Doctors treating brain cancer have a limited tool kit. They can cut tumours out with a knife, burn them with radiation or try to poison them with drugs.

NovoCure, a closely held Israeli company, has added a fourth option for hard-to-treat tumours.

It's an array of electrodes resembling a tight-fitting helmet that bathes the cancer in a faint electric field, scrambling the inner workings of the rampaging cells and preventing them from multiplying.

The helmet, powered by a six-pound battery pack, is designed to zap deadly glioblastomas, the malignancy that killed Senator Ted Kennedy in August 2009.

A study reports that it helped patients with recurrent tumours live 7.8 months, compared with a median 6.1 months for patients given best available chemotherapies or Avastin.

The technology is so different from other treatments, it was difficult to convince patients and doctors to try it, said Philip Gutin, primary investigator for the study.

"This new data actually shows that it's effective," said Gutin, chair of neurosurgery at Memorial Sloan-Kettering Cancer Center, New York. "People will ask for this now."

The electric fields resonate at a frequency designed to do no harm to healthy brain tissue. In the test, the only side-effect was mild scalp irritation, Gutin said. "If it continues to look as good as it does, it will be used in lots of different treatments. There's no downside to it."

The study, reported at the American Society of Clinical Oncology in Chicago, followed 237 very sick patients whose cancers had returned after prior treatment.

The study was designed to show that patients using the helmet fared significantly better than those taking chemotherapy and Avastin. On this basis, it was a failure because more than 50 patients either died or dropped out before they completed the first round of treatment, said Eilon Kirson of NovoCure.

But when those patients are excluded from both arms of the analysis, the helmet performed better than other treatments, Kirson said. Under either analysis, the trial found the helmet to be at least as good as other approaches, but without the vomiting, fatigue and infections associated with chemotherapy. Patients wore NovoCure's helmet for about 20 hours a day.