Thyroid nodules are common, with the overall prevalence of 36.9% in the Chinese population . In recent years, the detection rate of thyroid nodules is up to 68% with the widespread application of ultrasonography . Only 5–10% of thyroid nodules are malignant, and the majority are benign that need no clinical treatment until the occurrence of compression or cosmetic concerns [10, 11]. Currently, the efficacy and safety of EA and thermal ablation on cystic or predominantly cystic thyroid nodules have been validated [1, 2, 12–14].
EA causes a sterile inflammatory response by directly inducing cell dehydration, protein coagulation, denaturation and necrosis using alcohol. It inhibits the secretory function of epithelial cells and blocks tumor blood supply, thus leading to the adhesion and closure of the nodular cavity and tumor shrink. However, the recurrence rate remains high after EA for predominantly cystic nodules, and the efficacy of multi-injection of alcohol is controversial . Generally speaking, the number of alcohol injection is determined by the initial volume of the tumor, treatment response and follow-up findings, and most cases require 2–3 times of injection. Nevertheless, the risk of complication increases with the treatment cycle. In the present study, the median treatment cycle in EA group was 2.5 (1.0-3.3) times, and 8 patients (26.67%) complained of mild pain, which were slightly higher than MWA group. The high incidence of pain during ablation procedure and follow-up period may be attributed to the difficulty in controlling the diffusion of alcohol solution to the thyroid nodule and the surrounding tissue space, leading to the hemorrhage of normal thyroid glands and surrounding tissues caused by stimulation, destruction, and even necrosis. VRR of cystic thyroid nodules after EA was significantly higher than that of predominantly cystic ones. Our previous study has reported the higher VRR of cystic thyroid adenomas after EA than that of solid ones . It is suggested that the therapeutic efficacy of EA is less influenced by the cystic component of nodules.
The frequency of microwave radiation ranges 900–2450 MHz. The interaction between radiation-induced oscillating charges and water molecules causes vibration, and the violent friction between the molecules based on the frequency of microwave radiation in turn produces heat. Electromagnetic microwaves produce friction and heat through water molecules in the surrounding tissues, thus inducing cell death via coagulative necrosis . Due to the controllable power, ablation time, and ablation range, electromagnetic microwaves remarkably reduce the stimulation and damage to the normal thyroid and surrounding tissues. Our previous studies have shown the acceptable efficacy and safety of MWA on benign thyroid nodules, papillary thyroid microcarcinoma (PTMC), and primary hyperparathyroidism (PHPT) [4, 17, 18]. The median VRR of benign thyroid nodules at 6 months and 12 months of MWA reaches 75.9% and 86.67%, respectively . However, the efficacy of MWA on cystic or predominantly cystic thyroid nodules remains unclear. In the present study, the median VRR at 3 months and 12 months of MWA was 75.76% and 89.34%, respectively. Luo et al.  conducted a 3-year follow-up in patients with thyroid nodules and found that the VRR of solid nodules at 1, 3 and 6 months postoperatively is significantly lower than that of cystic and predominantly cystic ones, while no significant correlation is identified between cystic component and VRR at 12, 24 and 36 months postoperatively. It is indicated that the component of thyroid nodules may not influence the long-term efficacy of MWA.
Our study proved that both EA and MWA significantly shrank cystic or predominantly cystic thyroid nodules. VRR and ER were higher in EA group during the follow-up period than those of MWA group, although significant difference was only detected in VRR at 3 months after ablation. The higher ER of EA group may be attributed to the significantly smaller volume of thyroid nodules at baseline compared with that of MWA group. Previous evidence has supported the negative correlation between the therapeutic efficacy of ablation and initial volume of nodules [19, 20]. Currently, the comparison between the clinical efficacy of EA and MWA has been rarely reported. Liu et al.  reported the similar efficacy of EA and MWA on simple thyroid cysts, although the incidence of intraoperative pain was lower in MWA group than that of EA group. Zhou et al.  demonstrated the superb efficacy of MWA with less damage in treating benign solid-cystic thyroid nodules compared with that of EA. The combination of EA and MWA has been proposed in a latest study, which is reported to significantly shorten the procedure time and reduce the recurrence in patients with predominantly cystic thyroid nodules . In the present study, we compared VRR of cystic and predominantly cystic thyroid nodules after EA and MWA, which was higher in EA, although no significant difference was obtained. Moreover, ER was comparable between EA and MWA group either in cystic or predominantly cystic thyroid nodules.
This study was limited by the small sample size, short follow-up period and differences in baseline characteristics. We did not perform propensity score matching to eliminate potential interventions in control group. Our findings should be further validated in randomized clinical trials with a large sample size, and the therapeutic efficacy of EA combined with MWA in treating thyroid nodules should be investigated in the future.
Taken together, EA and MWA are both effective and safe in the treatment of cystic or predominantly cystic thyroid nodules. Although EA is more cost-effective, it requires more times of treatment and may pose a higher risk of postoperative pain than MWA.