The association between volume reduction ratio and time after ultrasound-guided microwave ablation for benign thyroid nodules of different compositions.

Background: Ultrasound-guided microwave ablation (MWA) is effective in the treatment of benign thyroid nodules (BTNs); however, there are large individual differences in the degree of lesion absorption and shrinkage speed. Although the internal components of nodules are important influential factors, few studies have reported temporal changes in the volume reduction ratio (VRR) in BTNs with different compositions. Therefore, this study aimed to explore how the thyroid nodule VRR changes over time and determine when it increases most significantly for BTNs of different compositions after MWA.

Methods: This is a retrospective study. We consecutively collected patients that received MWA for BTNs from July 2020 to June 2021, with 12 months of follow-up, and VRR >50% as effective treatment indicator. A generalized additive model was used to analyze temporal changes in the VRR in BTNs of different compositions, and a log-likelihood ratio test of the segmented regression model was used to determine a potential threshold. We evaluated the consistency of two operators in MWA of BTNs.

Results: In total, 221 treatment-effective nodules (216 patients) were included, in which 21 were predominantly cystic, 61 were predominantly solid, and 139 were solid nodules. A non-linear relationship was detected between VRR and time. In predominantly cystic nodules, the VRR increased by 36% per month until 2.0 months and by 1.4% per month after 2.0 months. In predominantly solid nodules, the VRR increased by 30.6% per month until 2.3 months and by 0.7% per month after 2.3 months. In solid nodules, the VRR increased by 17.4% per month until 4.3 months and by 0.3% per month after 4.3 months. Intraclass correlation coefficient (ICC) was 0.839 (95% confidence interval: 0.773, 0.887), indicating good consistency between the two operators.

Conclusions: The VRR after ultrasound-guided MWA for different components of BTNs was non-linearly related with follow-up time, and there was a threshold effect.