Thermal Ablation:

Rationale and Objectives

To compare thermal dosimetry metrics for specified diameters of coagulation achieved using three different ablation energy sources.

Materials and Methods

204 ablations measuring 20, 30, or 40 ± 2 mm were created in an ex-vivo bovine liver model using 1) 2.5 cm cluster RF electrodes (n = 114), 2) 3 cm microwave antennas (n = 45), and 3) 3 cm laser diffusing fibers (n = 45). Continuous temperature monitoring was performed 5–20 mm from the applicators to calculate: a) the area under the curve (AUC), b) cumulative equivalent minutes at 43°C (CEM43), and c) Arrhenius damage integral (Ω) for the critical ablation margin (DOC), with results compared by multivariate analysis of variance and regression analysis.

Results

The end temperatures at the margin of coagulation varied, and was lowest for the RF cluster electrode (33–58°C), higher for laser (52–72°C), and covered the widest range for microwave (42–95°C). These end temperatures correlated with applied energy, as linear functions (r² = 0.74–0.96). The total heat needed to achieve ablation (AUC) varied with applied energy and coagulation diameter as negative exponential (RF and laser) or negative power (microwave) functions (r² = 0.82–0.98). Similarly, CEM43 values varied exponentially with energy and distance (r² = 0.52–0.76) over a wide range of values (10¹²). Likewise, Ω varied not only based upon energy source and DOC, but also as a positive linear correlation to applied energy and with sigmoid correlation to duration of ablation (r² = 0.85–0.97).

Conclusion

Our study demonstrates that the thermal dosimetry of ablation is not based solely on a fixed end temperature at the margin of the coagulation zone. Thermal dosimetry is not constant, but dependent on the type and amount of energy applied and distance suggesting the need to take into account the rate of heat transfer for ablation dosimetry.

Key Words: Thermal ablation, radiofrequency tumor ablation, thermal dose, image-guided intervention, hyperthermia