larger ablation zone by preventing the formation of char and vaporization. To further increase ablation zones, other applicator styles have been developed. Cluster electrodes consist of three single electrode needles in one applicator. The diameter of ablation using these devices is approximately 3 cm. The umbrella-style expandable array electrode consists of multiple individual tines that are deployed within the tumor, creating 5- to 6-cm zones of ablation. Some of these electrodes also employ instillation of interstitial saline from the tips of the applicator to spread thermal energy more efficiently while increasing tissue ionicity, which allows for greater flow of current. Finally, the simultaneous use of multiple single probe applicators placed at different locations in the tumor can be used to treat larger neoplasms ( Fig. 25-2 ).

In addition to new applicator technology, new generator designs have allowed more efficient production of larger ablation zones ( Box 25-2 ). Energy pulsing has been developed to augment overall energy transfer while avoiding vaporization and charring. The rapid alternation of low- and high-energy deposition allows preferential cooling of tissue nearest the probe while maintaining continual heating of more distant tissue. Combining energy pulsing with an internally cooled RF applicator synergistically produces greater tissue ablation than either method alone. Connecting multiple probes to a generator (switch box technology) can also be used. The most recent innovation that will likely have a large impact on thermal ablation technology is the development of bipolar RFA, wherein an ablation zone can be created between two electrodes. Bipolar percutaneous RF electrodes are not yet commercially available in the United States.

The length of treatment time to create tissue necrosis varies depending on which type of applicator is used and the size of the tumor being treated. Most single treatments are 10-16 minutes in soft tissue. The goal of treatment is to gain a 5- to 10-mm treatment margin beyond the suspected tumor borders identified on preprocedural imaging. To accomplish this goal, it may be necessary to plan multiple overlapping ablations following intervening applicator repositioning, which can significantly lengthen the total procedure time ( Fig. 25-3 ).

One of the principal advantages of RFA is that it is a minimally invasive outpatient procedure with patients often being treated and discharged on the same day ( Table 25-1 ). In addition to being preferred by patients over a prolonged hospital stay, this leads to significant cost savings to the health care system. There are fewer complications when compared with major surgery and the use of general anesthesia. RFA has an intrinsic cautery effect which may decrease bleeding complications, a major consideration in coagulopathic patients. RFA has also been shown to be synergistic with traditional treatments, such as radiation therapy, which can improve patient survival.

There are two primary factors inherent to RFA which may impede complete tumor necrosis ( Box 25-3 ). Tissue boiling and charring occurs with temperatures greater than 100°C and acts as an electrical insulator, limiting heat conduction to tumor beyond the area of charring. The ability to obtain adequate thermal heating may also be limited by adjacent large blood vessels. This is referred to as the heat sink effect , wherein rapidly flowing blood carries heat away from the treatment zone. Methods to counteract the heat sink effect have been developed, including preprocedural hepatic artery embolization or temporary occlusion of the hepatic artery and portal vein during the procedure (Pringle maneuver). Other methods showing promise to increase thermal necrosis through synergistic effects include concurrent chemoembolization, external beam radiation, and brachytherapy.