**1. Introduction**

Hepatocellular carcinoma (HCC) occurs predominantly in patients with chronic liver disease and limited hepatic functional reserve. Therefore, surgical removal of HCC is feasible only in 15–20% of cases and non-surgical modalities play a relatively important role in HCC manage‐ ment. There are several non-surgical methods; however, ablation therapy has become a mainstay in particular for early-stage HCC because of its superb local control capability and high safety profile [1].

Ablation modalities currently available include percutaneous ethanol injection (PEI), radio‐ frequency ablation (RFA), microwave ablation (MWA), cryoablation, laser ablation (LA), and irreversible electroporation.

PEI was one of the first effective ablative techniques to be widely adopted for the treatment of small HCC. Ethanol causes dehydration and subsequently necrosis [2]. As far as PEIs are concerned, the 5-year survival rates in patients with HCCs measuring less than 3 cm range from 47% to 65% and in a recent study of 685 Japanese patients, the 5-, 10-, and 20-year survival rates—49%, 18%, and 7.2%, respectively, were similar to those observed in patients with cirrhosis who did not have HCC [3]. PEI maintains the advantage of allowing the treatment of tumors near sensitive organs and tissues; however the applicability of PEI in larger HCC has been shown to produce incomplete necrosis mainly due to the heterogeneous consistency of these tumors [4]. Moreover, PEI is of little benefit in infiltrating HCC or in metastases.

Current limitations of PEI can be overcome with RFA. Radiofrequency current induces ionic agitation that in turn results in heating. The superiority of RFA to PEI in prolonging patient survival has been shown in a randomized controlled trial [5]. The 3-year survival rates were 48%–67% following PEI and 63%–81% following RFA. Moreover, Chen et al. performed a randomized control trial between RFA and hepatectomy in patients who had HCC ≤ 5 cm and found the same overall and recurrence-free survival between the two patient groups [6]. A major disadvantage of RFA is mainly the difficulty to target HCC located in "problem" areas of the liver, for instance tumors adjacent to blood vessels, settings in which the diffusion of heat is less advisable [7]. This phenomenon is also known as the heat-sink effect.

In the last two years, MWA has gained acceptance as a favorable alternative and in some cases a preferred choice of ablation alternative. In MWA, the mechanism of heat generation is based on rapid frictional movement of water molecules in high-frequency (900–2500 MHz) electro‐ magnetic field. The tissue's polar molecules are forced to continuously realign with the oscillating electric field, increasing their kinetic energy, and hence the temperature of the tissue [8]. Unlike RFA, microwaves are capable of effectively heating and propagating through many types of tissue, even those with low electrical conductivity, high impedance, or low thermal conductivity. Moreover, they can readily penetrate through the charred or desiccated tissues that tend to build up around all hyperthermic ablation applicators, resulting in limited power delivery for non-microwave energy systems [9].

MWA has several theoretical advantages, including greater penetration of energy into tissues resulting in a larger area of ablation, higher intratumoral temperatures, faster ablation times, less susceptibility to the heat-sink effect, no need for grounding pads, and low sensitivity to local variation in tissue physiological properties [10]. In some studies, MWA has been compared with RFA for the treatment of HCCs of different sizes (< 3 cm and < 5 cm) and despite the theoretical advantages of MWA, no significant differences have been observed in either setting with regard to the completeness of tumor necrosis, disease recurrence, survival, or complication rates [11, 12].

**1. Introduction**

224 Recent Advances in Liver Diseases and Surgery

high safety profile [1].

irreversible electroporation.

Hepatocellular carcinoma (HCC) occurs predominantly in patients with chronic liver disease and limited hepatic functional reserve. Therefore, surgical removal of HCC is feasible only in 15–20% of cases and non-surgical modalities play a relatively important role in HCC manage‐ ment. There are several non-surgical methods; however, ablation therapy has become a mainstay in particular for early-stage HCC because of its superb local control capability and

Ablation modalities currently available include percutaneous ethanol injection (PEI), radio‐ frequency ablation (RFA), microwave ablation (MWA), cryoablation, laser ablation (LA), and

PEI was one of the first effective ablative techniques to be widely adopted for the treatment of small HCC. Ethanol causes dehydration and subsequently necrosis [2]. As far as PEIs are concerned, the 5-year survival rates in patients with HCCs measuring less than 3 cm range from 47% to 65% and in a recent study of 685 Japanese patients, the 5-, 10-, and 20-year survival rates—49%, 18%, and 7.2%, respectively, were similar to those observed in patients with cirrhosis who did not have HCC [3]. PEI maintains the advantage of allowing the treatment of tumors near sensitive organs and tissues; however the applicability of PEI in larger HCC has been shown to produce incomplete necrosis mainly due to the heterogeneous consistency of these tumors [4]. Moreover, PEI is of little benefit in infiltrating HCC or in metastases.

Current limitations of PEI can be overcome with RFA. Radiofrequency current induces ionic agitation that in turn results in heating. The superiority of RFA to PEI in prolonging patient survival has been shown in a randomized controlled trial [5]. The 3-year survival rates were 48%–67% following PEI and 63%–81% following RFA. Moreover, Chen et al. performed a randomized control trial between RFA and hepatectomy in patients who had HCC ≤ 5 cm and found the same overall and recurrence-free survival between the two patient groups [6]. A major disadvantage of RFA is mainly the difficulty to target HCC located in "problem" areas of the liver, for instance tumors adjacent to blood vessels, settings in which the diffusion of

In the last two years, MWA has gained acceptance as a favorable alternative and in some cases a preferred choice of ablation alternative. In MWA, the mechanism of heat generation is based on rapid frictional movement of water molecules in high-frequency (900–2500 MHz) electro‐ magnetic field. The tissue's polar molecules are forced to continuously realign with the oscillating electric field, increasing their kinetic energy, and hence the temperature of the tissue [8]. Unlike RFA, microwaves are capable of effectively heating and propagating through many types of tissue, even those with low electrical conductivity, high impedance, or low thermal conductivity. Moreover, they can readily penetrate through the charred or desiccated tissues that tend to build up around all hyperthermic ablation applicators, resulting in limited power

MWA has several theoretical advantages, including greater penetration of energy into tissues resulting in a larger area of ablation, higher intratumoral temperatures, faster ablation times,

heat is less advisable [7]. This phenomenon is also known as the heat-sink effect.

delivery for non-microwave energy systems [9].

Laser thermal ablation is another technique that has been associated with high rates of complete necrosis (an average of 95%) in HCCs measuring less than 3 cm [13]. Unfortunately, there are only a few centers that use this type of ablation and therefore the amount of data is limited. Moreover, it is based on sophisticated technology, requires much more substantial operator experience, and involves placement of multiple optical fibers within the neoplastic lesion according to a programmed spatial distribution scheme [14]. Although more expensive to set up and support than RF, LAs are a little more predictable.

To date, there are only a few studies comparing LA with RFA in hepatocellular carcinoma. In their randomized controlled prospective study, Ferrary et al. [15] treated 81 cirrhotic patients with 95 biopsy proven ≤ 4 cm HCCs comparing LA with RF ablation. Two matched groups were randomized to US-guided RF or LA under general anesthesia. The authors adopted multiple fiber techniques using 5 W per fiber delivering a maximum of 1800 J per fiber per single illumination. They reported no significant overall differences in survival rates between the two methods with cumulative rates of 91.8%, 59%, and 28.4% at 1, 3, and 5 years, respec‐ tively. However, they demonstrated a statistically significant higher survival rate for RF over LA for Child A patients (p=0.9966) and nodules ≤ 2.5 cm (p=0.01181). In a randomized prospective trial in a single center with three years of follow-up, the authors treated 140 patients with 157 biopsy-proven HCCs to compare LA and RFA (70 patients with 77 nodules and 70 patients with 80 nodules, respectively). Median follow-up in RFA and LA groups was 21 and 22.5 months, respectively. Complete response was observed in 97.2% and in 95.8% of RFA and LA group patients, respectively. Median time to tumor recurrence was 25.6 and 37.8 months in RFA and in LA groups, respectively (P = 0.129). Estimated probability of survival at 1, 2, and 3 years was 94%, 88%, and 66% in the RFA group and 94%, 81%, and 59% in the LA group, respectively (p = 0.693). No major complications or significant treatment-related morbidity were observed in both groups. The authors concluded that LA was non-inferior to RFA either in obtaining the complete ablation of HCC nodules or in the long-term outcome [16].

Another type of percutaneous tumor ablation is represented by cryoablation (CRYO). Percu‐ taneous CRYO is a promising local ablation technique, which is believed to ablate cancer cells by several mechanisms including intracellular ice formation, solute-solvent shifts that cause cell dehydration and rupture, and small-vessel obliteration with resulting hypoxia. Perhaps, the main advantage of CRYO relative to RFA is its precise intraprocedural monitoring of iceball formation via various imaging techniques [17]. There are a few studies comparing CRYO with other types of tumor ablation techniques; however Wang C et al. report the results of a randomized, controlled multicenter trial comparing percutaneous CRYO and RFA in patients with cirrhosis, Child-Pugh class A or B liver function, and 1-2 HCC nodules measuring ≤ 4 cm. The primary endpoints were local tumor progression at 3 years and safety. As for the former, CRYO proved to be significantly superior to RFA in patients with larger tumors (i.e., those that were 3.1 to 4 cm in diameter). The two methods were not significantly different in terms of complication rates, which were less than 4% in both groups, or survival (overall and tumorfree) at 1, 3, and 5 years [18]. The superiority of CRYO over RFA in the larger tumors suggests that CRYO has the ability to necrotize larger volumes of tissue, hence increasing the chances of ablating microsatellite lesions that are always possible with lesions of this size.

Irreversible electroporation (IRE) is a new treatment method with certain advantages over the existing ablative techniques that have gained widespread attention. With IRE, cell death is induced with electric energy. Under image guidance electrodes are placed around the tumor and through multiple and short high-voltage electric pulses, the existing cell membrane potential is disturbed. As a consequence, nanoscale defects appear in the lipid bilayer of the cell membrane. Although IRE is believed to destroy all cells within the ablation zone effec‐ tively, the non-thermal nature of IRE results in relative preservation of the extracellular matrix. Hence, the structural integrity of vessels and bile ducts remain intact. Moreover, IRE is not affected by the heat-sink effect [19]. All these advantages suggest that IRE may be more suitable for the treatment of HCCs ineligible for surgical resections or thermal ablation because of unfavorable location.

Currently, there are no published clinical trials for the treatment of hepatic tumors using IRE. In a recent review, Scheffer J. et al. included 221 patients with 325 lesions in different organs: 227 hepatic tumors, 70 unresectable pancreatic adenocarcinoma, 17 renal tumors, 8 pulmonary tumors, 1 presacral tumor, and 2 lymph nodes. Most of the patients were treated by IRE owing to tumor proximity to bile ducts, bronchi, renal pelvis, presacral neural plexus or large vessels, making the tumor unsuitable for surgery or thermal ablation. They concluded that IRE is a safe procedure with a promising early efficacy on smaller hepatic tumors near vascular structures and portal triads, with reported ablation success reaching 90%, but rapidly decreas‐ ing with increasing tumor size [20].

Tremendous efforts have been made in the last decades to improve the currently available techniques. However, given that there is not a single method available that meets all the requirements of an ideal ablation system, based on what has been discussed above and on data from the vast literature available, we can reasonably draw some conclusions.

Firstly, all differences between the techniques in terms of results are modest. Secondly, one technique may be more difficult than another and more rapid than another. Thirdly, each technique has its own major advantages and disadvantages. Finally, the rate of recurrence is still high after tumor ablation despite the major advances in tumor ablation devices, optic fibers, and improved imaging guidance. A major limitation in its overall effectiveness is due to the difficulties of heating large tumors. Small regions of viable tumor may still remain even after apparently good tumor ablation. Moreover, simple heating techniques have trouble discriminating between tumors and surrounding healthy tissues leading to many side effects. In order to overcome these major limitations, numerous groups are investigating the use of different types of nanoparticles, including carbon nanotubes, gold nanoparticles, and magnetic nanoparticles, placed/ introduced within tumor tissues to facilitate localized heating.
