Embodiments herein relate to medical devices and methods for using the same to treat cancerous tumors within a bodily tissue. A medical device system is included having at least one electric field generating circuit configured to generate one or more electric fields; 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 at least one electric field generating circuit; and two or more electrodes to deliver the electric fields to the site of a cancerous tumor within a patient. At least one electrode can be configured to be implanted. At least one electrode can be configured to be external. The control circuitry can cause the electric field generating circuit to generate one or more electric fields at frequencies selected from a range of between 10 kHz to 1 MHz.