Thyroid nodules are a usual clinical condition, with an overall incidence rate of approximately 38.0 % in China [1]. Most thyroid nodules are asymptomatic and do not in need of therapy. However, some benign nodules may cause compressive symptoms when they progressively grow [2]. The usual course of treatment for thyroid disorders is surgery, but it is expensive and may result in lifelong thyroid hormone replacement therapy. Additionally, permanent complications may occur after surgery, including voice changes related to recurrent laryngeal nerve and superior laryngeal nerve injury, and airway obstruction related to bilateral vocal cord dysfunction, hematoma, or infection [3,4].

In recent years, ultrasound-guided intervention therapies for thyroid nodules have rapidly developed, and non-invasive or minimally invasive treatments have become a trend [5]. For cystic thyroid nodules, first-line therapy is advised to be ethanol ablation due to significant efficacy and low cost [6]. However, for large or predominantly vascular cystic thyroid nodules, the efficacy of ethanol chemical ablation is limited [7], and studies have reported a high recurrence rate of up to 38.3 % after a single ethanol ablation treatment [8]. Polidocanol injection is a new type of sclerosing agent that has been widely used for sclerotherapy of various vascular tumors, venous malformations, and various cystic diseases, and its safety has been widely recognized by physicians both domestically and internationally [[9], [10], [11]]. Currently, many clinicians in China have started to explore the use of polidocanol as a substitute for ethanol for benign cystic thyroid nodules [12,13]. But during chemical ablation process, some studies directly withdraw the sclerosing agent, while others leave it in the cyst cavity, and the retention dose is not consistent. Some studies use 25–33 % (maximum volume of 35 mL) of the withdrawn cyst fluid, while others use 30–50 % (maximum volume of 20 mL) [12,14]. It is worth noting that there is currently rare study that clarifies the influence of various polidocanol retention doses on efficacy of thyroid nodule treatment. Therefore, seeking the optimal polidocanol retention dose is of significant clinical significance. This study aimed to explore influence of retention doses of ultrasound-guided polidocanol chemical ablation on therapeutic efficacy of benign cystic thyroid nodules.