P.M. MERTYNA¹, Z. Liu¹, T. Brody¹, M.W. Dewhirst², S..N. Goldberg¹; ¹Boston/US, ²Durham/US

Purpose
To determine the critical thermal dosimetry necessary to create RF - induced coagulation in an ex-vivo tissue model.

Material and methods
Ablations were performed in ex-vivo bovine liver using a 500-kHz monopolar RF generator (model CC-1; Valleylab). A total of 240 RF trials were performed (n = 114 for 2 cm internally cooled electrodes [Valleylab]; n = 126 for cluster electrodes). Real time temperatures were monitored continuously during the application of RF by thermocouples at 10, 15, 20 and 25mm away from the active electrode. For each electrode, RF energy was applied using a constant current ranging from 800 to 1600mA varied in 100 mA intervals. Duration of RF ablation was also varied 3 – 60 min to achieve defined coagulation diameters (20 to 50 mm). Recorded temperatures and coagulation diameters were used for mathematical calculations of thermal endpoints: 1) maximum temperature at the margin of ablation, 2) total amount of heat delivered to tissues during RF trials (AUCs), and 3) Cumulative Equivalent Minutes at 43°C (CEM₄₃). These endpoints were correlated with ablation size and analyzed as functions of current delivered to the tissue.

Results
The total amount of heat needed to achieve ablation, (AUC), varied with RF current as a negative exponential function. R2 ranged from 0.82 - 0.97 for each electrode and diameter of coagulation. The relationship between duration of RF application and current for each coagulation diameter was also best expressed as negative exponents with an even tighter fit (R² 0.92 - 0.99). Nevertheless, the temperatures achieved at the margin of ablation at the end of RF application were not constant, but ranged from 33°C - 76°C. Indeed, linear functions for each of the electrodes and coagulation diameters best described the “critical ablation temperature”’. The analysis of CEM43 values similarly was not constant, and showed an exponential relationship (R² 0.52 - 0.76) to current and distance, albeit with very wide range of values (10¹²).

Conclusion
In this ex-vivo model, different thermal doses were required to achieve coagulation of specified diameter. If verified for other tissues, this unexpected relationship of variable, current-dependent and distance-dependent thermal dosimetry would suggest that RF ablation may not be due to purely “thermal” effects. Thus, ablation may involve other interactions with tissue – quite possibly including electromagnetic properties of the RF field.