Electric fields with certain characteristics have been shown to be effective at inhibiting the growth of cancer cells (and other rapidly dividing cells). However, when the cancer is located in a target region beneath the surface of a body, it can be difficult to deliver the beneficial fields to the target region. This difficulty can be surmounted by positioning a biocompatible field guide between the surface of the body and the target region, positioning electrodes on either side of the field guide, and applying an AC voltage with an appropriate frequency and amplitude between the electrodes. This arrangement causes the field guide to route the beneficial field to the target region. In an alternative embodiment, one of the electrodes is positioned directly on top of the field guide.