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Radio Frequency (RF) ablation has become of considerable interest as a minimally invasive treatment for primary and metastatic liver tumors. Major limitations are small lesion size, which make multiple applications necessary and incomplete killing of tumor cells, resulting in high recurrence rates. RF ablation is typically carried out in the frequency range of around 500 kHz. Measurements have shown a marked difference in electrical conductivity between normal liver tissue and tumor tissue. The difference is most pronounced at frequencies below 100 kHz, where tumors exhibit around two times higher conductivity compared to normal tissue. Conductivity is similar for tumor and normal liver at 500 kHz. We created Finite Element Method (FEM) models of the RITA model-30 multi-prong probe. The probe was placed in a tumor of 20 mm diameter. We simulated 12 minute, temperature controlled RF ablation at 95°C at frequencies of 20 kHz and 500 kHz. At 20 kHz we observed increased current density within the tumor boundaries. This resulted in an increase in lesion size by 29% at a frequency of 20 kHz compared to 500 kHz.