This study demonstrates that microwave ablation with high-power triaxial antennas may be used to create larger zones of ablation than RF in an in vivo porcine kidney model. For a given number of applicators, ablations with triaxial antennas were significantly larger than RF ablations. In addition, ablation zones created using one or two triaxial antennas were comparable to those created using three RF electrodes. In other words, a given volume of tissue can be coagulated with fewer microwave antennas than RF electrodes. The faster heating and higher tissue temperatures achieved with microwaves is considered to be an important advantage when compared with RF ablation.
The superior performance of microwave ablation compared with RF ablation in the kidney may be attributed to the different mechanisms of heating of the two modalities. RF ablation is based on resistive heating caused by electrical current, which only dominates within approximately 1 cm of the electrode. Tissue further away from the electrode is heated via thermal conduction, making it more susceptible to perfusion-mediated cooling. In addition, treatment temperatures must be kept under 100 °C to prevent charring and impedance increases which limit RF energy deposition. On the other hand, microwave ablation is based on dielectric heating, and active tissue heating dominates in tissue up to 2 cm away from the antenna. Microwaves can also heat tissue an order of magnitude faster than RF and are not hindered by charring. Therefore, temperatures can be driven considerably higher, which creates a larger thermal gradient and increases thermal conduction. Given the high water content and perfusion rate of kidneys, microwave ablation may actually be more effective in the kidney when compared with other less hydrated organs such as the liver.
The triaxial microwave ablation system utilizes a resonant design to limit reflected power, thereby enabling higher input powers and maximizing energy deposition in tissue. Importantly, the design does not increase the physical dimensions above that of the single RF electrode (17-gauge). Therefore, the triaxial system can be used to increase the volume of tissue coagulated without increasing the invasiveness of the procedure. Also, arrays of triaxial antennas can be safely applied to further increase the volume of coagulation, as demonstrated in this study and previous studies in liver and lung (13
). Several multiple-antenna ablations were performed in this study, but were excluded from the statistical analysis because fewer samples were collected given that the size of the kidneys severely limited the growth of the ablation zones. Two- or three-antenna configurations allowed ablation of a significant portion of the kidneys and should be reserved for larger tumors given the possibility of greater complications with larger ablation zones if inappropriately applied.
Hope et al recently characterized the performance of a 915 MHz microwave ablation system in an in vivo porcine kidney model (20
). They established an optimal time (10 min) and power (45 W) combination, which resulted in ablation zones with a mean diameter of 2.0 cm. Longer ablation times (20 min) and larger powers (60 W) did not significantly increase the size of the ablation zone. In contrast, our study found that applying 90 W for 12 min resulted in much larger ablation zones (mean diameter, 3.6 cm). The power delivered to the tissue is a function of the generator output power, the losses within the coaxial cables feeding the power from the generator to the antenna, and the antenna design. As indicated above, the triaxial antenna used in our study was designed to minimize power reflected at the antenna-tissue interface, which maximizes the amount of input power deposited in tissue. In addition to increasing power delivered to the tissue, minimizing reflected power also enables the use of smaller antennas that are safer for percutaneous use—the triaxial antenna is 17 gauge compared to the 13-gauge antenna used by Hope et al.
Clinical experience with microwave ablation for kidney tumors is limited, but early results are promising. Clark et al performed microwave ablations in ten patients who subsequently underwent a nephrectomy (21
). They reported mean ablation zone dimensions of 4.1 × 2.7 × 2.2 cm using a 13-gauge antenna and a generator output of 60 W. In addition, histological analysis demonstrated uniform cell death within the ablation zone. Liang et al recently reported the results of a feasibility study in which they performed ultrasound-guided microwave ablation on 12 patients with renal cell carcinoma (22
). Complete tumor necrosis was achieved in all cases and no residual or recurrent tumor was detected at a median follow-up of 11 months.
A known complication of renal RF ablation is injury to the collecting system leading to obliteration of calyces or ureteral strictures (23
). While the underlying mechanism of injury is likely to be largely thermal, there may also be an electrical component given the electrolytic content of urine and the physics of RF ablation (ie, electrical current flowing through electrolytic tissue). Given that microwave ablation does not rely on electrical current flow, it may decrease the risk of damage to the collecting system during renal ablation. Further studies are needed to characterize the underlying mechanism of calyceal injury and to determine whether one modality is safer than the other.
The most significant limitation of this study was the use of an open surgical approach in a normal porcine kidney model. At the time of the study, a large animal kidney tumor model was not available. The highly vascular nature of renal cell carcinoma makes it likely that the results will translate to tumors. However, additional studies are still needed to optimize the performance of the microwave system in tumor models. Given the heterogeneous makeup of the kidney, the effects of antenna placement on coagulation size and shape also need to be investigated. Finally, fewer samples were collected in the multiple-antenna microwave groups because the small size of the kidneys severely limited the growth of the ablation zone. Further work characterizing the performance of multiple-antenna configurations is warranted.
In conclusion, microwave ablation with triaxial antennas created larger zones of ablation in normal porcine kidneys than RF ablation, and offers the potential to increase the effectiveness of tumor ablation for treating larger renal tumors. Microwave ablation may also reduce the invasiveness and complication rate associated with thermal ablation of kidney tumors by decreasing the size and number of applicators needed to achieve a given volume of coagulation.