Effects of microwave ablation of the kidney

Introduction: Thermal ablative therapy with cryotherapy and radiofrequency ablation is currently being utilized for treatment of small renal masses. Recently, microwave ablation (MWA) has been investigated for soft tissue destruction, with limited clinical experience for renal masses. This study was designed to evaluate the effectiveness of a new microwave system in the porcine kidney. Materials and Methods: MWAs were performed in vivo on six pigs using the Evident Microwave Ablation System (Valleylab, Boulder, CO) with a 3.7cm active tip percutaneous antenna. Ablations were performed in both upper and lower poles at 45 W for 3, 5, and 10 minutes. Three pigs died; three pigs survived for 7 days. After necropsy the kidneys were harvested, ablation zones were measured, and histological analysis was performed. Results: The ablated zones at each time interval were inconsistent. The gross appearance of the lesions in both groups was similar. The antenna tract was charred, the collecting system was damaged, and there was asymmetry of the zones of ablation. Histological analysis revealed coagulative necrosis in the area of the ablation with sloughed and denuded urothelium. Conclusion: MWA of the kidney utilizing this model yields inconsistent geometrical lesions when applied near the renal collecting system. Additionally, microwave energy may be preferentially absorbed by the increased water content in the collecting system resulting in damage to it. Further study is needed to assess the attenuation of the microwave field in the kidney at different locations due to the kidneys heterogeneity.