A comparison of microwave ablation and bipolar radiofrequency ablation both with an internally cooled probe: Results in ex vivo and in vivo porcine livers

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Abstract

Purpose

The purpose of this study was to compare the effectiveness of microwave (MW) ablation and radiofrequency (RF) ablation using a single internally cooled probe in a hepatic porcine model.

Materials and methods

In the ex vivo experiment, MW ablations (n = 40) were performed with a 2450 MHz and 915 MHz needle antenna, respectively at 60 W, 70 W power settings. Bipolar RF ablations (n = 20) were performed with a 3-cm (T30) and 4-cm (T40) active tip needle electrodes, respectively at a rated power 30 W and 40 W according to automatically systematic power setting. In the in vivo experiment, the 2450 MHz and 915 MHz MW ablation both at 60 W and T30 bipolar RF ablation at 30 W were performed (n = 30). All of the application time were 10 min. Long-axis diameter (Dl), short-axis diameter (Ds), ratio of Ds/Dl, the temperature data 5 mm from the needle and the time of temperature 5 mm from the needle rising to 54 °C were measured.

Results

Both in ex vivo and in vivo models, Ds and Dl of 915 MHz MW ablations were significantly larger than all the RF ablations (P < 0.05); the Ds for all the 2450 MHz MW ablations were significantly larger than that of T30 RF ablations (P < 0.05). 2450 MHz MW and T30 RF ablation tended to produce more elliptical-shaped ablation zone. Tissue temperatures 5 mm from the needle were considerably higher with MW ablation, meanwhile MW ablation achieved significantly faster rate of temperature rising to 54 °C than RF ablation. For in vivo study after 10 min of ablation, the Ds and Dl of 2450 MHz MW, 915 MHz MW and Bipolar RF were 2.35 ± 0.75, 2.95 ± 0.32, 1.61 ± 0.33 and 3.86 ± 0.81, 5.79 ± 1.03, 3.21 ± 0.51, respectively. Highest tissue temperatures 5 mm from the needle were 80.07 ± 12.82 °C, 89.07 ± 3.52 °C and 65.56 ± 15.31 °C and the time of temperature rising to 54 °C were respectively 37.50 ± 7.62 s, 24.50 ± 4.09 s and 57.29 ± 23.24 s for three applicators.

Conclusion

MW ablation may have higher potential for complete destruction of liver tumors than RF ablation.

Introduction

Hepatic resection remains the reference standard in the treatment of malignant liver tumors, however, a large number of patients with liver cancers are not amenable to surgical therapy because of unfavorable anatomy, the presence of multiple tumors or poor hepatic reserve [1], [2]. Therefore, there is a need for a simple, affordable, and more precise technique which has a lower morbidity rate for destroying liver tumors. With the advantages of sparing more normal liver tissue, minimal invasion, favorable efficacy and low complication rates, microwave (MW) and radiofrequency (RF) ablations are the currently widely used thermal ablation techniques [3], [4], [5], [6], [7].

RF ablation is the widely used and studied ablative technique worldwide. In RF ablation, a high frequency alternating electrical current (375–500 kHz) is used to create ionic agitation, which produces frictional heat and heat conduction to achieve subsequent tissue necrosis [8], [9]. MW ablation is one of the most recent and exciting advances in the field of thermoablative technology, which uses electromagnetic energy to rapidly rotate adjacent polar water molecules. However, MW ablation shares several theoretical advantages over RF ablation in consistently higher intratumoral temperatures, larger ablation volumes, faster ablation times, less dependency on the electrical conductivities of tissue and energy delivery less limited by the exponential rising electrical impedances of tumor tissue [9], [10], [11], [12], [13], [14].

The survival benefit of hepatocellular carcinoma patients undergoing resection or percutaneous tumor ablation is directly dependent on tumor size [15], [16]. This reflects the need of radical tumor ablation including a safety margin, so numerous efforts have been made to increase the volume of coagulation necrosis. Recent technical advances—internally cooled probes in MW and RF ablation have facilitated remarkable progress in obtaining larger ablation zone [3], [17]. In addition, for MW ablation, 915 MHz microwave equipment is a newly developed instrument which can penetrate more deeply than commonly adopted 2450 MHz microwave and may yield larger ablation zone [18]. Meanwhile, for RF ablation, commercially available devices were improved in efficacy by changing to a bipolar RF system, which does not require grounding pads and the electrical energy is deployed in the target tissue only, thus systemic heating effects are avoided and larger ablation zone could be achieved than that of monopolar RF devices [19].

The purpose of our study was to compare the two relatively new single probe products: the cool-tip bipolar RF ablation system with automatically pulsed power mode and the cooled-shaft MW ablation system with 2450-MHz and 915-MHz applicators in their effects on the extent of tissue coagulation and temperature rising rate achieved both in the ex vivo and in vivo liver models.

Section snippets

Microwave system

The microwave unit (KY-2000, Kangyou Medical, China) consists of a microwave generator, a flexible coaxial cable, a water-pumping machine and a needle antenna. The generator is capable of producing 100 W of power at 2450 MHz and 915 MHz, respectively. The needle antenna has a diameter of 1.9 mm (15G), a 20 cm shaft with a coating to prevent tissue adhesion. A narrow radiating segment of 3 mm is embedded on the shaft, 11 mm away from the tip for 2450 MHz applicator and 22 mm for 915 MHz applicator (Fig. 1a

Evaluation of lesion size and shape

All coagulation zones were ellipsoidal. The undesired extension of coagulation along the needle shaft was not included. A deeper color of arrow-shaped charring around the shaft for MW mode than that of RF mode was observed (Fig. 2, Fig. 3). In the ex vivo experiments designed to compare the efficacy of the cooled-shaft microwave antennae with 60 W, 70 W of power at 2450 MHz, 915 MHz and the cool-tip bipolar radiofrequency probes with a 3-cm (T30) and 4-cm (T40) active tip (Table 1), the ablation

Discussion

RF and MW ablation employ a common agent for tumor destruction – heat, and they share several common advantages in treating liver tumors. They both allow flexible treatment approaches, including percutaneous, laparoscopic, or open surgical access, with convenient ultrasonographic (US) or computed tomographic (CT) guidance. Furthermore, because of sparing more normal liver tissue, treatments are generally well tolerated, even in patients with limited hepatic reserve. With the technical

Acknowledgment

Supported by two grants respectively from the National Scientific Foundation Committee of China (30825010) and from the Ministry of Health of the People's Republic of China (2008ZX10002-026).

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