Factors associated with ablation zone resorption after microwave ablation in solitary T1aN0M0 papillary thyroid carcinoma: baseline lesion volume and microcalcification

Objectives To identify preoperative factors associated with ablation zone resorption after microwave ablation (MWA) in patients with solitary T1aN0M0 papillary thyroid carcinoma (PTC), and to explore their association with energy density.Materials and methods This retrospective study enrolled patients with solitary T1aN0M0 PTC who underwent MWA in our institution between September 2013 and January 2021. Ablation zone volume was measured at 1, 3, 6, 12, and 24 months post-ablation, and annually thereafter. Cox proportional hazards regression was performed to identify independent factors associated with ablation zone resorption. Kaplan-Meier analysis compared cumulative resorption rates among subgroups. Energy density was compared across subgroups.Results A total of 296 patients were included, with an overall complete resorption rate of 73.6%. Multivariable Cox analysis identified baseline lesion volume (HR per IQR: 0.788, 95% CI: 0.678–0.917, p = 0.002) and microcalcification (HR: 0.669, 95% CI: 0.508–0.881, p = 0.004) as independent factors associated with delayed ablation zone resorption. Both factors were significantly correlated with energy density: the large-volume group (>52.1 mm³) received lower energy density than the small-volume group (≤52.1 mm³) (median 18.3 vs. 60.5 J/mm³, p < 0.001), and energy density was also lower in the microcalcification group than in those without (median 25.6 vs. 35.2 J/mm³, p = 0.030).Conclusion MWA is an effective and safe treatment for solitary T1aN0M0 PTC. Baseline lesion volume and microcalcification are independent factors associated with delayed ablation zone resorption, and their correlation with energy density provides a rationale for optimizing energy delivery strategies to facilitate resorption.