Bimodal Electric Tissue Ablation-Modified Radiofrequency Ablation with a Le Veen Electrode in a Pig Model

Radiofrequency ablation (RFA) is a method of treating non-resectable liver tumors by use of a high-frequency alternating electrical current. Concerns have been raised as the local recurrence rates following treatment have been reported to be as high as 47%. The size of the ablation is limited by charring of adjacent tissues. It is hypothesized that by hydrating the liver, we can reduce charring, thus producing larger ablations, and that this can be achieved by addition of a direct electrical current to the electrical circuit.

Using a pig model, standard RFA control ablations were created in the left lobe of the liver. Ablations using the modified circuit were created in the right lobe. At the end of the procedure, the pig was killed by lethal injection and the liver harvested. From the explanted liver, the diameter of each ablation was measured and the modified ablations were compared with controls using restricted maximum likelihood variance analysis.

From 4 pigs, 14 controls and 12 modified ablations were produced. The mean diameter of the controls was 27.78 mm (± SE 3.37 mm). The mean diameter of the modified ablation was 49.55 mm (± SE 3.46 mm), which was significantly larger than the controls (P < 0.001).

This study has shown that by modification of the standard RFA circuit with the addition of a direct electrical current, significantly larger ablations can be produced. By using this technique, the number of ablations required to treat one tumor would be less and it is anticipated this could reduce the rate of local recurrence.

Introduction

It has been well established that patients with hepatomas or colorectal liver metastases have their best chance of long term survival where surgical resection is possible [1, 2]. Unfortunately, only 20% of patients are amenable to this form of treatment [2]. This has resulted in the development of ablative techniques that offer these patients an alternative treatment. Of these, radiofrequency ablation is currently one of the most popular. It works by delivering a high frequency, alternating electrical current through the tumor that causes oscillation of ions as they follow the alternating current [3]. This in turn leads to frictional heating of the tumor cells and this heating of the tumor induces coagulative necrosis [3, 4]. Previous reports have shown it to be safe and efficacious and it can be performed percutaneously or surgically [5, 6, 7, 8].

Reports are now emerging which suggest that the rates of local recurrence after treatment with radiofrequency ablation are unacceptably high and these have been reported to be as high as 47% [9, 10, 11, 12, 13, 14]. There are many associated factors with local recurrence but the most profound risk factor for local recurrence is the size of the tumor being ablated [9]. Tumors greater than 3 cm in diameter have been shown to have much greater rates of local recurrence [9, 12, 15, 16].

In a trial of four commonly used needle electrodes, the authors concluded that theoretically, the largest tumor that is able to be ablated with a 1 cm safety margin is 3 cm in diameter [17]. Larger tumors require multiple ablations to treat the one tumor [18]. This has the potential to lead to incomplete ablation of the tumor treated and, as a result, can lead to local recurrence [19].

The limiting factor for all electrodes is tissue desiccation and localized charring that occurs around the electrode itself [18]. Charring prevents further conduction of the electrical current [18]. It is hypothesized that if desiccation and charring could be prevented or at least delayed, the size of the ablation zone could be increased. Experiments with preinjecting the tissue to be ablated with hypertonic saline and the development of perfusion needle electrodes that allow infusion of saline through them into the tissues have shown modest improvements in the size of the ablative zones [20, 21].

Previous experimental work has shown that direct electrical currents have water attracted to the cathode or negative electrode [22, 23, 24]. It was therefore hypothesized that if a direct current could be combined with an alternating radiofrequency current, then potentially water could be drawn into the region where the electrode is performing an ablation and charring could be prevented. Consequently, a bimodal electrical circuit that combined the two electrical modalities was created and the hypothesis was tested in a pig model using a standard 35 mm multitine Le Veen electrode.