MW ablation is a promising ablation technology that is used extensively in Asia, but has not yet been widely applied in the United States. To date, only a single company (Vivant Medical, Mountain View, California) has FDA approval for MW ablation. The MW ablation mechanism of cell death is identical to that of RF, because heat is again utilized to denature and destroy cells. However, the transfer of energy is different and there are several theoretical advantages of MW over RF, which have not been optimized or fully explored to this point. MW ablation probes make use of electromagnetic waves (30-3000 MHz) that are identical to those used in a MW oven. These electromagnetic waves produce an electromagnetic field around the MW antenna and similar to RF, they produce ion agitation and heat. However, the electromagnetic field does not require an electrical circuit and is not dependent on direct physical contact for energy transfer. As a result, no grounding pads are required and theoretically, a much larger zone of active heating can be achieved (49,86). This also means that the transfer of energy is not affected nearly as much by the charring that occurs in the surrounding tissues. As a result, temperatures approaching 150°C can be achieved (87). Similar to cryoablation, multiple MW antennae can be placed simultaneously at the beginning of the procedure with the associated benefits discussed above. Overall, MW should theoretically achieve a more uniform and predictable zone of necrosis than RF; however, there are still significant technological and procedural limitations to be overcome.
Unfortunately, with current technology, the probe sizes remain quite large and the ablation sizes are relatively small. The most widely utilized system (Microtaze, Azwell, Osaka, Japan) uses a 14-gauge antenna, which is introduced through a 13-gauge cannula. Despite the large size of the probe, this system is only capable of producing a 1.6 cm diameter zone of ablation (88). Obviously technical improvements need to be made, and with this goal in mind, several other MW systems are currently in development (Vivant Medical, Mountain View, California; Microsulis, Ltd., Bath, England). As a result of these limitations, percutaneous image-guided MW ablation has only been applied on a limited basis for the treatment of RCC. MW has been used as an intraoperative nephron-sparing surgical technique for renal tumor enucleation in Asia (89). The results thus far have been comparable to other nephron-sparing surgical techniques. Although MW holds much promise as an ablation modality and rapid improvements in the technology are expected, it is not yet ready for widespread clinical application in the treatment of RCC.
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