2. Percutaneous ablation

Percutaneous ablation is a curative intent procedure reserved for patients with early stage disease HCC. It involves accessing the tumor percutaneously under ultrasound or CT guidance with probes. Three broad categories of percutaneous ablation exist: chemical, thermal, and non-thermal.

Chemical ablation induces cellular dehydration, protein denaturation, and blood vessel thrombosis causing coagulation necrosis via ethanol administration into the tumor. The ethanol has an unpredictable distribution in the surrounding tissue, leading to a high rate of tumor recurrence. This technique has largely been replaced by thermal ablation though still finds its uses where thermal ablation is risky such as in tumors in close proximity to vital organs.

Thermal ablation can be classified into three modalities: radiofrequency, microwave ablation, and cryoablation.

Radiofrequency ablation (RFA) is the most commonly performed procedure for hepatic tumors. There is an alternating electrical current within the device that causes agitated ions to generate heat and induce coagulative necrosis. This technique is reserved for tumors less than 3 cm due to the techniques poor conductive heating over greater distances.

Microwave ablation (MWA) uses electromagnetic waves to induce an alternating electrical field that produces heat. This has the ability to reach higher temperatures to overcome perfusionmediated tissue cooling when compared to RFA, making ablation of larger tumors possible along with faster ablation times. This technique is preferred for tumors near major vessels, such as the inferior vena cava or main hepatic veins, due to the attenuated heat sink effect [6].

Cryoablation consists of pumping high-pressure argon into a probe, which escapes at the very tip and causes rapid expansion of the gas leading to intense cooling and formation of an ice ball around the needle tip. This causes intracellular ice crystals and disrupts the cell membrane and cellular metabolism. The low temperature also causes vascular thrombosis. Multiple cycles of freezing and thawing are performed. The advantage of cryoablation is the ability to see the ice ball during the procedure on CT, which allows the physician to determine adequate ablation. The technique was historically not used because of reported cryoshock. Recent reports have demonstrated that hepatic cryoablation is feasible and safe.

Non-thermal ablation is performed through irreversible electroporation (IRE). This technique involves high voltage electrical impulses between parallel electrodes. The high voltage causes large pores in the cellular membranes leading to apoptosis. The advantage of IRE is the ability to ablate tumors that are in close proximity to vital structures such as the portal veins or bile ducts. IRE is a technically difficult procedure, as it requires multiple devices to be inserted in a near-perfect parallel conformation.
