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Radiofrequency Tumor Ablation: Insight into Improved Efficacy Using Computer Modeling

¹ Department of Radiology, Beth Israel Deaconess Medical Center, 1 Deaconess Rd., WCC 308B, Boston, MA 02215.
² Department of Electrical Engineering, University of New Mexico in Albuquerque, Albuquerque, NM.

View larger version (35K): [in a new window]   Fig. 1A. —ETherm computer simulation model template [5]. These computer simulations were used to generate heating profiles for 12 min of radiofrequency application using 3-cm tip, 17-gauge internally cooled electrode and 2,000-mA output generator. Radiofrequency electrode (blue line) is centered within variable-sized cylinder (colored rectangles, arrow) representing tumors of variable radius. Inner compartment radius and electrical conductivity were varied (r = 5-30 mm and [I] = 0.07-14 S/m = siemens per meter, respectively) compared with background electrical conductivity ([O] = 0.12 S/m = siemens per meter) of liver [9].

View larger version (14K): [in a new window]   Fig. 1B. —ETherm computer simulation model template [5]. These computer simulations were used to generate heating profiles for 12 min of radiofrequency application using 3-cm tip, 17-gauge internally cooled electrode and 2,000-mA output generator. Schematic depicts electrical field from ETherm simulation.

View larger version (28K): [in a new window]   Fig. 1C. —ETherm computer simulation model template [5]. These computer simulations were used to generate heating profiles for 12 min of radiofrequency application using 3-cm tip, 17-gauge internally cooled electrode and 2,000-mA output generator. Schematic depicts ETherm thermal map presented at 12 min. Temperature at 20 mm from the midpoint of electrode (T2 cm, red X) and 50°C isotherm at midpoint of electrode were calculated and used to construct response surface contours such as those presented in following figures.

View larger version (39K): [in a new window]   Fig. 2. —Response contours of 50°C temperature isotherms. Effective radiofrequency energy for ablation is considered to be thermal dose of 50-54°C for 4-6 min. We therefore selected 50°C isotherm to allow standardized means of comparison. Figure represents color-coded schematic depiction of 50°C temperature isotherms versus distance from 3-cm internally cooled electrode. Blue represents parameters that can successfully heat 3-cm zone of ablation, whereas red denotes greater than 7 cm.

View larger version (53K): [in a new window]   Fig. 3A. —Inner electrical conductivity response contours. S/m = siemens per meter. Response contour represents 3D relationship of temperatures (T) 2 cm from electrode with varying inner electrical conductivity (I) and radius.

View larger version (43K): [in a new window]   Fig. 3B. —Inner electrical conductivity response contours. S/m = siemens per meter. Response contour depicts 50°C isotherms for varying inner electrical conductivity from computer simulations. For given tumor radius or tumor conductivity, increasing either conductivity or radius first increases heating but then can decrease heating because of limitations in generator output.

View larger version (37K): [in a new window]   Fig. 4. —Effect of background tissue conductivity. 3D response contours of 50°C isotherms illustrating interaction between tumor volume and inner and outer electrical conductivity are shown for three different inner conductivities. Significant interaction between inner and outer electrical conductivity on radiofrequency heating is shown. S/m = siemens per meter.

View larger version (50K): [in a new window]   Fig. 5. —Effects on radiofrequency heating by alteration of thermal conductivity. Surface response at 0.5 watts/m-°C most closely approximates empirically determined results in liver, whereas plot at higher thermal conductivities approximates results in agar phantoms. Increasing thermal conductivity of entire system (i.e., "tumor" and surrounding tissue) can achieve bigger ablations. Left shift in response surface is caused by both increased thermal conductivity and current limitation, where thermal conductivity increases until it becomes limited by current. For region to right of maximum, greater energy is needed to obtain larger ablations. S/m = siemens per meter.

View larger version (27K): [in a new window]   Fig. 6. —Effect of tissue perfusion on radiofrequency ablation. Range of perfusion states (0-10 kg/m3-sec) are presented. Hypervascular tumor perfusion acts as significant thermal sink at all tumor sizes and electrical conductivities. Height of surface contour lowers as perfusion increases from 0 to 10 kg/m3-sec, while peak only shifts slightly. Surface responses show that perfusion significantly affects radiofrequency heating. S/m = siemens per meter.

View larger version (36K): [in a new window]   Fig. 7. —Thermal response contours versus radiofrequency generator output. This figure illustrates effects on radiofrequency heating based on available radiofrequency generator output. As expected, larger volumes of ablation are possible with increasing radiofrequency generator output. Simulated data for 4,000-mA generator not only shows increase in radiofrequency heating but also stronger interaction under conditions of greater electrical conductivity within larger volume of tissue surrounding electrode than is evidenced for low- and medium-power generators. There is increased height of surface contour and peak shifts to right. Note larger color scale for this figure. S/m = siemens per meter.

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