Research and development of a new RF-assisted device for bloodless rapid transection of the liver: computational modeling and in vivo experiments

An in silico assessment on the potential of using saline infusion to overcome non-confluent coagulation zone during two-probe, no-touch bipolar radiofrequency ablation of liver cancer

No-touch bipolar radiofrequency ablation (bRFA) is known to produce incomplete tumour ablation with a 'butterfly-shaped' coagulation zone when the interelectrode distance exceeds a certain threshold. Although non-confluent coagulation zone can be avoided by not implementing the no-touch mode, doing so exposes the patient to the risk of tumour track seeding. The present study investigates if prior infusion of saline into the tissue can overcome the issues of non-confluent or butterfly-shaped coagulation. A computational modelling approach based on the finite element method was carried out. A two-compartment model comprising the tumour that is surrounded by healthy liver tissue was developed. Three cases were considered; i) saline infusion into the tumour centre; ii) one-sided saline infusion outside the tumour; and iii) two-sided saline infusion outside the tumour. For each case, three different saline volumes were considered, i.e. 6, 14 and 22 ml. Saline concentration was set to 15% w/v. Numerical results showed that saline infusion into the tumour centre can overcome the butterfly-shaped coagulation only if the infusion volume is sufficient. On the other hand, one-sided infusion outside the tumour did not overcome this. Two-sided infusion outside the tumour produced confluent coagulation zone with the largest volume. Results obtained from the present study suggest that saline infusion, when carried out correctly, can be used to effectively eradicate liver cancer. This presents a practical solution to address non-confluent coagulation zone typical of that during two-probe bRFA treatment.

How does saline backflow affect the treatment of saline-infused radiofrequency ablation?

BACKGROUND AND OBJECTIVE

Saline infusion is applied together with radiofrequency ablation (RFA) to enlarge the ablation zone. However, one of the issues with saline-infused RFA is backflow, which spreads saline along the insertion track. This raises the concern of not only thermally ablating the tissue within the backflow region, but also the loss of saline from the targeted tissue, which may affect the treatment efficacy.

METHODS

In the present study, 2D axisymmetric models were developed to investigate how saline backflow influence saline-infused RFA and whether the aforementioned concerns are warranted. Saline-infused RFA was described using the dual porosity-Joule heating model. The hydrodynamics of backflow was described using Poiseuille law by assuming the flow to be similar to that in a thin annulus. Backflow lengths of 3, 4.5, 6 and 9 cm were considered.

RESULTS

Results showed that there is no concern of thermally ablating the tissue in the backflow region. This is due to the Joule heating being inversely proportional to distance from the electrode to the fourth power. Results also indicated that larger backflow lengths led to larger growth of thermal damage along the backflow region and greater decrease in coagulation volume. Hence, backflow needs to be controlled to ensure an effective treatment of saline-infused RFA.

CONCLUSIONS

There is no risk of ablating tissues around the needle insertion track due to backflow. Instead, the risk of underablation as a result of the loss of saline due to backflow was found to be of greater concern.

Unidirectional ablation minimizes unwanted thermal damage and promotes better thermal ablation efficacy in time-based switching bipolar radiofrequency ablation

Switching bipolar radiofrequency ablation (bRFA) is a thermal treatment modality used for liver cancer treatment that is capable of producing larger, more confluent and more regular thermal coagulation. When implemented in the no-touch mode, switching bRFA can prevent tumour track seeding; a medical phenomenon defined by the deposition of cancer cells along the insertion track. Nevertheless, the no-touch mode was found to yield significant unwanted thermal damage as a result of the electrodes' position outside the tumour. It is postulated that the unwanted thermal damage can be minimized if ablation can be directed such that it focuses only within the tumour domain. As it turns out, this can be achieved by partially insulating the active tip of the RF electrodes such that electric current flows in and out of the tissue only through the non-insulated section of the electrode. This concept is known as unidirectional ablation and has been shown to produce the desired effect in monopolar RFA. In this paper, computational models based on a well-established mathematical framework for modelling RFA was developed to investigate if unidirectional ablation can minimize unwanted thermal damage during time-based switching bRFA. From the numerical results, unidirectional ablation was shown to produce treatment efficacy of nearly 100%, while at the same time, minimizing the amount of unwanted thermal damage. Nevertheless, this effect was observed only when the switch interval of the time-based protocol was set to 50 s. An extended switch interval negated the benefits of unidirectional ablation.

The impact of additional margin coagulation with radiofrequency in liver resections with subcentimetric margin: can we improve the oncological results? A propensity score matching study

BACKGROUND

Whereas the usefulness of radiofrequency (RF) energy as haemostatic method in liver surgery has become well established in the last decades, its intentional application on resection margins with the aim of reducing local recurrence is still debatable. Our goal was to compare the impact of an additional application of RF energy on the top of the resection surface, namely additional margin coagulation (AMC), on local recurrence (LR) when subjected to a subcentimeter margin.

METHODS

We retrospectively analyzed 185 patients out of a whole cohort of 283 patients who underwent radical hepatic resection with subcentimetric margin. After propensity score adjustment, patients were classified into two balanced groups according to whether RF was applied or not.

RESULTS

No significant differences were observed within groups in baseline characteristics after PSM adjustment. The LR rate was significantly higher in the Control than AMC Group: 12 patients (14.5%) vs. 4 patients (4.8%) (p = 0.039). The estimated 1, 3, and 5-year LR-free survival rates of patients in the Control and AMC Group were: 93.5%, 86.0%, 81.0% and 98.8%, 97.2%, 91.9%, respectively (p = 0.049). Univariate Cox analyses indicated that the use of the RF applicator was significantly associated with lower LR (HR = 0.29, 95% confidence interval 0.093-0.906, p = 0.033). The Control Group showed smaller coagulation widths than the AMC group (p < 0.001).

CONCLUSIONS

An additional application of RF on the top of the resection surface is associated with less local hepatic recurrence than the use of conventional techniques.

Comparisons between impedance-based and time-based switching bipolar radiofrequency ablation for the treatment of liver cancer

Switching bipolar radiofrequency ablation (bRFA) is a cancer treatment technique that activates multiple pairs of electrodes alternately based on a predefined criterion. Various criteria can be used to trigger the switch, such as time (ablation duration) and tissue impedance. In a recent study on time-based switching bRFA, it was determined that a shorter switch interval could produce better treatment outcome than when a longer switch interval was used, which reduces tissue charring and roll-off induced cooling. In this study, it was hypothesized that a more efficacious bRFA treatment can be attained by employing impedance-based switching. This is because ablation per pair can be maximized since there will be no interruption to RF energy delivery until roll-off occurs. This was investigated using a two-compartment 3D computational model. Results showed that impedance-based switching bRFA outperformed time-based switching when the switch interval of the latter is 100 s or higher. When compared to the time-based switching with switch interval of 50 s, the impedance-based model is inferior. It remains to be investigated whether the impedance-based protocol is better than the time-based protocol for a switch interval of 50 s due to the inverse relationship between ablation and treatment efficacies. It was suggested that the choice of impedance-based or time-based switching could ultimately be patient-dependent.

Bipolar radiofrequency ablation treatment of liver cancer employing monopolar needles: A comprehensive investigation on the efficacy of time-based switching

Radiofrequency ablation (RFA) is a thermal ablative treatment method that is commonly used to treat liver cancer. However, the thermal coagulation zone generated using the conventional RFA system can only successfully treat tumours up to 3 cm in diameter. Switching bipolar RFA has been proposed as a way to increase the thermal coagulation zone. Presently, the understanding of the underlying thermal processes that takes place during switching bipolar RFA remains limited. Hence, the objective of this study is to provide a comprehensive understanding on the thermal ablative effects of time-based switching bipolar RFA on liver tissue. Five switch intervals, namely 50, 100, 150, 200 and 300 s were investigated using a two-compartment 3D finite element model. The study was performed using two pairs of RF electrodes in a four-probe configuration, where the electrodes were alternated based on their respective switch interval. The physics employed in the present study were verified against experimental data from the literature. Results obtained show that using a shorter switch interval can improve the homogeneity of temperature distribution within the tissue and increase the rate of temperature rise by delaying the occurrence of roll-off. The coagulation volume obtained was the largest using switch interval of 50 s, followed by 100, 150, 200 and 300 s. The present study demonstrated that the transient thermal response of switching bipolar RFA can be improved by using shorter switch intervals.

Role of saline concentration during saline-infused radiofrequency ablation: Observation of secondary Joule heating along the saline-tissue interface

Infusion of saline prior to radiofrequency ablation (RFA) is known to enlarge the thermal coagulation zone. The abundance of ions in saline elevate the electrical conductivity of the saline-saturated region. This promotes greater electric current flow inside the tissue, which increases the amount of RF energy deposition and subsequently enlarges the coagulation zone. In theory, infusion of higher concentration of saline should lead to larger coagulation zone due to the greater number of ions. Nevertheless, existing studies on the effects of concentration on saline-infused RFA have been conflicting, with the exact role of saline concentration yet to be fully elucidated. In this paper, computational models of saline-infused RFA were developed to investigate the role of saline concentration on the outcome of saline-infused RFA. The elevation in tissue electrical conductivity was modelled using the microscopic mixture model, while RFA was modelled using the coupled dual porosity-Joule heating model. Results obtained indicated that the presence of a concentration threshold to which no further elevation in tissue electrical conductivity and enlargement in thermal coagulation can occur. This threshold was determined to be at 15% NaCl. Analysis of the Joule heating distribution revealed the presence of a secondary Joule heating site located along the interface between wet and dry tissue. This secondary Joule heating was responsible for the enlargement in coagulation volume and its rapid growth phase during ablation.

Thermal and thermal damage responses during switching bipolar radiofrequency ablation employing bipolar needles: A computational study on the effects of different electrode configuration, input voltage and ablation duration

Recent studies have demonstrated the effectiveness of switching bipolar radiofrequency ablation (bRFA) in treating liver cancer. Nevertheless, the clinical use of the treatment remains less common than conventional monopolar RFA - likely due to the lack of understanding of how the tissues respond thermally to the switching effect. The problem is exacerbated by the numerous possible switching combinations when bRFA is performed using bipolar needles, thus making theoretical deduction and experimental studies difficult. This article addresses this issue via computational modelling by examining if significant variation in the treatment outcome exists amongst six different electrode configurations defined by the X-, C-, U-, N-, Z- and O-models. Results indicated that the tissue thermal and thermal damage responses varied depending on the electrode configuration and the operating conditions (input voltage and ablation duration). For a spherical tumour, 30 mm in diameter, complete ablation could not be attained in all configurations with 70 V input voltage and 5 minutes ablation duration. Increasing the input voltage to 90 V enlarged the coagulation zone in the X-model only. With the other configurations, extending the ablation duration to 10 minutes was found to be the better at enlarging the coagulation zone.

Shape-shifting thermal coagulation zone during saline-infused radiofrequency ablation: A computational study on the effects of different infusion location

BACKGROUND AND OBJECTIVE

The majority of the studies on radiofrequency ablation (RFA) have focused on enlarging the size of the coagulation zone. An aspect that is crucial but often overlooked is the shape of the coagulation zone. The shape is crucial because the majority of tumours are irregularly-shaped. In this paper, the ability to manipulate the shape of the coagulation zone following saline-infused RFA by altering the location of saline infusion is explored.

METHODS

A 3D model of the liver tissue was developed. Saline infusion was described using the dual porosity model, while RFA was described using the electrostatic and bioheat transfer equations. Three infusion locations were investigated, namely at the proximal end, the middle and the distal end of the electrode. Investigations were carried out numerically using the finite element method.

RESULTS

Results indicated that greater thermal coagulation was found in the region of tissue occupied by the saline bolus. Infusion at the middle of the electrode led to the largest coagulation volume followed by infusion at the proximal and distal ends. It was also found that the ability to delay roll-off, as commonly associated with saline-infused RFA, was true only for the case when infusion is carried out at the middle. When infused at the proximal and distal ends, the occurrence of roll-off was advanced. This may be due to the rapid and more intense heating experienced by the tissue when infusion is carried out at the electrode ends where Joule heating is dominant.

CONCLUSION

Altering the location of saline infusion can influence the shape of the coagulation zone following saline-infused RFA. The ability to 'shift' the coagulation zone to a desired location opens up great opportunities for the development of more precise saline-infused RFA treatment that targets specific regions within the tissue.

Computer modeling and in vitro experimental study of water-cooled microwave ablation array

INTRODUCTION

Microwaves (MWs) quickly deliver relatively high temperatures into tumors and cover a large ablation zone. We present a research protocol for using water-cooled double-needle MW ablation arrays for tumor ablation here.

MATERIAL AND METHODS

Our research program includes computer modeling, tissue-mimicking phantom experiments, and in vitro swine liver experiments. The computer modeling is based on the finite element method (FEM) to evaluate ablation temperature distributions. In tissue-mimicking phantom and in vitro swine liver ablation experiments, the performances of the new device and the single-needle MW device currently used in clinical practice are compared.

RESULTS

FEM shows that the maximum transverse ablation diameter (MTAD) is 4.2 cm at 100 W output and 300 s (assessed at the 50 °C isotherm). In the tissue-mimicking phantom, the MTDA is 2.6 cm at 50 W and 300 s in single-needle MW ablation, and 4 cm in double needle MW ablation array. In in vitro swine liver experiments, the MTAD is 2.820 ± 0.127 cm at 100 W and 300 s in single-needle MW ablation, and 3.847 ± 0.103 cm in MW ablation array.

CONCLUSION

A new type of water-cooled MW ablation array is designed and tested, and has potential advantages over currently used devices.

A computational model to investigate the influence of electrode lengths on the single probe bipolar radiofrequency ablation of the liver

BACKGROUND AND OBJECTIVES

Recently, there have been calls for RFA to be implemented in the bipolar mode for cancer treatment due to the benefits it offers over the monopolar mode. These include the ability to prevent skin burns at the grounding pad and to avoid tumour track seeding. The usage of bipolar RFA in clinical practice remains uncommon however, as not many research studies have been carried out on bipolar RFA. As such, there is still uncertainty in understanding the effects of the different RF probe configurations on the treatment outcome of RFA. This paper demonstrates that the electrode lengths have a strong influence on the mechanics of bipolar RFA. The information obtained here may lead to further optimization of the system for subsequent uses in the hospitals.

METHODS

A 2D model in the axisymmetric coordinates was developed to simulate the electro-thermophysiological responses of the tissue during a single probe bipolar RFA. Two different probe configurations were considered, namely the configuration where the active electrode is longer than the ground and the configuration where the ground electrode is longer than the active. The mathematical model was first verified with an existing experimental study found in the literature.

RESULTS

Results from the simulations showed that heating is confined only to the region around the shorter electrode, regardless of whether the shorter electrode is the active or the ground. Consequently, thermal coagulation also occurs in the region surrounding the shorter electrode. This opened up the possibility for a better customized treatment through the development of RF probes with adjustable electrode lengths.

CONCLUSIONS

The electrode length was found to play a significant role on the outcome of single probe bipolar RFA. In particular, the length of the shorter electrode becomes the limiting factor that influences the mechanics of single probe bipolar RFA. Results from this study can be used to further develop and optimize bipolar RFA as an effective and reliable cancer treatment technique.

Theoretical and experimental study on RF tumor ablation with internally cooled electrodes: when does the roll-off occur?

The Cool-tip is one of the most widely employed electrodes in radiofrequency (RF) ablation (RFA) of hepatic tumors. This electrode creates reliable geometry and coagulation zones. Despite the advantages of this electrode, during the ablation is produced a phenomenon called roll-off in which impedance increases, energy deposition completely stops and the lesion size cannot be increased. Consequently, the thermal lesion size is smaller and the tumors which can be ablated are smaller too. In this research we studied theoretical and experimentally the electrical-thermal performance of the Cool-tip electrode during RFA of hepatic tissue. Mainly, we were interested in the occurrence of the roll-off and its relationship with the tissue temperatures around the electrode. The theoretical model included the vaporization of the tissue and the variation of the thermal and electrical conductivities with temperature. The model was solved numerically using COMSOL Multiphysics software. For the experimental part we conducted a study in ex vivo liver tissue. The experimental and theoretical results showed that the roll-off is totally related when temperatures around 100 °C surrounds the tissue close to the center of the Cool-tip. The knowledge of this fact brings a powerful tool to analyze alternative methods or techniques to avoid the roll-off.

[Sutureless hepatic transection using a new radiofrequency assisted device. Theoretical model, experimental study and clinic trial]

The ideal instrument for performing hepatic transection should combine safe and rapid haemostasis in a single tool. We present a new multidisciplinary investigation designed to develop a hepatic transection device assisted by radiofrequency (RF); the investigation included: a computerised theoretical model, and experimental study and a clinical trial of this device. The theoretic modelling was performed by computer, based on the Finite Elements Method (FEM), with the objective of studying the distribution of electrical energy and temperature in the tissue, and to assess the effect of the characteristics of the instrument. The experimental study, based on an in vivo porcine model, suggested that the new instrument would allow the transection velocity of the hepatic parenchyma to be increased with lower bleeding per transection area compared with other techniques extensively used in liver surgery. These data should enable the first phase of clinical trial to be conducted, with preliminary results that suggest that the new device is safe and effective.

Experimental and theoretical study of an internally cooled bipolar electrode for RF coagulation of biological tissues

Although some types of bipolar electrodes have been broadly employed in clinical practice to coagulate biological tissue by means of radiofrequency (RF) currents, there is still scanty available information about their electrical-thermal behaviour. We are focused on internally cooled bipolar electrodes. The goal of our study was to know more about the behavior of this kind of electrodes. For that, we planned an experimental and theoretical model. The experimental study was based on bovine hepatic ex vivo tissue and the theoretical model was based on the Finite Element Method (FEM). In order to check the feasibility of the theoretical model, we assessed both theoretically and experimentally the effect of the internal cooling characteristics of the bipolar electrode (flow rate and coolant temperature) on the impedance progress during RF heating and coagulation zone dimensions. The experimental and theoretical results were in good agreement, which suggests that the theoretical model could be useful to improve the design of cooled bipolar electrodes.