The electric fields' different effects on normal and dividing cells mostly have to do with geometry. A dividing cell has what Palti calls "an hourglass shape rather than a round shape." The electric field generated by the NovoCure device passes around and through round cells in a uniform fashion. But the narrow neck that pinches in at the center of a dividing cell acts like a lens, concentrating the electric field at this point. This non-uniform electric field wreaks havoc on dividing cells. The electric field tears apart important biological molecules, such as DNA and the structural proteins that pull the chromosomes into place during cell division. Dividing cells simply "disintegrate," says Palti.

Palti, who for years has been studying the effect of electric fields on cancer and normal cells, says that he has verified this mechanism in computer models and experiments in the lab. "The physics are solid," says David Cohen, associate professor of radiology at Harvard Medical School.

Patients in the glioblastoma clinical trial wear the device almost constantly, carrying necessary components in a briefcase. A wire emerging from the briefcase connects to adhesive electrodes covering the skull. Alternating electric fields pass through the scalp, into the skull, and on to the brain. The Food and Drug Administration approved the device for late-stage clinical trials for glioblastoma following promising results from a pilot study in 10 patients, one of whom had a complete recovery.