A wearable medical device that delivers electrical fields through the scalp helped to extend the survival of patients with lethal brain tumors, according to data presented Sunday.

In a study involving major medical centers in the United States and abroad, the novel treatment was used to administer alternating, low-intensity “tumor-treating fields” to newly diagnosed glioblastoma patients who also were getting chemotherapy. Such electrical fields may block the division of cancer cells and cause their demise, according to Roger Stupp, the study’s lead investigator and a neuro-oncologist at the Northwestern University Feinberg School of Medicine.

Stupp, who has worked on the device for several years, acknowledged that it has prompted considerable skepticism from some experts in the field. Although the new data have not yet been peer-reviewed, he said he believes the results “firmly establish the survival benefit.”

The study, presented at the annual meeting of the American Association for Cancer Research in Washington, D.C., involved 695 adult patients who were enrolled between July 2009 and November 2014. It found that the median survival for patients who were randomly assigned to use the device while getting the chemo drug temozolomide lived a median of 21 months compared with 16 months for those assigned to receive chemo without the device.

“I think this is fascinating, but the improvement is still quite modest,” said George Demetri, an oncologist at the Dana-Farber Cancer Institute who was on the AACR committee that accepted and highlighted the abstract. Exploring non-drug therapies is important given that glioblastoma patients survive, on average, less than 15 months following their diagnosis.

Some critics and even some supporters have said Stupp should have designed the trial so that one group of patients would have gotten a placebo in the form of a device without any electrical current. Stupp counters that such a sham device would have been impractical since patients would have known the difference because of the warming sensation the actual treatment causes on the scalp.

Two prominent brain-cancer specialists declined to discuss the study, signaling that they remain dubious.

The technology, called Optune, is manufactured by Novocure Inc. Interim results for the first 315 patients enrolled in the study, which were published in 2015 in JAMA, prompted an independent data and safety monitoring committee to recommend that the trial be ended early so that all patients could get the device if they wanted. The latest data show that the two-year survival rate for the full group of patients was 43 percent for those using the device while getting temozolomide, compared with 31 percent for those only receiving chemo. The five-year rate was 13 percent vs. 5 percent.

Optune has several components. The device has a generator that can be plugged into an electrical outlet or powered by a battery. It produces an electrical field that is conveyed via wires to four adhesive patches taped in place on a patient’s shaved scalp. Each patch has an array of nine electrode discs. The electrical field being delivered changes direction rapidly, which Stupp speculates disrupts the rapidly dividing cancer cells but does not affect healthy cells.

The device was initially approved by the Food and Drug Administration in 2011 to treat patients with glioblastoma who had returned after chemotherapy. In 2015, the agency approved the treatment for newly diagnosed patients who also were getting temozolomide.

At $21,000 a month, it’s costly. Standard Medicare doesn’t cover it, and some other insurers are balking.