Z. Liu, M. Ahmed, A. Sabir, S. Humphries, S..N. Goldberg; Boston/US

Purpose
To determine and characterize the effect of both tumor and background tissue perfusion on RF ablation heating and ablation time with different electrodes for a range of tumor sizes.

Material and methods
Computer simulation of RF heating using 2-D and 3-D finite element analysis (Etherm) was performed using a two-compartment model. Systematic simulated RF application (modeled on clinically relevant application parameters: 5 - 30 minutes, 2,000 mA maximum generator output) was applied for a range of inner tumor perfusion (0-5 kg/m³-s) and outer normal surrounding tissue perfusion (0-5 kg/m³-s) for internally-cooled 3 cm single and 2.5 cm cluster electrodes for a range of simulated tumor sizes (diameters 2-5 cm) (n=432 parameters). Tissue heating patterns and the time required to achieve complete ablation (tumor heating >50°C for the tumor ± a 5 or 10 mm margin) were assessed. Three dimensional surface response contours were generated, and linear and non-linear curve-fitting was performed.

Results
For both 3 cm single and 2.5 cm cluster electrodes, increasing overall tissue perfusion from 0 to 5 kg/m³-s exponentially decreased the overall coagulation size for all tumor sizes (R²=0.94). Equally as important, increasing overall perfusion over this range exponentially decreased the time required to achieve thermal equilibrium/maximum tumor ablation (R²=0.94). Thus, increasing tissue perfusion from 0 to 3.34 kg/m³-s decreased the 50ºC isotherm from 6.0 to 2.8 cm and 7.6 to 4.0 cm for a 3 cm single and 2.5 cm cluster electrode, respectively. However, the time to achieve these isotherms was 2430 and 2260 sec at 0 kg/m³-s, but only 481 and 392 sec, for 3.34 kg/m³-s. Furthermore, the relative effect of inner and outer perfusion varied with increasing tumor size in a two compartment model. At smaller tumor sizes (2 cm in diameter for a 3 cm single electrode and 2–3 cm in diameter for the cluster electrode), the ability and time to achieve tumor ablation was largely determined by the outer perfusion value. However, for larger tumors (4-5 cm in diameter for single and 5 cm diameter for cluster), the inner tumor perfusion had the predominant effect on the time to achieve maximum ablation. A mixed pattern (i.e. both tumor and background tissue perfusion having an effect on the time to achieve ablation) was noted for 3 cm tumors using a single electrode and 4 cm tumors using a cluster electrode.

Conclusion
Computer modeling demonstrates that blood flow reduces not only RF coagulation but also the time to achieve thermal equilibrium. Accordingly, shorter ablation times could be contemplated for higher perfusion tumors. These results further show the importance of considering not only tumor blood flow, but also size (in addition to background tissue blood flow) when attempting to predict the effect of perfusion on RF heating and ablation times.