Pulmonary Thermal Ablation: Comparison of Radiofrequency and Microwave Devices by Using Gross Pathologic and CT Findings in a Swine Model

Abstract

Purpose:To compare the performance of equivalently sized radiofrequency and microwave ablation applicators in a normal porcine lung model. Materials and Methods:All experiments were approved by an institutional animal care and use committee. A total of 18 ablations were performed in vivo in normal porcine lungs. By using computed tomographic (CT) fluoroscopic guidance, a 17-gauge cooled triaxial microwave antenna (n ϭ 9) and a 17-gauge cooled radiofrequency (RF) electrode (n ϭ 9) were placed percutaneously. Ablations were performed for 10 minutes by using either 125 W of microwave power or 200 W of RF power delivered with an impedance-based pulsing algorithm. CT images were acquired every minute during ablation to monitor growth. Animals were sacrificed after the procedure. Ablation zones were then excised and sectioned transverse to the applicator in 5-mm increments. Minimum and maximum diameter, cross-sectional area, length, and circularity were measured from gross specimens and CT images. Comparisons of each measurement were performed by using a mixed-effects model; P Ͻ .05 was considered to indicate a significant difference. Results:Mean diameter (3.32 cm Ϯ 0.19 [standard deviation] vs 2.70 cm Ϯ 0.23, P Ͻ .001) was 25% larger with microwave ablation and mean cross-sectional area (8.25 cm 2 Ϯ 0.92 vs 5.45 cm 2 Ϯ 1.14, P Ͻ .001) was 50% larger with microwave ablation, compared with RF ablation. With microwave ablation, the zones of ablation were also significantly more circular in cross section (mean circularity, 0.90 Ϯ 0.06 vs 0.82 Ϯ 0.09; P Ͻ .05). One small pneumothorax was noted during RF ablation but stabilized without intervention. Conclusion:Microwave ablation with a 17-gauge high-power triaxial antenna creates larger and more circular zones of ablation than does a similarly sized RF applicator in a preclinical animal model. Microwave ablation may be a more effective treatment of lung tumors.

Standards of Practice: Guidelines for Thermal Ablation of Primary and Secondary Lung Tumors

“…Moreover, microwaves are not limited by tissue boiling, lower thermal conductivity of lung parenchyma, or increased impedance of charred tissues [25,59]. An experimental study comparing the relative effectiveness of microwave ablation versus RFA in the lung shows that coagulation produced by microwave was larger in diameter and more circular than the coagulative area achieved with a similar-sized radiofrequency applicator [60]. Microwave ablation for clinical uses generally operates with electromagnetic waves at frequencies ranging from 915 and 2,450 MHz, resulting in dielectric heating to cytotoxic levels through rapid rotation of water molecules [25].…”

Thermal Ablation of Lung Tumors: Focus on Microwave Ablation

“…In contrast, RFA is hampered by the high electrical resistivity of lung tissue which limits energy deposition. Tissue changes caused by ablation, such as carbonization and desiccation, also increase tissue resistivity, thus further hindering the expansion of the ablation zone [33,34]. The high electrical resistivity of a ventilated lung and the ablation-induced tissue inhomogeneity make the expansion of the ablation zone particularly reliant on thermal conduction, especially at its periphery.…”

“…1 -4) [33,55,58,59]. If contrast agent is administered, the outer rim usually shows circular benign periablational enhancement [60].…”

Thermal Ablation of Malignant Lung Tumors

“…This procedure can generate higher temperatures in larger zones than RFA. Zones of ablation up to 50% larger than with RFA have been achieved in animal testing (19 …”