Treatment planning considerations for IRE in the lung: placement of needle electrodes is critical

Abstract

Purpose The poor electrical conductivity of the lung parenchyma, the difference in conductivity between tumor and healthy tissue and the absence of strong post-ablation imaging findings makes it difficult to plan and deliver irreversible electroporation (IRE) treatment in this organ. We use numerical modeling and simulations to arrive at guidelines for planning IRE ablations in the lung. Materials and Methods A numerical model of the Laplace equation for electric field distribution in biological tissue was constructed using Comsol Multiphysics as a 1.5 cm diameter spherical tumor surrounded by normal lung tissue. Simulations were performed considering use of two treatment needles. Two variables were given consideration for their effect on outcomes. First, the ratio of electrical conductivity between tumor and normal lung (conductivity:-0.1 S/m) tissue was varied between 3 and 10 times that of the latter. Second, the location of the two treatment needles were varied with respect to each other, where both were placed within the tumor, both were placed in healthy tissue but closely bracketing the tumor, and finally one needle was placed within the tumor while the other was located outside it. Treatments were simulated at field strength of 1500 V/cm. The critical ablation zone was considered to be 600 V/cm and used to demarcate the ablated zone. Concurrently, retrospective clinical cases were analyzed to determine similar needle placement conditions and the corresponding conductivity changes reported by the IRE console. Results Simulations indicate the strong role played by the conductivity ratio of tumor and surrounding healthy tissue on ablation outcome. This factor is normally not considered in the treatment planning software available on the clinically used IRE console. Second, the relative location of the needles played an important role in ablation outcomes, with needle electrodes placed in healthy tissue causing incomplete treatment in the surrounding region. Finally, it was found that the conductivity changes reported by the console are not an independent indicator of treatment success. Conclusion Simulation modeling can be used to assist treatment planning for application of IRE to ablate tumors in the lung.