In RFA, a high-frequency alternating electrical current (375–500 kHz) is used to create ionic agitation, which produces frictional heat and heat conduction to achieve subsequent tissue necrosis [25, 26]. MWA uses electromagnetic energy to rapidly rotate adjacent polar water molecules in the surrounding tissue, which flip back and forth 2–5 billion times a second depending on the frequency of the microwave energy. The rapid friction produces heat, thus inducing cellular death via coagulation necrosis. Ionic polarization as another mechanism responsible for heat generation is a far less important mechanism in living tissue.

The difference between MWA and RFA characteristics are summarized as follows (Table 16.2): (1) the active tissue heating of RFA is limited to a few millimeters surrounding the active electrode, with the remainder of ablation zone relying on the conduction of electricity into the tissue [26]. Microwave uses electromagnetic energy with the much broader field of power density (up to 2 cm surrounding the antenna) to rapidly rotate adjacent polar water molecules to achieve primarily active heating, which can get a much broader zone of active heating [27]. (2) RFA is limited by the increase in impedance with tissue boiling and charring [26], because water vapor and char act as electrical insulators. MWA does not seem to be subject to this limitation. Therefore, temperature greater than 100 °C is readily achieved [28]. (3) Owing to the active heating ability, MWA can achieve higher intratumoral temperatures, larger ablation volumes, and shorter ablation time [28–31]. Because the cooling effect of blood flow is most pronounced within the zone of conductive rather than active heating, MWA is less affected by blood vessel-mediated cooling (the heat-sink effect). These benefits have the potential to allow for a more uniform tumor kill in the ablation zone, both within the targeted zone and perivascular tissue [31, 32]. (4) MWA allows for simultaneously multiple probes deployment to reduce the duration of therapy and increase the diameter of ablation zone [26–28]. (5) MWA does not require the placement of grounding pads, and the electrical energy is deployed in the target tissue only, which avoids applied energy losing and skin burns. Moreover, MWA is not contraindicated by the metallic materials like surgical clips or pacemaker.

For the comparison of two techniques in HCC, only a limited number of studies have been performed, most of which are retrospective studies and all with a relatively small samples and short-term follow-up (Table 16.3). The only reported randomized controlled trial was performed by Japanese researches in 2002 [33]. Most of the researches showed MWA and RFA achieved similar tumor necrosis effect and survival [34–37]. But Japanese researchers thought RFA had tumor control advantage in small liver lesions [38, 39]. However, randomized controlled trial with large sample and long-term follow-up is lacking and strongly recommended to provide evidence of evidence-based medicine.