Purpose
Both radiofrequency (RF) and microwave (MW) ablation use applicators that produce most heat in the proximity of the applicator with little direct power deposition at larger distances. We created Finite Element computer models of a cooled RF needle electrode, and a dipole MW antenna. We determined relative contribution of direct power deposition by RF and MW, and compared to heating due to thermal conduction from the hot region close to the applicator.

Material and methods
We created axi-symmetric Finite Element computer models of the Cool-Tip needle electrode (17 gauge), and a dipole antenna (2.5 mm diameter). We simulated RF ablation for 12 min with power controlled to keep maximum tissue temperature at 100 ºC. We simulated MW ablation for 6 min with 75 W of power applied. For both models we considered change in electric and thermal tissue properties depending on tissue temperature. We considered temperature dependent tissue perfusion where perfusion ceased above 50 ºC. We determined tissue temperature profile at the end of the ablation procedure. In addition we calculated relative contribution of direct heating by RF or MW, thermal conduction, and perfusion to tissue heating.

Results
After 12 min RF ablation, maximum tissue temperature was 100 ºC and ablation zone radius was 15 mm. After 6 min MW ablation, maximum tissue temperature was 177 ºC and ablation zone radius was 22 mm. Initially direct heating is dominating everywhere for both RF and MW ablation. For MW ablation, direct heating due to dielectric losses is dominating up to a radius of 20 mm over the 6 min MW ablation procedure; further away thermal conduction and direct heating have similar contributions. For RF ablation, thermal conduction is dominating in the range from 12mm to 19mm from the electrode over the 12 min procedure, while direct heating due to resistive losses is dominating elsewhere. Tissue cooling due to perfusion is highest in the same region during RF heating, as the perfusion stops at 50 ºC and therefore perfusion losses are maximum near the ablation zone boundary where tissue temperatures just below 50ºC. This discontinuity in perfusion promotes high thermal gradients, and increased thermal flux near the ablation zone boundary.

Conclusion
During RF ablation thermal conduction contributes more towards tissue heating compared to MW ablation, especially in the region around the ablation zone boundary. In addition, tissue cooling due to perfusion is more significant during RF ablation, in part due to the longer treatment times. This may in part explain the superior performance of MW ablation close to large vessels compared to RF.