I have been through the wringer with the FDA in respect to this problem and am well aware of their considerations. I now sell zappers only for the purpose of killing some bacteria and protozoa in water that is not being used for humans.
While I have defeated skin cancer myself using an electro-therapy device, my reason for responding is to post a warning in regards to placing electrodes on or near the head.
It is possible to cause damage to the optic nerve if the electrodes are too near the eyes. To test, close your eyes and have the machine turned on and off a few times. If you can see light or visual patterns appear, then the electrical current may be affecting the optic nerve.
Also, it may not be safe to place electrodes on the forehead and the back of the skull near the base at the same time as this can reportedly cause other problems.