When a surgeon removes a tumor, some cancer cells may get left behind, threatening to seed another malignant growth. Researchers have just begun the first clinical trial of a new anticancer tool that they hope will kill these stubborn cells: a plasma scalpel.

The pen-size scalpel emits a small jet of helium whose charged particles glow with a vivid lilac hue. An electrode at the scalpel's tip splits some of the helium atoms into a plasma soup of positive ions and electrons.

Unlike in the sun's blazing plasma, the scalpel's ions are relatively slow-moving—so the jet feels like a cool breeze to the touch. But its fast electrons are packed with energy and can convert atmospheric oxygen and nitrogen into reactive forms, including superoxide, nitric oxide and atomic oxygen. These substances can interrupt key metabolic processes and hamper cell reproduction, and researchers have found that cancer cells are much more vulnerable to such effects than healthy cells are. The scalpel can be used on a tumor site for just a few minutes during surgery, says Jerome Canady, a surgeon in Washington, D.C., and part of the team that developed the tool. “We just spray that area with plasma to kill any microscopic tumors,” he says.

Cold plasma is already used to treat infections and sterilize wounds, and more energetic plasma can neatly cut or cauterize tissue. Turning it against cancer has long been a goal, and the new trial is a major milestone, says Mounir Laroussi, who studies the biological effects of cold plasma at Old Dominion University. “I think this is huge,” he says.

In the past few years doctors have used plasma scalpels on three cancer patients on a “compassionate use” basis, after all other treatment options had failed. The plasma successfully killed residual cancer cells in these people, Canady says, but a full clinical trial will provide vital data about safety and longer-term effects. As Scientific American went to press, Canady and his colleagues were due to perform the first surgery of their trial in late October 2019. They aim to use the plasma scalpel on 20 patients with late-stage solid cancers, including those affecting the pancreas, ovary or breast.

Laroussi says it took more than a decade of laboratory work on cell cultures and animals to prepare the plasma scalpel for the clinic. The process involved identifying the chemicals it generates, measuring their penetration into tissue and understanding how the disruption of cancer cells works. “You also have to stay below a certain dose—otherwise you kill both cancer cells and healthy cells,” he says. Laroussi hopes the trial will show that the device can be fine-tuned to take out its cancerous quarry without causing unwanted damage.