Radiofrequency ablation (RFA) has become increasingly popular for the treatment of both benign and malignant tumors in various organs [1-5]. This technology generates a localized alternating electric field in the tissue, causing ion agitation and friction; the resulting high local temperatures lead to irreversible cell damage, tumor apoptosis, and coagulation necrosis [2]. However, this procedure carries a risk of thermal injury to adjacent sensitive structures, including nerves, bile ducts, and the diaphragm, potentially leading to a variety of complications [6-8].

The hydrodissection technique, also termed artificial ascites, is widely used as a thermoprotective adjunct to minimize thermal injury in RFA of abdominal organs. This is achieved through the injection of fluids, such as 5% dextrose and 0.9% saline [9,10]. The liquid is administered between the target site and adjacent critical structures to create a physical and thermal barrier during thermal ablation procedures [9-11]. While the efficacy of this technique in reducing complications from thermal injury has been established in clinical settings, the best choice of fluid to provide heat insulation during thyroid ablation has yet to be conclusively determined.

Some research suggests that in 0.9% saline, the alternating electric field generated by RFA may induce ion agitation and produce frictional heat. This phenomenon could increase the temperature and diminish the solution’s capacity to insulate against heat during RFA of the thyroid and liver [10,12,13]. In contrast, 5% dextrose contains macromolecular substances that are unaffected by the electric field and exhibit negligible electrical conductivity. When made to surround the target organ, this solution may offer dual protection against both electrical and thermal damage [10,14]. Laeseke et al. [12] observed that instilling 5% dextrose into the peritoneal cavity before hepatic RFA reduced the risk and severity of injuries to the diaphragm and lungs compared to 0.9% saline in domestic pigs. However, several clinical studies have demonstrated the efficacy of 0.9% saline in providing adequate heat insulation during various procedures, including RFA of liver tumors [9,11] and thyroid nodules [15].

The discrepancies in these findings may stem from several factors. First, studies have varied in the power and energy levels applied with the ablation needles [1,3,14], as well as in the distances covered by the liquid within the thyroid [14,16] and liver [17,18]. Higher energy levels and shorter liquid distances are potentially associated with an increased risk of thermal injury. Second, the thermal sensitivity of the tissues under protection differs. In RFA of liver tumors, the muscle tissues of the diaphragm and gastrointestinal tract are the primary tissues to be safeguarded, with a denaturation temperature of approximately 50°C [2,13]. In contrast, in RFA of thyroid nodules, nerves represent the structures most vulnerable to heat injury and are extremely temperature-sensitive, with neuroendothelial edema and collagen deposition occurring at a threshold of 42°C [19-21]. Consequently, to draw a more scientifically valid comparison of the heat insulation efficacy between 0.9% saline and 5% dextrose, it is crucial to examine temperature variations and maximum temperatures under identical distances and heat field distributions. However, no analogous studies have yet been conducted.

In this study, the efficacy of heat insulation was assessed by comparing temperature variations and maximums reached when using 5% dextrose versus 0.9% saline. These comparisons were made under the predominant thermal conditions of the surroundings observed during RFA of the thyroid. The objective was to determine which agent offers superior heat insulation during thyroid RFA, based on scientifically sound evidence.

The experimental design of this study was divided into two parts, as shown in Fig. 1.

This study compared the temperature variations and maximum temperature differences in samples insulated with 0.9% saline and 5% dextrose under identical distances and heat field distributions. For both solutions, the temperature curves decreased as the distance from the edge of the ablated tissue increased. However, the 0.9% saline samples reached higher maximum temperatures than the 5% dextrose samples at 0, 1, 3, and 5 mm from the tissue edge. Considering a 42°C threshold for nerve injury, the proportion of samples reaching this critical temperature was also greater for 0.9% saline.

This study involved the use of an 18-gauge RFA electrode with a 7-mm tip. A fixed ablation technique was applied for 3 minutes in continuous mode at 30 W, reflecting the most common thermal environment of the surroundings during thyroid RFA [2,14,22-24]. The moving-shot technique has been recommended as the preferred clinical method for complete ablation in the RFA of benign thyroid nodules [10,14]. However, due to the limited size of the porcine liver tissue in this study, achieving precise control over the heat field distribution using the moving-shot technique would have been challenging and may have introduced uncontrollable variables, such as displacement of the liver tissue and temperature sensors. In contrast, fixed ablation enabled a controlled distribution of the heat field without movement of the needle tip or liver tissue, while maintaining a consistent distance between the temperature sensors and the tissue [23,24]. After preliminary testing, a cuboid tissue sample measuring 10 mm across was selected to simulate a scenario in which the heat field precisely coincided with the posterior medial thyroid capsule. At this location, the heat field closely borders the recurrent laryngeal nerve, representing a situation of high vulnerability that is commonly encountered in clinical practice. Furthermore, the use of fixed ablation technique with prolonged duration and higher ambient temperatures increases the risk to adjacent structures, thus requiring greater insulation efficacy [6,7]. In conclusion, the size of the porcine liver tissue was adjusted to match the boundaries of the heat field. The heat insulation efficacy of 0.9% saline and 5% dextrose on vital adjacent structures was then compared within the RFA heat field, with fixed ablation employed to control key parameters such as the field and the distance for temperature sensor variables.

For both 0.9% saline and 5% dextrose, the temperature curves decreased in conjunction with increasing distance from the edge of the ablated tissue. However, significant differences were observed between the temperature curves of the two groups. Specifically, the 0.9% saline group displayed higher maximum temperatures at the same distances (1, 3, and 5 mm), along with a greater proportion of samples reaching 42°C, compared to 5% dextrose. Notably, nerve injury may occur when the ambient temperature exceeds 42°C. Complications from nerve injuries, such as hoarseness, Horner syndrome, and brachial plexus injury, are well-documented and can necessitate prolonged recovery periods or extended hospital stays [6,7]. These findings suggest that 5% dextrose may offer greater insulation efficacy than 0.9% saline, thereby providing superior protection for thermally sensitive nerves during RFA of the thyroid.

The findings of this study align with an experimental investigation by Laeseke et al. [12], which demonstrated that the instillation of 5% dextrose into the peritoneal cavity during hepatic RFA reduces the risk and severity of diaphragm and lung injuries compared to 0.9% saline in domestic pigs. However, other clinical studies have indicated that artificial ascites established with either 0.9% saline or 5% dextrose can provide effective protection during RFA of hepatic carcinoma [17]. The discrepancies between these findings may stem from two factors. First, the gastrointestinal tract, gallbladder, and other liver-adjacent structures are less thermosensitive than the nerves. Second, the volume of artificial ascites injected during liver ablation was substantial, up to 1,000-2,000 mL [9,17], which increased the distance between the adjacent structures and the edge of the heat field. In such cases, 0.9% saline can also provide satisfactory heat insulation. This difference highlights the superior nerve protection achieved with the use of 5% dextrose as a hydrodissection fluid during RFA in anatomically confined areas such as the neck (for instance, the thyroid and lymph nodes).

This study had several limitations. First, it utilized porcine liver tissue for in vitro experiments, with the procedure reflecting the specific experimental setting and heat field conditions. However, this tissue differs in density, conductivity, and specific heat capacity relative to thyroid and kidney tissues. Second, due to the non-conductivity of 5% dextrose, the porcine liver tissue needed to maintain constant contact with the bottom of the stainless-steel cup during the ex vivo experiment. In contrast, the human body exhibits excellent electrical conductivity, and the electrical conductivity of hydrodissection fluid may exceed that observed in vitro, particularly with 5% dextrose. Nerve protection during RFA of benign thyroid nodules is a multifaceted process that involves hydrodissection, the maintenance of a precise distance between the nerve and the edge of the ablation zone, and considerations regarding ablation method, duration, and power settings. This study solely compared the heat insulation properties of 5% dextrose and 0.9% saline under identical conditions of distance, ablation method, time, and power. Although the findings are limited in scope, they still hold clinical significance.

At 1 to 5 mm, the heat insulation efficacy of 5% dextrose surpasses that of 0.9% saline when both are tested at the same distance and under thermal conditions of the surrounding structure typical for thyroid RFA.