Radiofrequency thermal ablation: Computer analysis of the size of the thermal injury created by overlapping ablations

OBJECTIVE. The purpose of this study was to perform a computer analysis of the size of the thermal injury created by overlapping multiple thermal ablation spheres. MATERIALS AND METHODS. A computer-assisted design system was used to create three-dimensional models of a spherical tumor, a spherical tissue volume consisting of the tumor plus a 1-cm tumor-free margin, and individual spherical ablations. These volumes were superimposed in real-time three-dimensional space in different geometric relationships. The effect of the size and geometric configuration of the ablation spheres was analyzed with regard to the ability to ablate the required volume of tissue (tumor plus margin) without leaving untreated areas or interstices. RESULTS. The single-ablation model showed that if a 360° 1-cm tumor-free margin is included around the tumor targeted for ablation, radiofrequency ablation devices producing 3-, 4-, and 5-cm ablation spheres can be used to treat 1-, 2-, and 3-cm tumors, respectively. The six-sphere model, in which six ablation spheres are placed in orthogonal planes around the tumor, showed that the largest tumor that may be treated with a 3-cm ablation device is 1.75 cm, whereas 4- and 5-cm ablation spheres can be used to treat tumors measuring 3 and 4.25 cm, respectively. The 14-sphere model showed that addition of eight more spheres to the six-sphere model increased the treatable tumor size to 3, 4.6, or 6.3 cm, depending on the diameter of the ablation sphere used. For treating larger tumors, we found a cylindrical model to be less efficient but easier to control. CONCLUSION. Our computer analysis showed that the size of the composite thermal injury created by overlapping multiple thermal ablation spheres is surprisingly small relative to the number of ablations performed. These results emphasize the need for a methodic tumor ablation strategy.