## 4 Cancer Treatment  
There are several intriguing evidences that the reactive oxygen species (ROS) originated in the plasma can kill cancer cells selectively, preserving healthy ones. This has been attributed, in analogy to other redox therapies, to the faster formation and loss level of these species in cancer cells as compared to healthy ones, leading to higher baseline concentrations. The plasma action will raise ROS concentration in both cell types, but only in cancer cells this will go beyond the threshold leading to apoptosis [11]. Reactive nitrogen species are also considered to be important in determining the plasma action, although their role is less understood [12].  
In the paper of [PERSON] and co-workers, a pre-clinical study on the use of argon plasma as adjuvant therapy on progressive head and neck cancer is described [13]. The plasma was produced by the certified medical device kINPen MED, operating at radiofrequency with an argon flow. The response of healthy and tumour cells of head and neck cancer to CAP exposure was addressed. In addition, tissue samples from 10 patients with histologically proven cancers of the maxillofacial region were treated and then investigated for induction of apoptosis and secreted proteins with antitumor activity. The viability of cancer cells was found to be strongly reduced by the plasma action, although no clear selectivity of cancer cells could be observed. However, induction of apoptosis was superior in tumour tissue than in healthy mucosal tissue. Furthermore, CAP treatment significantly decreased cell motility in squamous cell carcinoma cells only but not in non-malignant keratinocytes. Overall, these results point to CAP treatment as a promising adjuvant treatment option to eliminate minimal residual cancer cells after radical surgery of carcinoma.  
The antitumor effect of the reactive species generated by the plasma can be displayed through direct plasma application, as in the previous study, or through the application of liquids previously treated with the plasma. This is the theme of the second paper of this section, by [PERSON] and co-workers, which shows that plasma-activated medium can be stored for up to six months in a freezer and then exhibit cytocidal effects on human cervical cancer HeLa cells, similar to those of a direct plasma treatment [14]. The treated medium was Dulbecco's Modified Eagle Medium supplemented with 10% fetal bovine serum and antibiotics, and the treatment was performed with a micro plasma-jet nozzle operating in air flow. The cytocidal effect was attributed to H\({}_{2}\)O\({}_{2}\) and nitrite/nitrate formed in the liquid by the plasma action.  
Apart from the previously mentioned mechanism, a new paradigm is also taking momentum as a possible way of inducing cancer cell death through the plasma action. This is the immunogenic cancer cell death, which proposes stimulation of an immune response against apoptotic tumor cells, and is the theme of the third paper, by [PERSON] and co-workers [15]. In this work, the role of plasma-treated murine melanoma cells in modulating murine immune cells' activation and marker profile was investigated. The results indicate a tumor-static action in terms of metabolic activity and cell motility and a negligible protective effect of protein present during the treatment. A role of plasma-mediated activation of splenic immune cells and a modulation of inflammatory parameters, in agreement with a pro-immunogenic role of plasma treatment, were also observed.