Microwave ablation technology avoids problems that plague RFA, offers promise for new applications

By Christopher L. Brace, Ph.D.
Assistant scientist
Departments of radiology and engineering
Principal investigator
Department of radiology
University of Wisconsin at Madison
The rapidly expanding field of tumor ablation now includes a variety of heat-based ablation modalities using laser, microwave, radiofrequency, and ultrasound energy sources. Among these, RF ablation has found the greatest clinical utility worldwide.

RFA devices are minimally invasive (14 to 17 gauge is typical). They provide zones of ablation large enough to treat primary and secondary tumors of the liver, lung, kidney, and bone, and new systems allow multiple-electrode operation in monopolar or bipolar modes. The multiple-electrode system based on switching has allowed both treatment of larger tumors and "simultaneous" treatment of multiple tumor sites using a relatively short treatment time.Even as RFA technology continues to advance, several drawbacks inherent to RF heating loom. RF heating is a result of electrical current flow through the ionic fluid that permeates tissue, typically between an electrode and ground pad. The high current density adjacent to the electrode generates enough heat to raise tissue temperature and cause cell death. As the temperature approaches 100°C, however, water in the tissue begins to vaporize and the electrical conductivity of the tissue rapidly decreases. This causes the familiar impedance spike or "rolloff" that precedes the end of active heating. While multiple-electrode RFA systems exist, they require current to be switched between electrode, so each electrode is active less than 100% of the time. Off-time allows the tissue to rehydrate and reestablish an electrical current path but means that procedure time is wasted. RFA has also been plagued by high local recurrence rates in perivascular regions, probably due to the inability of RF current to generate heat faster than it can be drawn away by flowing blood. This is known as the "heat-sink effect." Finally, with the exception of bipolar or multipolar systems, RFA still requires ground pads.Microwave ablation shares the advantages of RFA but offers several additional benefits due to the physics of microwave heating:

• faster heat generation over a larger volume of active heating
• true simultaneous, multiple-antenna capability with each antenna active 100% of the time
• less susceptibility to heat-sinks
• no ground pads

Active research to improve multiple-antenna techniques and device efficiency may add other benefits to this list.Despite these advantages, microwave ablation has remained in the shadow of RFA. This may be due to insufficiencies in current microwave ablation systems, such as relatively large antenna diameter (about 13 to 14 gauge), unwanted heating along the proximal portions of the antenna that lead to tear-drop-shaped zones of ablation, and a perception that using multiple antennas requires multiple generators.

MICROWAVE PHYSICS

Microwave heating occurs through a process known as dielectric hysteresis. An applied electromagnetic (EM) field causes molecules with intrinsic dipole moments (such as water) to alternate along with the field. Because the dipoles cannot alternate as fast as the field, some of the microwave energy is converted to heat. At powers sufficient for microwave ablation, heat may be generated 100 times faster than in RFA. Unfortunately, the same heating process causes unwanted heating of the power distribution cables and proximal portions of the antenna. While this heating is less severe than in tissue, it has become the main engineering challenge to making a safe commercial product.The means to couple microwave energy into the tissue is an antenna. Unlike the RF electrode that couples electrical current to the tissue, antennas radiate EM fields, and microwave heating occurs in a larger volume around the antenna. The fields themselves are responsible for heat generation. This is beneficial for tissue ablation, because tissue desiccation does not inhibit EM field propagation. Thus, the impedance spike that cuts off RF heating does not affect microwave heating. Problematic tissues for RFA - normal lung, cystic fluid, bone, - may therefore be more effectively heated with microwaves. EM fields from separate antennas are relatively independent, so multiple antennas may be operated at the same time without the need for switching. Microwave power may be easily divided, allowing any number of antennas to be driven from a single high-power source.

MICROWAVE ABLATION ADVANTAGES

Traditional RFA applications, such as liver and kidney tumors, will also be well suited for microwave ablation. But the applications of microwave ablation may extend beyond what can be treated with RF. RFA has been less efficacious, for example, in the lung than in solid organs. This is primarily due to the extremely low conductivity of normal lung tissue, which limits current flow and makes an ablative margin difficult to achieve. Thus, recurrences can occur near the tumor boundaries. Microwave propagation is actually better in normal lung, so tumor boundaries may be more effectively treated with microwaves. Similarly, microwave's volume heating ability may be more appropriate for bone therapies, although growing evidence supports cryoablation as more advantageous in bone.Cystic lesions have also presented challenges for RF ablation. The higher thermal and electrical conductivity of cystic fluid allows current to flow through the mass without effectively heating it or retaining the heat generated. Microwave heating of cystic masses may actually be enhanced due to their higher conductivity. Since heat can be generated faster than with RF, it may be fast enough to overcome any thermal sinking.Perivascular tumors have been associated with higher recurrence rates with RFA. This is likely due to the inability of RF to heat cells nearest to the vessel to temperatures sufficient to induce cell death. The vessel simply sinks heat away faster than it can be generated. Microwave heating provides faster heat generation and seems to be less susceptible to thermal sinks. In some cases, preferential heating of the perivascular space has been observed. This makes microwave ablation a good candidate for tumors located near major vessels.

FUTURE OF MICROWAVE ABLATION

Despite all of these potential advantages, only one device has been FDA approved for microwave tumor ablation (Vivant Medical/Valleylab, Boulder, CO). This system is not currently available but may be released in the near future. Other devices are in the early stages of development as microwave ablation research continues to grow. Microwave ablation may become increasingly significant in the next two to five years, depending on commercial development efforts and receptiveness of the clinical community to this new and exciting technology. Until then, researchers will continue to refine the devices and techniques for microwave ablation in preclinical settings, work that we hope will lay the foundation for a new era in tumor ablation.

BIBLIOGRAPHY

Brace CL, Laeseke PF, Sampson LA, et al. Microwave ablation with small-gauge triaxial antennas. Using multiple antennas to create large volumes of ablation in vivo. Radiology: accepted for publication.Brace CL, Laeseke PF, Sampson LA, et al. Microwave ablation with small-gauge triaxial antennas. Single-antenna ablation in an in vivo porcine liver model. Radiology: accepted for publication.Brace CL, Laeseke PF, van der Weide DW, Lee FT. Microwave ablation with a triaxial antenna: Results in ex vivo bovine liver. IEEE Transactions on Microwave Theory and Techniques 2005;53:215-220.Cioni R, Armillotta N, Bargellini I, et al. CT-guided radiofrequency ablation of osteoid osteoma: long-term results. Eur Radiol 2004;14:1203-1208.Davis KW, Choi JJ, Blankenbaker DG. Radiofrequency ablation in the musculoskeletal system. Semin Roentgenol 2004;39:129-144.Duck FA.

Physical properties of tissue: A comprehensive reference book.

London: Academic Press, 1990.Hines-Peralta AU, Pirani N, Clegg P, et al. Microwave ablation: results with a 2.45-ghz applicator in ex vivo bovine and in vivo porcine liver. Radiology 2006; Feb 16 [Epub ahead of print] Laeseke PF, Sampson LA, Brace CL, et al. Multiple-electrode RF ablation: Simultaneous production of separate zones of coagulation in an in vivo porcine liver model. J Vasc Interv Radiol 2005;16(12):1727-1735.Laeseke PF, Sampson LA, Haemmerich D, et al. Large volume multiple-probe RF ablation: results in in vivo porcine liver. Radiology 2005: accepted.Lee FT Jr., Haemmerich D, Wright AS, et al. Multiple probe radiofrequency ablation: pilot study in an animal model. J Vasc Interv Radiol 2003;14:1437-1442.Simon CJ, Dupuy DE. Current role of image-guided ablative therapies in lung cancer. Expert Rev Anticancer Ther 2005;5:657-666.Simon CJ, Dupuy DE, Mayo-Smith WW. Microwave ablation: principles and applications. Radiographics 2005;25 Suppl 1:S69-83.Wright AS, Lee FT, Jr., Mahvi DM. Hepatic microwave ablation with multiple antennae results in synergistically larger zones of coagulation necrosis. Ann Surg Oncol 2003;10:275-283.Wright AS, Sampson LA, Warner TF, et al. Radiofrequency versus microwave ablation in a hepatic porcine model. Radiology 236(1):132-139.Yu NC, Lu DS, Raman SS, et al. Hepatocellular carcinoma: microwave ablation with multiple straight and loop antenna clusters - pilot comparison with pathologic findings. Radiology 2006.