The role of superb microvascular imaging in evaluating the efficacy of microwave ablation for primary hyperparathyroidism

Incomplete ablation is one of the main factors leading to operative failure in microwave ablation (MWA), which is used as an alternative to surgery for treating primary hyperparathyroidism (PHPT). Contrast-enhanced ultrasonography (CEUS) is commonly used for evaluating complete and incomplete ablation following MWA. Superb microvascular imaging (SMI) is a new ultrasound (US) technique that accurately demonstrates tissue microvascularity. Compared to CEUS, SMI is noninvasive, requires no contrast agents, is more cost-effective, and is easily accessible. This prospective, cross-sectional study, conducted at a single center with a multidisciplinary approach, aimed to investigate the role of SMI in evaluating the efficacy of MWA for PHPT. Our study was approved by the local ethics committee. A total of 41 patients who underwent MWA for PHPT between September 2022 and February 2024 were included in the study. All patients underwent clinical, laboratory, and radiological assessments before ablation. The MWA procedures were monitored, and the technical parameters and complications were documented. After MWA, all patients were followed up for 6 months. Serum parathyroid hormone (PTH) and calcium levels were measured on the first day, at the first month, and at the sixth month of follow-up after MWA. Radiological assessment was repeated at the first-month follow-up. The radiological evaluations included the assessment of the parathyroid lesion's location, maximum diameter, and volume using gray-scale US. The lesion boundaries were then delineated using a free-style ROI (Region of Interest) in SMI, followed by the measurement of the vascularity index (VI). In SMI, post-ablation, the absence of vascularity within the lesion was defined as complete ablation, whereas the presence of residual vascularity was considered indicative of incomplete ablation. Clinical success was defined as maintaining serum PTH and calcium levels within the normal range at the six-month follow-up after MWA. The study included 41 parathyroid lesions from 41 patients, consisting of 35 women and 6 men, aged between 34 and 79 years. Each MWA treatment was performed in a single session. The technical success rate was 100%. In SMI, complete ablation was achieved in 75.6% of the lesions, while incomplete ablation was detected in 24.4%. The clinical success rate was 53.7%. At the sixth-month follow-up after the MWA, the normocalcemia rate was 80.5%. A statistically significant relationship was found between SMI findings and clinical success (p=0.002). The clinical success rate in patients with incomplete ablation detected by SMI (10%) was statistically significantly lower than in those with complete ablation (67.7%) (p=0.002). Based on the clinical success outcomes, the sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of SMI in evaluating MWA efficacy were calculated as 47.4%, 95.5%, 90%, 67.7%, and 73.2%, respectively. In the pre-ablation US, the lesions' SMI VI values ranged from 4 to 52.96%, with a mean of 20.45 ± 9.25% and a median of 19.55%. In both clinically successful and unsuccessful patients, the reductions in the lesion's maximum diameter, volume, and SMI VI in post-ablation US compared to pre-ablation US were statistically significant (p=0.001). In both clinically successful and unsuccessful patients, reductions in serum PTH and calcium levels on the first day, first month, and sixth month post-ablation compared to pre-ablation levels, were statistically significant (p<0.05). The pre-ablation serum PTH levels in clinically unsuccessful patients were statistically significantly higher than those in clinically successful patients (p=0.014). The MWA power in clinically successful patients was statistically significantly higher than in clinically unsuccessful patients (p=0.003). In conclusion, SMI is observed to be an effective imaging method that contributes to the assessment of treatment efficacy by demonstrating incomplete ablation following MWA in patients with PHPT. Keywords: Superb microvascular imaging, Vascularity index, Microwave ablation, Ultrasonography, Primary hyperparathyroidism