An increase in the temperature of a material changes its state from solid to liquid and then to gas. A further increase in temperature ionizes the gaseous molecules, producing electrons and ions. This ionized gas is called plasma. Over the past 20 years, plasma has been studied for potential applications in cancer therapy. Since plasma is generated using a high-voltage discharge, it contains an electric field and electric current. Additionally, a variety of reactive oxygen and nitrogen species are produced due to molecular collisions with electrons accelerated by the electric field. These factors influence cancer cells. Some in vitro and in vivo experiments have also demonstrated the possibility that plasma can induce antitumor immunity.

　This review introduces cancer treatment using plasma, with a particular focus on several mouse model experiments conducted by the author’s group suggesting induction of antitumor immunity: (i) the abscopal effect induced by tumor irradiation, (ii) long-term systemic antitumor effects demonstrated by a re-challenge experiment, (iii) the abscopal effect induced by normal tissue irradiation, (iv) the combined effect of plasma with an immune checkpoint inhibitor (ICI), and (v) the inhibition of local recurrence.

　The abscopal effect in (i) refers to an antitumor effect on a tumor following irradiation of a distant tumor. The re-challenge experiment in (ii) was conducted to support the hypothesis that the abscopal effect observed in (i) is mediated by antitumor immunity in mice. The experiment in (iii) demonstrates that the abscopal effect can be achieved even when plasma is applied to normal tissue in tumor-bearing mice. The combination experiment in (iv) suggests a potential synergistic effect between plasma and ICI. In (v), plasma irradiation of the tumor resection site shows the possibility of reducing the local recurrence rate by half.