Residual vital ratio: predicting regrowth after radiofrequency ablation for benign thyroid nodules

Purpose To determine a novel quantitative index, residual vital ratio(RVR) by contrast-enhanced ultrasound(CEUS) with conventional Ultrasound(US), to early predict nodule regrowth after radiofrequency ablation (RFA)for benign thyroid nodules. Methods This retrospective study evaluated 186 patients with 206 benign thyroid nodules underwent RFA. Patients were followed at 1, 3, 6, 12 months and every 12 months thereafter by conventional US, CEUS and clinical evaluation. RVR was defined as the initial ratio of residual vital volume to the total volume calculated by CEUS and conventional US at the first follow-up period after RFA. The relationship between RVR and regrowth was investigated. Results The mean volume of thyroid nodules was 10.09 +/- 12.90 ml (range 0.40-71.39 ml), which decreased significantly to 2.33 +/- 4.65 ml (range 0-36.75 ml) (p < .001) after a mean follow-up time of 22.50 +/- 13.29 months (range 6-68 months) with a mean VRR as 85.26 +/- 15.02% (range 32.23-100%). The overall incidence of regrowth was 12.62% (26/206) and the mean timing of regrowth was 20.77 +/- 12.03 months (range 6-48 months). Multivariate logistic regression revealed that RVR (OR = 1.050, 95%CI 1.025-1.075), initial volume(OR = 1.033, 95%CI 1.000-1.066), location close to critical structures (OR = 5.967, 95%CI 1.898-18.760) and vascularity (OR = 2.216, 95%CI 1.185-4.143) were independent factors associated with regrowth. According to receiver-operating characteristic curve, the area under curve for RVR to regrowth was 0.819 (95% CI 0.740-0.897,p < .001) with the optimal cutoff value of 44.5% (sensitivity 80.8%, specificity 74.7%). Conclusion RVR was not only an independent factor but also an early quantitative predictor for regrowth. If RVR was larger than 44.5%, the nodule tended to regrowth in the follow-up.