Objective: Microwave ablation (MWA) is an effective technique for the treatment of benign thyroid nodules. The aim of this study was to investigate whether it is necessary to study changes in variables other than the volume reduction between follow-up intervals. This study also introduced the index R (S-surface/V) to explore whether it could reflect features of shape between follow-up intervals.Materials and methods: A total of 236 complex benign thyroid nodules in 230 patients who underwent MWA in our department from October 2013 to February 2017 were included. The D-max, volume (V), volume reduction ratio (VRR) and index R (S-surface/V) parameters were evaluated before treatment and postoperatively at 1, 3, 6 and >12months. Multiple comparisons of these parameters were performed between the baseline and each follow-up period for the entire patient group and subgroups based on diameter and volume.Results: All of the thyroid nodules underwent a significant decrease in size after MWA. The mean decrease in the volume of the thyroid nodules was from 17.4022.52mL to 1.31 +/- 2.71mL, with a mean percent decrease of 0.90 +/- 0.15% after 12months. Index R increased over time from 2.55 +/- 1.08 to 8.10 +/- 5.01, which increased the effectiveness of shape parameterisation between the follow-up periods after the three-month time point, regardless of the initial volume size. For the nodules with a baseline D(max)3.4cm, the V, VRR and index R demonstrated similar capabilities in the evaluation of efficiency before the six-month follow-up visit.Conclusions: Long-term follow-up emphasises changes in the shape and spatial structure of complex benign thyroid nodules, rather than size variations, after effective MWA. Index R, which is the surface area to volume ratio, can also reflect the shape change to some degree.