The blood supply of the liver is enormous and blood has a high effective heat capacity. The heat delivered from the needle is therefore rapidly taken away and no cell damage occures after a specific distance from the needle. In fact the ablation zone fits pretty close the dimension of the prongs.

As you can see in the left picture taken during experiments with a liver model, there is a sharp boundary between dead tissue and unaffected surrounding tissue. Thus liver tumors can be ablated without much sacrifice of surrounding normal liver tissue