Electric field delivery and ablation of target tissue regions, including cancerous cells and solid tumors. Methods and systems include delivering an electric field to a target tissue, and may include positioning a first electrode or plurality to at least partially define a first treatment volume in the target tissue; positioning a second electrode or plurality to at least partially define a second treatment volume, the first volume is disposed in the second volume; and establishing a first current flow extending through the first volume and a second current flow extending through the second volume.