Study of calculating method for ablation area and program for electrode placement by ultrasound-guided radio-frequency ablation of large liver tumors

Objective To investigate the relationship among diameter of treated area,the expendable size of the ablation electrode, and number of overlapping ablations and to design the program for electrode placement by the ultrasound-guided radio-frequency ablation(RFA) of large liver tumor.Methods Two mathematical models were created for preoperative planning of the ablation in spherical tumors. The models were based on the overlapping sphere principle and were deduced and calculated by the regular prism way and regular polyhedron way. A map was achieved in detail for the optimal placement of ablation electrode. According to assumption of complete destruction of tumor and its 0.5 cm margin by 5.0 cm ablation device with this method, calculating results were obtained as follows: a tumor of 4.1 ～ 4.3 cm in diameter required 4 ablation at least by the regular tetrahedron way, a tumor of 4.4 ～ 5.6 cm in diameter required 5～8 ablation at least by the regular prism way, a tumor of 5.7 ～ 6.0 cm required 12 ablation at least by the three layers overlapping way. The treatment plan for ellipsoidal and irregular tumors were also mentioned. Results In clinic, 84.4 % of tumor which larger than 3.5 cm in diameter achieved complete ablation.Conclusions To those large tumors which were bigger than one single ablation could cover, this method can provide an effective plan of complete ablation.