Electrodes of radiofrequency ablation for malignant lung tumors: a narrative review of advances, applications, and prospects.

Introduction

Background
Lung cancer has been a leading cause of cancer-related deaths worldwide. The incidence and mortality rates of this disease were 12.4% and 18.7%, respectively, in 2022, ranking first among all kinds of cancers globally (1). Moreover, the lungs are the most common sites for metastases in malignant tumors such as colorectal cancer, renal cell carcinoma and breast cancer owing to their extensive vascular network, posing a serious threat to human health (2,3). While surgical resection remains to be regarded as the first choice for both primary lung cancer and pulmonary metastases treatment, a considerable number of patients are ineligible for it due to severe cardiopulmonary function impairment or other comorbidities. Therefore, there is an urgent need for other effective alternative therapies.
With reference to guidelines published by the National Comprehensive Cancer Network (NCCN) and Society of Interventional Radiology (SIR), image-guided thermal ablation (IGTA), including radiofrequency ablation (RFA), microwave ablation (MWA), and cryoablation (CA), has been suggested as one of the standard alternative minimally invasive treatments for lung tumors, especially for inoperable non-small cell lung cancer (NSCLC) and pulmonary metastases (4-6). Among them, RFA has become the most extensively studied ablation modality due to its convenient operation, technical maturity, and broad indications. Since Dupuy et al. (7) first reported the successful treatment of lung malignancies with RFA in 2000, RFA has been increasingly applied to patients with inoperable malignant lung tumors. RFA relies on the delivery of thermal energy through electrodes inserted into the tissue to induce coagulative necrosis. With its profound clinical accumulation and technical evolution in 20 years, RFA shows stronger controllability of ablation zone and safety of clinical application compared with other IGTA methods (8-11).

Rationale and knowledge gap
The radiofrequency (RF) electrode is a specific medical device used in RFA system for energy delivery within biological tissues. It is typically made of conductive materials and has a certain degree of rigidity and flexibility to allow safe puncture, accurate placement, and adaptation to various clinical scenarios. As the core component of the RFA system, the electrode geometry (e.g., active tip length and exposed configuration) and functional integration (e.g., cooling and multi-tined arrays) directly affect the current distribution, thereby influencing the morphology and extent of the ablation zone (12-15). Given the poor thermal conductivity and loose structure of lung tissue, as well as its proximity to key anatomical structures such as large blood vessels and bronchi, the design, selection, and application of this device must be more sophisticated to avoid the risk of incomplete ablation or collateral injury (16).
With the development of image guidance and finite element simulation technology, researchers have been able to assess the performance of different RF electrodes including the thermal field distribution and the ablation volume achieved in lung tissue more accurately, which provides a theoretical basis for parametric optimization and individualized design of electrodes. Currently, the types of electrodes used in clinical and scientific research have become increasingly diverse. Systematically summarizing their design features and optimization results in lung tissue is essential for advancing lung tumor ablation.

Objective
This article aims to systematically sort out the research status of RF electrodes in pulmonary ablation, focusing on their ablation mechanism, structural classification, and ablation effect in malignant lung tumors, and to explore the challenges and future development trend of their clinical application. We present this article in accordance with the Narrative Review reporting checklist (available at https://jtd.amegroups.com/article/view/10.21037/jtd-2025-1975/rc).

Methods
A comprehensive literature search was conducted using PubMed, Web of Science, China National Knowledge Infrastructure (CNKI), and Google Scholar databases to identify relevant studies published from January 2000 to January 2026. Articles related to the structure, design, or ablation performance of RF electrodes in lung tumors were included. The search terms included “radiofrequency ablation”, “lung tumor”, “lung cancer”, “pulmonary metastasis”, “electrode”, “ablation device”, and “needle”, among others related to the topic. The search strategies are summarized in Table 1, with an example for PubMed provided in Appendix 1.

Fundamental principles of RFA for lung tumors
The RFA system primarily consists of an RF power generator, RF electrode, and grounding pads placed on the patient’s skin to complete the electrical circuit (Figure 1). During the treatment procedure, the RF electrode is inserted into the tumor mass under image guidance, and the generator delivers RF energy (typically a high-frequency alternating current in the range of 375–500 kHz) through the electrode. Ions in the tissue around the tip of the electrode oscillate and generate Joule heat, which increases the local temperature. When tissue temperature reaches 43 °C, the proteins in the cells begin to denaturate. As temperature rises over 60 °C, the intracellular proteins suffer irreversible denaturation, resulting in coagulative necrosis in the target area. Furthermore, if temperature rises rapidly more than 100 °C, tissue vaporizes and carbonizes, which may rise the tissue impedance around the electrode, thereby limiting further heat transfer (17,18). Consequently, novel RF electrodes with specific functional enhancements have been developed and introduced into clinical practice, and real-time monitoring and feedback adjustment are performed by temperature-controlled or impedance control systems to further ensure the safety and efficacy of the ablation process.
The generally accepted criterion for RFA is homogeneous necrosis of the entire tumor and 0.5–1 cm of its surrounding non-tumorigenic tissue to maintain the integrity of the ablation border (19,20), as shown in Figure 2. This objective, however, must be pursued considering the lungs’ unique physiology, which presents both advantages and challenges for RFA therapy. The low conductivity and insulating properties of the air surrounding lung tumors facilitate the efficient delivery of RF energy within low-impedance lesions (21,22). Under the same energy conditions, the ablation volume formed in the lung is significantly larger than that in subcutaneous tissue, kidneys, and other solid organs (23). However, this unique high-impedance physical property and the fact that lung tissue is rich in blood vessels and bronchi which may bring significant heat sink effects during RFA process, making it challenging for RF electrodes to achieve complete thermal coverage of the margin in large-sized tumors, thereby leading to an increased risk of local recurrence (24,25). Additionally, the lung tissue exhibits high dynamism due to its susceptibility to respiratory movements, imposes more stringent requirements on the positional stability and thermal-field controllability of the RF electrodes. Therefore, rationally selecting and optimizing the electrode design and ablation parameters based on the unique characteristics of the lungs is an important direction of current research on lung RFA technology.

Types of RF electrodes for lung tumors
According to the differences in clinical treatment requirements and tumor characteristics, the design of RF electrodes has undergone a gradual diversification concerning both structure and function (26). RF electrodes available for lung ablation can be mainly classified into monopolar, bipolar, and multipolar electrodes, as shown in Figure 3. Among them, the monopolar electrode remains the prevailing form of current clinical application due to its relatively simple structure and wide range of adaptability, which can be further subdivided into multi-tined expandable, internally cooled, and wet electrodes, etc. In bipolar systems, the applied RF current forms a circuit between the two interstitial electrodes, eliminating the need for ground pads and providing higher localized energy deposition (27). On this basis, some electrodes have evolved to possess a multipolar structural design, permitting the application of RF energy in time-sharing and segmentation to enhance the therapeutic coverage of large or irregular lesions. Furthermore, the advent of multifunctional integrated technology has led to the emergence of novel composite electrodes that are being utilized in the ablation of complex lung tumors, exhibiting enhanced adaptability.
This chapter will focus on the types mentioned above and systematically review the structural characteristics, mechanism of action, and research progress of these, aiming to provide a reference basis for the selection of clinical electrodes and the design of new electrodes.

Monopolar systems

Conventional single electrode
The conventional single electrode has been extensively utilized for the ablation of small-diameter lesions (≤2 cm) due to its simple structure and convenient puncture operation. The design of the electrode consists of a straight insulated needle with an exposed metallic tip, which is specially treated to ensure superior electrical conductivity, tissue penetration capability, and under-image visualization (Figure 3A). The Soloist™ Single Needle Electrode (Boston Scientific) is one of the most commonly used single electrodes in clinical practice, primarily for small ablation zones of 1.5 cm length and 1 cm diameter. Its tri-faceted needle tip design enhances the echogenicity and ease of needle insertion into solid lesions. However, conventional single electrodes exhibit a limited effective ablation zone, restricting their application to small tumors in uncomplicated anatomical locations. For larger tumors, multiple punctures and multipoint ablation are necessary to achieve complete tumor coverage; this approach, however, increases operative risks and may lead to tumor recurrence or metastasis (28).

Multi-tined expandable electrodes
The multi-tined expandable electrodes represent an improvement over conventional monopolar designs, featuring multiple fine metallic sharpened tines stored in a coaxial needle cannula (29). After inserting the RF electrode into the target area, the tines are deployed according to the tumor size (as shown in Figure 4, where the tines are usually unfolded in an umbrella shape). The synergistic action of multi-tines optimizes the spatial energy distribution by increasing the contact area between electrode and tumor tissue, increasing the heating rate, and producing ellipsoidal necrotic zones with a diameter of approximately 3–5 cm, reducing the need for overlapping ablations in large tumors (30-32). Research has demonstrated that the distance between the diameter of the electrode array and that of the lung tumor constitutes a pivotal factor in local progression (33). Therefore, for larger lung tumors, needle deployment must extend beyond the tumor edge (Figure 4B) to ensure an adequate safety margin and effective ablation. And this stable array deployment provides distinct advantages in lung ablation, as once the lesion is engaged by tines, it is less likely to displace during manipulation, especially in the case of an enlarging pneumothorax (34). The LeVeen™ Needle Electrode (Boston Scientific) represents a widely adopted clinical example of multi-tined expandable electrodes with variable deployment lengths and diameters of the array. It is based on the impedance feedback system, which is designed to accurately monitor the extent of tissue desiccation and permit continued delivery of RF energy until complete ablation is achieved.
Notably, the multi-tined expandable configuration is more invasive than non-expandable designs. Improper deployment or retrieval of tines can increase the risk of bleeding, pneumothorax, and other complications (35). Furthermore, the current design of expandable electrodes mostly adopts fixed sizes, and the shape of ablation zone formed by it does not match the intrinsic tumor morphology, resulting in issues of localized over-ablation or incomplete ablation. This “mismatch” not only limits its application in complex anatomical areas but also raises concerns about the safety and efficacy of the treatment (36). Therefore, future designs of multi-tined expandable electrodes should focus on optimizing the flexibility and controllability of tine deployment mechanisms, using highly elastic materials with favorable biomechanical properties to better suit the complex pulmonary tissue environment.

Internally cooled electrodes
Internally cooled electrodes have been widely used in RFA for malignant lung tumors in clinical practice with the strategy of limiting tissue charring around the electrode tip and improving heat transfer during ablation. The internally cooled electrode features a hollow structure with an inner sleeve that divides the internal cavity into coaxial inner and outer lumens. The cooling medium (water, saline or gas) is circulated through the inner lumen channel to cool down the electrode tip, and then returns to the outer collection unit through the outer lumen. Although cooling the tip may seem contradictory to the heating effect of ablation, this cooling effectively delays overheating of tissues and reduce the impedance around the electrode tip, thereby prolonging the duration of RF current application and providing more precise control over the ablation process (31). Schutt et al. (37) compared the thermal field distributions of internally cooled and multi-tined expandable electrodes under varying blood perfusion conditions using finite element analysis models. The results showed that internally cooled electrodes exhibit greater thermal stability. Moreover, multiple studies have confirmed that, compared to conventional electrodes, internally cooled electrodes significantly decrease impedance and increase the extent of the coagulative necrosis area (38,39). Currently, this electrode type is represented by the Cool-tip™ RF System (Medtronic). There are multiple electrode lengths (10, 15, 25 cm) with different active-tip exposure lengths (1, 2, 2.5, 3, 4 cm), allowing for the control of varying lesion sizes. This system is specifically designed for versatility, flexibility, and patient safety and has demonstrated favorable therapeutic efficiency for tumors located adjacent to the pleura or pulmonary hilum (40). When a larger ablation zone is required, three electrodes can be placed in parallel as cluster electrodes within the target area to increase the localized energy deposition (41). Given the anatomical and physiological characteristics of lung tissue, internally cooled electrodes offer significant clinical value in lung RFA therapy due to their high thermal field control precision and low carbonization rates.

Wet electrodes
In pulmonary RFA, large blood vessels (>3 mm) and airways adjacent to the tumor carry away heat and causes the local temperature to drop. This “heat sink effect” phenomenon heavily limits the efficient heat transfer in the target tissue (6,42). To address this challenge, it is necessary to modulate the physical properties of tissue to improve energy deposition of RFA. Wet electrodes, also known as perfusion electrodes, feature one or more small holes at the tip of the electrode to improve electrical/thermal conduction by injecting isotonic or hypertonic saline solution into the tissue. Lee et al. (43) investigated the feasibility of saline infusion during RFA in an animal model and found that the average diameter of the ablation zone was significantly increased. And Liu et al. (44) conducted a retrospective clinical study to evaluate the efficacy of expandable electrodes with perfusion systems in patients with stage I NSCLC. The results showed that the application of this multifunctional electrode type is a feasible and effective procedure in completely treatment of large tumors. Nonetheless, despite the technology’s demonstrated therapeutic potential, there are still certain safety concerns that must be addressed such as the improper implementation of saline-infused, which may not only lead to incomplete tumor ablation, but also cause unnecessary thermal damage to adjacent tissues (45). Jin et al. (46) used a single wet electrode for RFA in 16 patients with NSCLC and found that while saline infusion can achieve a larger ablation range, the uneven diffusion of large amounts of saline solution may lead to increased risk of distal heating and ablation in non-tumor areas. Consequently, in clinical applications, it is recommended to combine real-time imaging technology [e.g., computed tomography (CT) or ultrasound] with a perfusion rate control system to dynamically monitor the diffusion range and concentration of saline, thereby enabling precise control of the ablation range and improving treatment safety.

Bipolar systems
Bipolar electrodes are typically composed of a single needle with two active electrodes separated by insulating material at the distal end, as shown in Figure 5. The RF current oscillates between two interstitial electrodes, enabling energy conduction without the need for grounding pads. This configuration concentrates energy primarily in the target area between the two electrodes, which improves the local deposition efficiency of thermal energy (47). Bipolar electrodes effectively reduce the risk of complications such as skin burns caused by poor grounding pad contact compared with monopolar systems (48). In addition, it also provides a feasible treatment option for patients with metal implants (such as cardiac stents or pacemakers). In the clinical landscape, Olympus Corporation’s CELON ProSurge series of RF electrodes stands as a representative example of bipolar systems. Its precise electrode spacing design integrated with internally-cooled function maintains larger and more uniform ablation zone while reducing collateral damage.
However, research on bipolar electrodes in RFA for lung tumors remains relatively limited. Current literature focuses primarily on their physical mechanisms and applications in organs such as liver, while systematic evaluations targeting the unique anatomical structure of lung tissue are still insufficient. This has, to a certain extent, limited its promotion and optimization in the field of pulmonary RFA. Furthermore, given the current path’s limitation to the two electrodes, the resulting coagulative necrosis area’s shape is relatively fixed, necessitating precise puncture angle and depth measurements. Improper operation may result in incomplete ablation of the target tissue or displacement of the ablation zone. Consequently, it is imperative to rely on high-precision image guidance systems and extensive operator expertise when using bipolar systems.

Multipolar systems
The multipolar electrodes consist of two or more RF electrodes inserted into the target tissue. They are connected to the same RF power generator to form a synergistic effect, and each electrode is activated through a sequential strategy or short-term switching strategy (Figure 6) (36). Studies have shown that the utilization of multiple electrodes for lung RFA constitutes a safe and effective treatment modality for larger-diameter lung tumors measuring between 2.0 and 5.0 cm (49,50). Notably, when larger tumors are adjacent to blood vessels or bronchi, it is unsuitable to use the multi-tined expandable electrodes due to the invasiveness of the tines. In contrast, multipolar electrodes, as an alternative with adjustable energy distribution and non-deployable structure, not only effectively improve the treatment coverage for medium to large lung tumors but also exhibit favorable safety in such high-risk regions (30).
In clinical practice, the ablation efficiency of multipolar electrodes is highly dependent on the precise matching between the number of electrodes and the spatial configuration, physicians should formulate an individualized electrode-placement strategy based on the specific structure and anatomical location of the tumors. Typically, two RF electrodes are used for tumors measuring 2.0–3.0 cm, while three electrodes are employed for those ranging from 3.0 to 5.0 cm with precise spatial arrangement to enhance the coverage of the targeted ablation area. When three electrode needles are required to ablate a large tumor, they should be arranged approximately in an equilateral triangle configuration to obtain more uniform energy distribution and improve the ablation effect. Taking the CELON ProSurge multipolar system as an example, this system allows the placement of two to three bipolar electrodes either within the tumor or in a peritumoral (non-touch) configuration. The peritumoral approach improves the predictability of the ablation volume and is theoretically associated with a lower risk of tumor-cell seeding along the applicator tract, as direct tumor puncture can be avoided (51).

Composite electrodes
The composite electrode constitutes a significant innovation in RF electrode technology in recent years. Its core concept lies in integrating multiple functions into a single device to address the limitations of traditional electrodes in complex clinical scenarios, aiming to achieve more controllable thermal distribution and reduce collateral damage. Common composite strategies include the combination of advanced electrode geometries (e.g., adjustable active-tip or expandable configuration) with internal cooling designs, saline infusion capability, drug delivery function, and other technologies. Feng et al. (52) performed a clinical investigation into RFA combined with intra-tumoral chemotherapy for NSCLC patients using a multi-tined expandable electrode integrated with saline perfusion, drug delivery, and real-time temperature monitoring function. The results revealed favorable therapeutic efficacy accompanied by a low complication incidence, validating the potential of multifunctional integrated designs in complex lesions. Another study by Iezzi et al. (39) evaluated a novel internally cooled wet electrode incorporating both internal cooling and saline-infused functions in the context of lung tumors. Their findings demonstrated that such multifunctional integration effectively mitigates tissue charring and enlarges the ablation volume.
Although composite electrodes show great prospects in terms of functional integration and thermal field control, relevant clinical research on their use in lung malignances remains relatively limited, with most studies focusing on computer simulations, animal experiments, or small-scale clinical observations. Future research should further optimize the electrode designs, conduct more clinical studies, and explore the application potential of various electrodes in different types and stages of lung tumors, thereby providing stronger support for the development of a minimally invasive treatment system for lung tumors.

Discussion and prospect
The performance of RFA relies primarily on the conduction of electrical and thermal energy within tissues, which is intrinsically sensitive to tissue impedance and electrode design, placing higher demands on the thermal field control and formation in the aerated lung tissue (53). Conversely, MWA utilizes dielectric heating that is generally less dependent on tissue electrical conductivity, enabling larger ablation volumes in shorter time under appropriate conditions (54,55). Although MWA exhibits superior physical efficiency, RFA can provide better controllability of the ablation zone and safety with the continuous evolution of RF electrode technology (56). As discussed above, progress has been made in RF electrodes regarding structural optimization, energy transmission, and multifunctional integration, which helps mitigate key challenges unique to lung ablation. For instance, multi-tined expandable designs allow for larger ablation zone with stable array anchoring in lung tissue; internally-cooled function limits tissue charring and maintains stable energy delivery; wet (perfusion) electrodes can increase tissue conductivity and improve heat distribution under high-impedance conditions; and bipolar/multipolar configurations can achieve larger ablation zone with potentially reduced collateral damage. Furthermore, the key characteristics of representative commercially available RF electrodes are summarized in Table 2, along with their lung-specific considerations. Nevertheless, there is no global consensus on the selection of the “best” electrode type for lung tumors, and its selection is highly dependent on tumor size, location (e.g., proximity to pleura, vessels, or bronchi) and operator experience.
To overcome existing challenges and further enhance procedural performance in lung tumors, persistent refinement of electrode technology is needed. Accordingly, future research could be pursued in the following directions. (I) Electrode structural and functional optimization: future electrode development should be optimized toward controllable, conformal, margin-oriented ablation in the aerated lung tissue. Promising approaches include the design of expandable tips employed with controllable deployment length and orientation, which can better match irregular target tissue and protect adjacent structures. In parallel, multifunctional integration (such as integrating local drug-delivery and sensing elements into electrode platforms) has been proved to achieve better ablation safety and controllability without sacrificing ablation volume (58,59). Importantly, these approaches require dedicated lung-specific validation in the future research. (II) Transitioning to intelligence and precision: in addition to the optimization of electrode structure and function, combining advanced RF electrode designs with precision and intelligent systems may substantially enhance procedural performance. Specifically, robotic navigation-assisted RFA for lung tumors has been proved feasible to improve placement accuracy and repeatability of electrode needles, shortening puncture time, and reducing the procedure-related complications (60). Meanwhile, integrating impedance-based control with multi-point thermometry within the monitoring and feedback mechanism has the potential to deliver more stable closed-loop energy modulation in aerated lung tissue, supporting more conformal ablation and safer margin achievement while reducing non-target heating near critical structures.

Conclusions
In summary, the geometric and conductive characteristics of RF electrodes have been proven to significantly influence energy distribution and procedural safety in RFA for malignant lung tumors. Progress has been made in RF electrodes regarding structural refinement, more stable energy delivery, and integrated functions. By continuously exploring electrode structural and functional optimization and developing intelligent control technologies, the electrode technology is poised for further improving the controllability and uniformity of ablation zones in lung tumors with reduced thermal damage to adjacent structure. Propelled by advances in electrode technology and their clinical validation, RFA will offer higher precision, stronger efficacy, and personalized minimally invasive treatment options to lung patients.

Supplementary
The article’s supplementary files as