A medical device (300; 400; 600; 700; 800) for treating a cancerous tumor comprising: an electric field generating circuit configured to generate one or more electric fields at or near a site of the cancerous tumor; control circuitry in communication with the electric field generating circuit, the control circuitry configured to control delivery of the one or more electric fields from the electric field generating circuit; wherein the control circuitry causes the electric field generating circuit to generate one or more electric fields having a field strength selected from a range of from 0.25 V/cm to 1000 V/cm; a first supply wire in communication with a first supply electrode and a second supply wire in communication with a second supply electrode, the first and second supply wires (202, 214) in electrical communication with the electric field generating circuit, wherein a current generated by the electric field generating circuit flows through the first supply wire and the second supply wire, wherein the first and second supply electrodes (206, 210) are configured to deliver an electric field at or near the site of the cancerous tumor; and a first sensing wire in communication with a first sensing electrode and second sensing wire in communication with a second sensing electrode, the first and second sensing wires (224, 234) in electrical communication with the control circuitry, wherein substantially none of the current generated by the electric field generating circuit flows through the first sensing wire and the second sensing wire; and wherein the first and second sensing electrodes (228, 230) are configured to measure an impedance of the cancerous tumor at at least two different electric field strengths during application of the one or more electric fields. an electric field generating circuit configured to generate one or more electric fields at or near a site of the cancerous tumor; control circuitry in communication with the electric field generating circuit, the control circuitry configured to control delivery of the one or more electric fields from the electric field generating circuit; wherein the control circuitry causes the electric field generating circuit to generate one or more electric fields having a field strength selected from a range of from 0.25 V/cm to 1000 V/cm; a first supply wire in communication with a first supply electrode and a second supply wire in communication with a second supply electrode, the first and second supply wires (202, 214) in electrical communication with the electric field generating circuit, wherein a current generated by the electric field generating circuit flows through the first supply wire and the second supply wire, wherein the first and second supply electrodes (206, 210) are configured to deliver an electric field at or near the site of the cancerous tumor; and a first sensing wire in communication with a first sensing electrode and second sensing wire in communication with a second sensing electrode, the first and second sensing wires (224, 234) in electrical communication with the control circuitry, wherein substantially none of the current generated by the electric field generating circuit flows through the first sensing wire and the second sensing wire; and wherein the first and second sensing electrodes (228, 230) are configured to measure an impedance of the cancerous tumor at at least two different electric field strengths during application of the one or more electric fields.