Computer modeling of factors that affect the minimum safety distance required for radiofrequency ablation near adjacent nontarget structures.

PURPOSE: To use computer modeling of radiofrequency (RF) ablation to evaluate the effects of (i) composition and varying perfusion of intervening tissue and (ii) electrode orientation and type on the required distance to avoid heating damage of adjacent nontarget structures. MATERIALS AND METHODS: Systematic three-dimensional finite-element computer simulation of RF heating (6-20 minutes) was performed (3,128 simulations). The distance (5-25 mm) between the electrode and the potentially injured structure and tissue composition as layers of tumor/soft tissue, fat, and/or fluid was varied (thermal conductivity, 0.46, 0.23, and 0.7 W/m- degrees C; electrical conductivity, 0.5, 0.1, and 1 S/m, respectively). Varying perfusion (0-5 kg/m(3)-s), electrode orientation (parallel or perpendicular), and electrode type (ie, noncooled and internally cooled 3-cm single or 2.5-cm cluster) were also studied. The time required to reach various temperatures (eg, the time to reach 50 degrees C designated as t50) and the distances at which the temperatures occurred and the distances required to avoid threshold temperatures at the margin of adjacent structures were compared. RESULTS: In all cases, increasing the amount of intervening fat increased t50 compared with tumor/soft tissue and/or fluid. With no perfusion, 9 mm of fat or 14 mm of tumor/soft tissue or fluid was required for perpendicular insertion (internally cooled single electrode) to prevent a temperature of 50 degrees C with 12 minutes of heating, compared with 12 mm of fat or 23 mm of tumor/soft tissue or fluid for parallel insertion. Less intervening fat was needed for noncooled electrodes (<8 mm parallel, <5 mm perpendicular), with more intervening tissue required for cluster electrodes (>13 mm) for an RF application of 20 minutes. Finally, the amount of intervening tissue required to prevent damage also decreased linearly with increasing perfusion for each tissue and electrode (r(2) = 0.74 for parallel; r(2) = 0.98 for perpendicular). CONCLUSIONS: In the computer model described in the present study, thermal and perfusion characteristics between the electrode and adjacent nontarget structures (specifically the presence of fat) and the electrode characteristics themselves (including parallel versus perpendicular insertion) have been shown to affect the minimum safe distance required for the prevention of thermal injury.

Computer modeling of the combined effects of perfusion, electrical conductivity, and thermal conductivity on tissue heating patterns in radiofrequency tumor ablation.

PURPOSE: To use an established computer simulation model of radiofrequency (RF) ablation to characterize the combined effects of varying perfusion, and electrical and thermal conductivity on RF heating. METHODS: Two-compartment computer simulation of RF heating using 2-D and 3-D finite element analysis (ETherm) was performed in three phases (n = 88 matrices, 144 data points each). In each phase, RF application was systematically modeled on a clinically relevant template of application parameters (i.e., varying tumor and surrounding tissue perfusion: 0-5 kg/m(3)-s) for internally cooled 3 cm single and 2.5 cm cluster electrodes for tumor diameters ranging from 2-5 cm, and RF application times (6-20 min). In the first phase, outer thermal conductivity was changed to reflect three common clinical scenarios: soft tissue, fat, and ascites (0.5, 0.23, and 0.7 W/m- degrees C, respectively). In the second phase, electrical conductivity was changed to reflect different tumor electrical conductivities (0.5 and 4.0 S/m, representing soft tissue and adjuvant saline injection, respectively) and background electrical conductivity representing soft tissue, lung, and kidney (0.5, 0.1, and 3.3 S/m, respectively). In the third phase, the best and worst combinations of electrical and thermal conductivity characteristics were modeled in combination. Tissue heating patterns and the time required to heat the entire tumor +/-a 5 mm margin to >50 degrees C were assessed. RESULTS: Increasing background tissue thermal conductivity increases the time required to achieve a 50 degrees C isotherm for all tumor sizes and electrode types, but enabled ablation of a given tumor size at higher tissue perfusions. An inner thermal conductivity equivalent to soft tissue (0.5 W/m- degrees C) surrounded by fat (0.23 W/m- degrees C) permitted the greatest degree of tumor heating in the shortest time, while soft tissue surrounded by ascites (0.7 W/m- degrees C) took longer to achieve the 50 degrees C isotherm, and complete ablation could not be achieved at higher inner/outer perfusions (>4 kg/m(3)-s). For varied electrical conductivities in the setting of varied perfusion, greatest RF heating occurred for inner electrical conductivities simulating injection of saline around the electrode with an outer electrical conductivity of soft tissue, and the least amount of heating occurring while simulating renal cell carcinoma in normal kidney. Characterization of these scenarios demonstrated the role of electrical and thermal conductivity interactions, with the greatest differences in effect seen in the 3-4 cm tumor range, as almost all 2 cm tumors and almost no 5 cm tumors could be treated. CONCLUSION: Optimal combinations of thermal and electrical conductivity can partially negate the effect of perfusion. For clinically relevant tumor sizes, thermal and electrical conductivity impact which tumors can be successfully ablated even in the setting of almost non-existent perfusion.

Radiofrequency tumor ablation: insight into improved efficacy using computer modeling.

OBJECTIVE: To use computer modeling of the Bio-Heat equation to demonstrate factors influencing RF ablation tissue heating. CONCLUSION: Computer modeling demonstrates the importance of energy deposition, tumor and background tissue electrical and thermal conductivity, and perfusion on RF ablation outcomes.