Embodiments herein relate to medical device systems including electric field shaping elements for use in treating cancerous tumors within a bodily tissue. In an embodiment, a medical device system for treating a cancerous tumor is described. The medical device system can include one or more electric field generating electrodes and an electric field shaping element configured to be implanted along with the one or more electric field generating electrodes. The electric field shaping element can be made from a material that alters the spatial area of tissue exposed to the electric field. Other embodiments are also included herein.