Comparison of two ablation methods in benign thyroid nodules: Radiofrequency ablation and non-cooled microwave ablation

INTRODUCTION

The American Thyroid Association defines a thyroid nodule as a distinct lesion within the thyroid gland that differs radiologically from the surrounding parenchyma. It may be solitary, multiple, cystic, or solid.^[1] Autopsy data indicate that thyroid nodules >1 cm are present in 50% of individuals without clinical symptoms, while the increasing use of ultrasound (US) in recent years has revealed thyroid nodules in 67% of the general population.^[2] The prevalence is 4 times higher in females than in males and is positively correlated with age and body mass index.^[3]

According to the Thyroid Imaging Reporting and Data System (TIRADS), fine-needle aspiration biopsy (FNAB) is initially recommended for all nodules >1 cm classified as suspicious, incidentalomas detected on positron emission tomography, and nodules >5 mm in patients at high clinical risk.^[4-6] Approximately 98% of thyroid nodules are diagnosed as benign; however, up to 3% of these benign nodules still carry a risk of harboring malignant cells.^[7]

Although benign thyroid nodules typically do not require surgical treatment, intervention may be necessary in cases where their size or location leads to compressive symptoms or cosmetic concerns.^[8] The treatment options include surgery, thermal ablation techniques, and percutaneous ethanol ablation for cystic nodules.^[9]

Percutaneous ablation techniques are advantageous due to their short procedure time, ability to be performed under local anesthesia, same-day discharge, and the absence of surgical incisions. The most serious complication, recurrent laryngeal nerve injury, occurs at rates of 0–12% in surgery and 0–1.6% in thermal ablation techniques.^[10,11] Hydrodissection, which isolates the thyroid tissue from surrounding critical structures by creating a protective fluid barrier, is an effective and safe method to prevent complications associated with thermal ablation.^[12]

The goal of thermal ablation therapy is to induce irreversible cellular damage by heating live tissue above 60°C, creating a well-defined coagulation zone. In radiofrequency ablation (RFA), an oscillating electrical current close to the radio broadcasting frequency band generates thermal energy due to the body’s poor conductivity.^[13,14] In contrast, microwave ablation (MWA) relies on the acceleration of hydrogen atoms within the tissue by an electromagnetic field, generating heat.^[15] Modern MWA devices include both cooled and non-cooled probe systems. Cooled systems use continuous cooling with +4°C 0.9% isotonic sodium chloride to prevent complications such as skin and deep tissue burns. Non-cooled systems, on the other hand, achieve the desired tissue temperature using lower generator power and lower frequency, minimizing such complications. Advantages of the non-cooled system include reduced vapor formation, improved visualization of ablation progression and surrounding structures, and easier probe manipulation.^[16]

This study aims to compare the effectiveness and complication rates of the non-cooled MWA system with the RFA system in the treatment of benign thyroid nodules and to identify factors influencing the success of thermal ablation therapies.

MATERIAL AND METHODS

RESULTS

In this study, RFA and MWA were performed on 38 (61.2%) and 24 nodules (38.8%), respectively. Table 1 presents the demographic characteristics of the patients and the morphological features of the thyroid nodules.

In the RFA group, the mean nodule volume before ablation was 18.97 ± 19.15 mL, decreasing to 11.00 ± 14.86 mL at 3 months, 9.85 ± 13.67 mL at 6 months, and 8.99 ± 12.56 mL at 12 months. The VRR was calculated as 49.68 ± 19.79% at 3 months, 57.03 ± 18.59% at 6 months, and 62.11 ± 18.63% at 12 months. At the end of 12 months, the minimum VRR was 28% and the maximum was 93%. The treatment response calculated by VRR in the RFA group was statistically significant (P < 0.001). In the MWA group, the mean nodule volume before ablation was 18.65 ± 20.68 mL, which decreased to 9.37 ± 15.43 mL at 3 months, 6.88 ± 12.16 mL at 6 months, and 4.72 ± 7.37 mL at 12 months. The VRR for this group was 59.94 ± 14.48% at 3 months, 71.21 ± 15.84% at 6 months, and 78.31 ± 12.59% at 12 months. The minimum VRR at 12 months was 59%, and the maximum was 96%. The treatment response calculated by VRR in the MWA group was also statistically significant (P < 0.001). The data presented above have been shown in Table 2.

When comparing the efficacy of RFA and MWA, nodules in the MWA group showed a greater volume reduction than those in the RFA group at 3, 6, and 12 months, with statistical significance (P = 0.022, P = 0.002, and P < 0.001) [Figure 1]. In univariate regression analysis, factors such as age, sex, and the presence of a multinodular goiter component did not significantly affect treatment response at 3, 6, or 12 months (P > 0.05 for each). TI-RADS score did not significantly influence treatment response at 3 months (P = 0.65), but showed a statistically significant effect at 6 and 12 months (P = 0.009 and P = 0.004). When considering nodule composition, it had no significant effect on treatment response at 3 months (P = 0.242), but played a significant role in volume reduction at 6 and 12 months (P = 0.029 and P = 0.023). In the MWA group, nodule composition did not significantly influence treatment response at 3 and 6 months (P > 0.05), but became significant at 12 months (P = 0.043). In the RFA group, nodule composition had no significant effect at 3 months (P > 0.05), but was statistically significant at 6 and 12 months (P = 0.044 and P = 0.046). The VRR values during the follow-up process, according to nodule structure, have been presented in graphical form in Figure 2.

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Figure 1:

Comparison of volume reduction ratio between a non-cooled microwave ablation system and a radiofrequency ablation system using the Mann–Whitney U test at 3, 6, and 12 months of follow-up.

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Figure 2:

Comparison of volume reduction ratio (VRR) according to thyroid nodule structure. Differences in VRR among the nodule structure groups at 3, 6, and 12 months were analyzed using the Mann–Whitney U test.

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DISCUSSION

When comparing the treatment efficacy of RFA and non-cooled MWA, this study found that the non-cooled MWA system demonstrated higher treatment success at the 3-, 6-, and 12-month follow-ups compared to the RFA system. One of the factors influencing the effectiveness of thermal ablation treatments is carbonization, which occurs when the nodule undergoes charring at high temperatures. Carbonized tissue cannot effectively conduct heat, preventing uniform heating of all areas within the nodule. In addition, it induces dense fibrosis and capsule formation, creating a secondary barrier in the ablated area. To mitigate these issues, techniques and systems need to be optimized. In non-cooled MWA, the use of lower power ensures lower target tissue temperatures, and the system automatically shuts off when necessary based on impedance measurements, leading to reduced carbonization. This is likely the primary reason for the superior treatment response observed in our cases. Similarly, Cheng et al. investigated the impact of carbonization on treatment response in benign thyroid nodules by comparing RFA and cooled MWA systems. Their study concluded that both methods were effective in treating benign thyroid nodules at 12 months.^[17] However, when comparing VRR at 3, 6, and 12 months, there was no significant difference between the two groups at 3 months, whereas RFA demonstrated superior outcomes over MWA in the later follow-ups. This was attributed to the rapid increase in temperature within the target tissue in the cooled MWA system, leading to carbonization. When evaluating both studies together, it appears that reducing ablation energy minimizes carbonization, facilitates more homogeneous heat distribution in the thyroid tissue, and ultimately enhances long-term treatment response. In our study, the difference in treatment efficacy between ablation methods became apparent after the 3-month follow-up, favoring non-cooled MWA. This result may support the idea that non-cooled MWA leads to lower carbonization rates.

Another potential explanation for the difference in treatment response between thermal ablation systems may be related to differences in probe usage techniques. Jeong et al. argued that the fixed electrode method used in RFA is not suitable for thyroid ablation due to the small size of the thyroid and the presence of critical surrounding structures.^[18] Another study highlighted that the moving-shot technique used in RFA requires multiple probe manipulations, which may present challenges, particularly in large nodules, as microbubbles formed during ablation can reduce visibility.^[19] In contrast, the overlapping technique used in non-cooled MWA involves placing the probe along the desired treatment trajectory and performing ablation without repositioning until the nodule boundary is reached, requiring fewer repositioning maneuvers to cover the entire target tissue. In addition, the lower energy levels used in this system result in fewer microbubbles, thereby preserving the visibility of the ablated area.

It is well established that nodule composition significantly influences the effectiveness of thermal ablation treatments. Compact tissues exhibit greater resistance to electrical currents than soft tissues. Consequently, spongiform and semi-solid nodules respond better to RFA compared to purely solid nodules.^[20] Similarly, in this study, semi-solid and spongiform nodules demonstrated superior treatment response at 6 and 12 month follow-ups for both treatment methods compared to solid nodules. Negro et al. attributed this phenomenon to the fact that solid nodules have a higher density, which impairs heat conduction and requires a tissue-specific threshold temperature for cell death induction.^[21]

In line with previous literature, the most significant volume reduction in this study was observed at 12 months. Yıldırım and Karakas evaluated the effects of non-cooled MWA on nodule size and thyroid function in a study of 40 solid nodules and reported VRR values of 49.88± 12.49% at 3 months, 65.3± 11.78% at 6 months, and 79.06 ± 8.65% at 12 months.^[22] Our findings align with the literature, regardless of technical differences. In another study by Che et al., which compared surgical treatment and RFA, the 12-month volume reduction in the RFA group was 84.8 ± 17.1%, and the complication rate was lower in the RFA group. In Che et al.’s study, the mean nodule volume was 5.4 ± 7.1 mL, whereas in our study, it was 18.97 ± 19.15 mL.^[13] The volume reduction in our study was 62.11 ± 18.63% at 12 months, which is lower than in their study. This discrepancy may be because nodules larger than 20 mL may not respond as effectively to treatment, often requiring repeated ablation sessions.^[23]

A study by Javadov et al. comparing the efficacy of RFA and cooled MWA included 50 nodules treated with RFA and 50 nodules treated with MWA. At the 1-, 3-, and 6-month follow-ups, the mean volume reduction rates were 42%, 56%, and 65% in the RFA group, and 46%, 64%, and 68% in the MWA group, respectively.^[24] They concluded that there was no significant difference in treatment efficacy between the two groups. Similarly, in our study, RFA treatment success was found to be consistent with their findings.

Long-term follow-up studies have reported that treatment efficacy declines after 12 months, with some nodules exhibiting regrowth. A retrospective study evaluating the long-term efficacy of RFA in benign thyroid nodules showed that although volume reduction reached 72.8% by 12 months, it declined to 71.3% at 24 months and 62.9% at 48 months.^[25] Yan et al. conducted a similar study, showing that volume regression diminished at 24 and 36 months, with nodule regrowth observed in 5.6–38% of cases within 2–3 years.^[26] However, despite this regrowth, patients reported sustained improvements in cosmetic concerns and compressive symptoms.

This study has some limitations, including its single-center design, small patient cohort, and a follow-up period limited to 12 months.

CONCLUSION

Both RFA and non-cooled MWA are effective and safe options for the treatment of benign thyroid nodules. When comparing the volumetric efficacy of both methods, non-cooled MWA was found to be more effective than RFA. Nodule composition significantly affects treatment success, with spongiform and semi-solid nodules responding better to ablation than purely solid nodules.