Use of cold atmospheric plasma in the treatment of cancer Parker Babington, Kenan Rajjoub, Jerome Canady, Alan Siu, Michael Keidar, and Jonathan H. Sherman Citation: Biointerphases 10, 029403 (2015); doi: 10.1116/1.4915264 View online: http://dx.doi.org/10.1116/1.4915264 View Table of Contents: http://scitation.aip.org/content/avs/journal/bip/10/2?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Cold atmospheric plasma in cancer therapya) Phys. Plasmas 20, 057101 (2013); 10.1063/1.4801516 An evaluation of anti-oxidative protection for cells against atmospheric pressure cold plasma treatment Appl. Phys. Lett. 100, 123701 (2012); 10.1063/1.3693165 Selective killing of ovarian cancer cells through induction of apoptosis by nonequilibrium atmospheric pressure plasma Appl. Phys. Lett. 100, 113702 (2012); 10.1063/1.3694928 Atmospheric-pressure plasma-jet from micronozzle array and its biological effects on living cells for cancer therapy Appl. Phys. Lett. 98, 073701 (2011); 10.1063/1.3555434 Effects of atmospheric nonthermal plasma on invasion of colorectal cancer cells Appl. Phys. Lett. 96, 243701 (2010); 10.1063/1.3449575

Use of cold atmospheric plasma in the treatment of cancer Parker Babington Department of Neurological Surgery, The George Washington University Medical Center, Washington, District of Columbia 20037

Kenan Rajjoub Columbian College of Arts and Sciences, George Washington University, Washington, District of Columbia 20052

Jerome Canady US Medical Innovations, Takoma Park, Maryland 20912

Alan Siu Department of Neurological Surgery, The George Washington University Medical Center, Washington, District of Columbia 20037

Michael Keidar Department of Mechanical Engineering, The George Washington University, Washington, District of Columbia 20052

Jonathan H. Shermana) Department of Neurological Surgery, The George Washington University Medical Center, Washington, District of Columbia 20037

(Received 1 February 2015; accepted 5 March 2015; published 19 March 2015) Cold atmospheric plasma (CAP) is an emerging modality for the treatment of solid tumors. In-vitro experiments have demonstrated that with increasing doses of plasma, tumor cells assays display decreased cell viability. CAP is theorized to induce tumor cells into apoptosis via multiple pathways including reactive oxygen and nitrogen species as well as cell cycle disruption. Studies have shown CAP treatment can decrease mouse model glioblastoma multiforme tumor volume by 56%, increase life span by 60%, and maintain up to 85% viability of normal cells. Emerging evidence suggests that CAP is a viable in-vivo treatment for a number of tumors, including glioblastoma, as it appears to selectively induce tumor cell death while noncancerous cells remain C 2015 American Vacuum Society. [http://dx.doi.org/10.1116/1.4915264] viable. V I. INTRODUCTION A. Plasma

Plasma is one of the four states of fundamental matter in the universe, with solid, liquid, and gas being the other three. It is the most abundant form of matter, making up 99% of the visible universe. Plasma can be created by heating a gas or placing it under a strong electromagnetic field. It consists of a partially ionized gas with charged ions, electrons, and a collection of several other uncharged particles. The overall charge of plasma is close to zero, with plasma being described as an electrically neutral medium of unbound negative and positive particles. In a magnetic field the particles are free to move, which generates electrical currents.1,2 Plasma does not have a shape or volume unless contained, since it is electrically conductive. However, it will form structures such as a beam when placed in a magnetic field.3 There are two types of plasma, thermal and nonthermal, which are based on the temperature of the electrons compared to the other particles. In thermal plasma, the electrons and heavy particles are in thermal equilibrium, while in nonthermal plasma the ions and neutral particles are approximately at room temperature while the electrons reside at a a)

Author to whom correspondence should be addressed; electronic mail: [email protected]

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much higher temperature. Traditional thermal plasma temperatures exceed 3000  C at the target, making it ideal for metallurgy but unusable for human tissue treatment. Nonthermal plasma is also called cold plasma, and at the point of application, cold plasma has a temperature of less than 40  C, making it an ideal treatment for living tissue.4–8 B. Generation of cold plasma

Cold plasma may be formed via several methods including dielectric barrier discharge (DBD), plasma needle, and the atmospheric pressure plasma jet. It can be produced by a variety of gases including helium, nitrogen, argon, and air. DBD is the electrical discharge produced by two electrodes separated by a dielectric barrier. A gas contained between the two electrodes becomes ionized and creates plasma. It was first reported by von Siemens in 1857, and since the 19th century DBD has been known to decompose gaseous compounds. Its current uses include production of ozone in industrial scales, germicidal processes, high power lasers for welding and cutting, and plasma display panels.9 A more recent modification of DBD by Fridman et al. is the floating electrode DBD. In this variation of DBD the second electrode is active, not grounded, meaning it can be a biologic tissue such as skin or an organ. This technique has been used in living tissue sterilization as well as deactivation of Bacillus stratosphericus.10

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C 2015 American Vacuum Society V

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An atmospheric pressure plasma jet consists of a gaseous mixture flowing at a high rate between two electrodes. One electrode is powered by a radio frequency charge while the other electrode is grounded, creating a reactive species that exits the nozzle at high velocity. This method of plasma production was developed by Koinuma et al.11 In 2002, a miniature atmospheric pressure plasma jet was developed by Stoffels et al. and was called a plasma needle. The plasma needle operates at low voltages and low power consumption. The plasma needle has been applied to organic materials without causing thermal or electrical damage to the surface. The main difference between DBD and the various types of plasma jets is that in DBD all of the agents have direct contact with the treated tissue, while plasma jets can treat samples remotely.12 II. MECHANISM OF ACTION OF COLD PLASMA ON CANCER CELLS Cold plasma has been shown to be efficacious in sterilization of surfaces, and has been proposed as a technique for selectively targeting cancer cells. Early research using cold plasma on tumor cells has been promising; however, the exact mechanism of action is not fully understood. Several mechanisms have been proposed including apoptosis via reactive oxygen species or nitrogen species (ROS, RNS), activation of p53 protein, activation of the p21 CKS inhibitor, and cell cycle arrest. In tissue treated with cold plasma, intracellular levels of ROS increase and target cells subsequently undergo DNA damage and apoptosis. The ROS/RNS theory is supported by N-acetyl-L-cysteine (NAC) studies. NAC is a free radical scavenger, and when melanoma cells were exposed to NAC prior to cold plasma treatment, apoptosis decreased from a rate of 35%–20%. NAC without cold plasma treatment had no effect on the melanoma cells.9,13 ROS have also been shown to react with amino acids that can induce cell membrane damage and lipid peroxidation. ROS may activate intracellular signaling pathways resulting in apoptosis. Cold plasma exposure can create openings in the cell membrane allowing ROS to enter cells more quickly to cause intracellular damage. Yan et al. described elevated concentrations of nitric oxide (NO), lipid peroxide, and other ROS during cold atmospheric plasma (CAP) treatment. These elevated concentrations were correlated with increasing number of dead tumor cells. In addition to ROS causing apoptosis, mitochondrial dysfunction is another proposed mechanism of cold plasma induced apoptosis.14 Ahn et al. found that human cervical carcinoma cells treated with cold plasma had mitochondrial membrane dysfunction and underwent apoptosis. With pretreatment of the target cells with antioxidants, apoptotic cell death was impaired.15 CAP is also being evaluated as a modifier of the cell cycle. Volotskova et al. found that CAP induced a twofold G2/M increase in two cancer cell lines. This study also found expression of an oxidative stress reporter for S-phase damage was enhanced with the application of CAP.16 Biointerphases, Vol. 10, No. 2, June 2015

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III. SELECTIVITY OF COLD PLASMA IN NORMAL CELL LINE EXPERIMENTS One of the most exciting factors driving the development of cold plasma as a potential tumor treatment is the reported cell selectivity of the treatment. In order to better understand this selectivity, bacterial cells were analyzed against eukaryotic cells when treated with plasma. Using plasma to treat living tissues is preferable to UV radiation because the radiation can cause irreversible damage to host tissue DNA. When cold plasma is used, however, the short-living freeradicals (RNS/ROS) in the plasma allow treatment without harming the host cells. Stoffels et al. found that eukaryotic cells react similarly to bacterial cells when sterilized using plasma and UV, but the complexity of these reactions is much greater in eukaryotic cells. Furthermore, the eukaryotic cells require much higher dosage of treatment in order to damage the cells due to their more advanced defense mechanisms.17 Keidar et al. found that the dissipated heat released by cold plasma does not harm eukaryotic cells. The cold plasma increased the temperature by 2  C, which is not enough to thermally damage the cells. This research has allowed cold plasma to progress from a sterilization technique into a potential viable tumor treatment since the tumor cells appear to respond similarly to bacterial cells, which are induced into apoptosis by the cold plasma while the normal host cells remain viable.18 IV. FREE-RADICAL SPECIES-ROS/RNS The mechanism for cell selectivity during cold plasma treatment is theorized to be due to free radical interactions with tumor and host cells. Free radicals are vital in the formation of RNS and ROS. Halliwell et al. demonstrated that a free radical is formed when there is a lack of an electron filling an orbital. Each orbital can only hold two electrons and according to Hund’s rule, each will spin in opposite directions. Measuring spin resonance enables the determination of the amount of energy in each radical. After the radical has formed it can react with molecules in many different ways. Radicals can either bond with a hydrogen ion or they can covalently bond with another free-radical in order to fill the electron space.19 RNS are a derivative of NO, and can have a profound effect on cells similar to ROS. It has been determined that the reaction of NO and free radicals can form RNS. According to Patel et al., the best understood example of this reaction is the formation of peroxinitrite (ONOO–). ONOO– can initiate lipid peroxidation reactions. In order to achieve this, the hydrogen atom in the polyunsaturated fatty acid must be abstracted by the ONOO–. This occurs naturally during inflammation when the macrophages and neutrophils produce NO, causing more ONOO– to be formed. The ONOO– becomes part of the force defending the body against infection.20 RNS can also play a role in cell signaling, which controls growth factors and cellular responses. Patel et al. discuss the role of NO in lessening the effect of inflammatory responses and in activating the Guanosine-50 -triphosphate binding

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protein p21Ras by NO. Tyrosine nitration, performed by ONOO–, prevents phosphorylation and degrades modified proteins, further affecting cell signaling pathways. Additionally, tyrosine nitration has been related to the permanent inhibition of enzymes.20 The effect of RNS on cell signaling pathways can in turn affect apoptosis. Apoptosis is vital to the homeostasis of organisms and when disrupted, pathological conditions and diseases can develop. ONOO– activates apoptosis in certain types of cells including thermocytes, HL-60 leukemia cells, and cortical cells. This initiation of apoptosis by ONOO– can cause inflammation which is theorized to predispose cells to become cancerous. However, ONOO– production can cause beneficial changes as well, such as the induction of apoptosis in tumor cells through ONOO– formation. The production of ONOO– is stimulated by flavone acetic acid (an antitumor agent), which causes apoptosis in the tumor cell, lessening or eradicating the disease burden.20 ROS are similar to RNS in operation and function, although the species from which it is created is different. ROS can be formed from several different species, including hydroxyl (OH), alkoxyl (RO), and peroxyl (ROO–). These form the free radicals of superoxide (O2), nitroxyl radical (NO), and nonradicals such as hydrogen peroxide (H2O2). ROS, like RNS, can cause damage to DNA, thus acting as a defense against bacteria and infection. ROS defend the cell against phagocytes when nicotinamide adenine dinucleotide phosphate-oxidase oxidase is activated.17 Like RNS, ROS activates apoptosis in cells, particularly in inflammatory cells. H2O2, for example, induces apoptosis in neutrophils, its production accounting for the short generation of mature neutrophils. ROS generation is also vital in the operation of cell death receptors, which come from the tumor necrosis factor (TNF)/nerve growth factor (NGF) family. ROS has a functional role in the activation of the caspase cascade, which activates the cell death receptors in the TNF/NGF family.17 V. USE OF CAP FOR TUMOR TREATMENT Although cold plasma has repeatedly been shown to be associated with cell death, it can be a viable cancer treatment only if it selectively targets tumor cells over normal host tissue. A multitude of studies have investigated cold plasma application in tumors including neuroblastoma, melanoma, pancreatic carcinoma, cervical carcinoma, lung carcinoma, lymphoblastic leukemia, colon carcinoma, breast CA, and glioblastoma (GBM). In 2007 Fridman et al. used plasma treatment on melanoma cancer cells. Low dose CAP caused apoptosis after several hours while high dose CAP caused necrosis.10 Thiyagarajan et al. noted CAP induced cell death in leukemia cancer cells with a dose dependent cell death response.21 Kim et al. studied melanoma tumors in mice. Mice were injected with B16F0 cells and the tumor was allowed to reach a size of 40 mm3. The mice were then treated with 5 s of CAP either once or four times a day for 4 days. No antitumor effect was noted after a single treatment; however, with four times a day treatment, tumor growth was Biointerphases, Vol. 10, No. 2, June 2015

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inhibited.22 Keidar et al. treated mice with subcutaneous bladder cancer and B16 melanoma cells. Five minutes of CAP was applied to the bladder cancer group. Smaller tumors were found to be completely ablated, while larger ones decreased in size. The ablated tumors did not grow back, whereas the large tumors began growing after 1 week but had not reached original size even 3 weeks post-treatment. The melanoma group was treated with CAP for 5 min with markedly decreased tumor growth and increase in median survival from 24.5 to 33.5 days compared to the control group.18 Walk et al. noted apoptosis and a reduced number of viable cancer cells with CAP treatment of neuroblastoma. Mice were injected with Neuro2a cells then treated with 5 min of CAP while a control group of mice received no CAP after cell injection. The median survival of the treatment group compared to the control group increased from 15 to 28 days. These studies as a whole are promising regarding the efficacy and selectivity of CAP treatment in cancer cells.23 VI. TREATMENT OF GBM WITH CAP GBM remains one of the most difficult tumor subtypes to treat, with the average life expectancy less than 2 years after diagnosis even with maximal surgical and medical therapies. Cold plasma has been postulated as a potential adjunct treatment with many studies over the past several years demonstrating benefit. In 2010, Vandamme et al. tested the safety and effectiveness of plasma treatment on mice with U87 glioblastoma cells. Pulsed DBD plasma increased subcutaneous temperature and caused a cutaneous skin pH reduction without any long term cutaneous effects. Long time plasma treatment (20 min for three consecutive days) produced a superficial burn. Thus, they concluded that the treatment was safe in the mouse model. Using bioluminescence as a marker of efficacy, plasma treatment was found to dramatically reduce tumor volume the U87 glioma-bearing mice.24 In 2011 Vandamme et al. injected U87 glioblastoma cells subcutaneously into female athymic nude mice. These tumors were tracked until they reached a volume of 150 6 50 mm3. In order to track the changes of the treatment of the experimental group, the control group was not treated. Treatments consisted of 200 Hz treatments in three intervals of 2 min. Over a period of five consecutive days with CAP treatment it was noted that the U87 malignant glioma cells shrunk significantly. Tumor volume decreased a total of 56% by the end of treatment and life span increased by 60% (Fig. 1).25 Koritzer et al. further investigated the use of CAP on glioma cells. The treatment groups consisted of O(6)Methylguanine-DNA-methyltransferase methylated and unmethylated cell lines. The goal was to determine if the combination of CAP and Temazolamide (TMZ) would stop cell growth. In order to test the cells, Koritzer et al. seeded 6 cm dishes and treated each dish with different time intervals, 0, 30, 60, and 120 s. They then allowed the cells to proliferate for 12 days. After treating multiple different glioblastoma cell lines (LN18, LN229, and U87MG) with CAP followed by TMZ, there was a strong induction of cell

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FIG. 1. CAP treatment in vivo. Representative BLI imaging of control and plasma treated mice. Reprinted with permission from M. Vandamme et al., “Antitumor effect of plasma treatment on U87 glioma xenografts: preliminary results,” Plasma Proc. Polym. 7.3–4, 264 (2010).

cycle arrest. This illustrated that TMZ was more effective in combination with CAP treatment than alone. There was a dose dependent inhibition of cell proliferation by CAP across each of the cell lines.26 Cheng et al. investigated the mechanism of CAP therapy on U87 glioblastoma cells and the correlation to 100 nm gold nanoparticles (AuNPs). Cells were cultured with AuNPs injected into the media, allowing them to penetrate the cells within 1 or 24 h. As higher flows of CAP were applied during treatment, higher ratios of RNS and ROS species were demonstrated and cell viability decreased. The use

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of CAP was associated with up to 30% increase in target cell death (Fig. 2).1 Cheng also reported that varying the composition of the CAP impacted the survival of target cells. Duration treatment, input/output voltage, flow rate, and composition of feed gas were found to create different specific ROS and RNS at different concentrations. A “plasma dosage” was calculated using these variables, and the data showed that with a higher plasma dosage, cell viability was decreased to a greater extent.27 Yan et al. investigated the use of CAP on GBM cells using fetal bovine serum (FBS) in culture medium to modify the killing of cancer cells. Four different concentrations of FBS were used (0%, 10%, 20%, and 30%). When the culture media contained FBS, the killing capability of the CAP decreased. In fact, it was found that CAP stimulated media with FBS actually enhanced the viability of U87 cells compared to FBS culture without CAP treatment. It was theorized that a slow reactive nitrogen species consuming reaction occurred in the FBS media treated with CAP, which counteracted the typical tumor killing effect of CAP. This study demonstrated the feasibility of controlling the killing capability of CAP stimulated media on GBM cells by regulating the concentration of FBS.28 Siu et al. have preliminary results using CAP on three glioma cell lines, normal astrocytes, and normal endothelial cell lines. CAP treatment was applied for different time periods and the cell viability examined. CAP treatment showed a dose-dependent relationship to tumor cell death with the ID60 for the three glioma cell lines being between 90 and 120 s (Fig. 3). Treatment with CAP for more than 120 s resulted in cell viability less that 35% 24 h post-treatment and less than 20% at 72 h. CAP treatment of 90–120 s to the normal astrocyte

FIG. 2. (a) Intracellular ROS generation in U87 cells under 0–60 s plasma treatment durations. Images were taken 30 min after the plasma treatment. (b) Quantification of the ROS intensity with Zen 2012 Lite. (Reprinted with permission from X. Cheng et al., “The effect of tuning cold plasma composition on glioblastoma cell viability,” PloS One 9.5, e98652 (2014). Biointerphases, Vol. 10, No. 2, June 2015

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ACKNOWLEDGMENTS This work was supported in part by a Cyrus and Myrtle Katzen Cancer Research Center grant. 1

FIG. 3. Results of 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide assay from U87 cells treated with varying durations of CAP. Viability was calculated relative to control. Plasma treatment of 90–120 s resulted in tumor cell viability less than 35% with normal cell line viability up to 85%.

and endothelial cell lines did not cause significant cell death with up to 85% of the cells remaining viable 72 h after treatment.29 VII. CONCLUSIONS Cold plasma is an emerging treatment modality, which is showing promising signs of selective targeting of tumor cells while allowing normal host cells to remain viable. It is theorized that this interaction is due to the formation of reactive oxygen and nitrogen species which preferentially induce apoptosis in tumor cells, as well as cell cycle disruption. Plasma has shown promising results in several different tumor types including neuroblastoma, melanoma, pancreatic carcinoma, cervical carcinoma, lung carcinoma, lymphoblastic leukemia, colon carcinoma, breast CA, and glioblastoma. Larger invivo studies are needed to further assess the selectivity and effectiveness of cold plasma in tumor treatment. The next step in the effort to treat GBM as well as other solid tumors with CAP will be more extensive in vivo treatment of the mouse xenograft. Based on flank studies and computer modeling, it is suggested that only a small portion of the tumor needs to be treated to effect the entire mass. The mouse xenograft model will be an intracranial tumor implantation model where a portion of the tumor will be treated with either the cold plasma directly or with cold plasma treated media. This can help to generate data to support human trials either with surgically unresectable tumors or the postoperative resection cavity.

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X. Cheng, W. Murphy, N. Recek, D. Yan, U. Cvelbar, A. Vesel, and J. H. Sherman, J. Phys. D: Appl. Phys. 47, 335402 (2014). 2 M. Keidar and I. Beilis, Plasma Enginerring: Application in Aerospace, Nanotechnology and Bionanotechnology (Academic, Waltham, MA, 2013). 3 G. Fridman, G. Friedman, A. Gutsol, A. B. Shekhter, V. N. Vasilets, and A. Fridman, Plasma Processes Polym. 5, 503 (2008). 4 N. Barekzi and M. Laroussi, Plasma Processes Polym. 10, 1039 (2013). 5 D. B. Graves, J. Phys. D: Appl. Phys. 45, 263001 (2012). 6 R. Guerrero-Preston, T. Ogawa, M. Uemura, G. Shumulinsky, B. L. Valle, F. Pirini, and B. Trink, Int. J. Mol. Med. 34, 941 (2014). 7 M. G. Kong, G. Kroesen, G. Morfill, T. Nosenko, T. Shimizu, J. Van Dijk, and J. L. Zimmermann, New J. Phys. 11, 115012 (2009). 8 G. E. Morfill, M. G. Kong, and J. L. Zimmermann, New J. Phys. 11, 115011 (2009). 9 C. Hoffmann, C. Berganza, and J. Zhang, Med. Gas Res. 3, 21 (2013). 10 G. Fridman, A. Shereshevsky, M. M. Jost, A. D. Brooks, A. Fridman, A. Gutsol, and G. Friedman, Plasma Chem. Plasma Process. 27, 163 (2007). 11 H. Koinuma, H. Ohkubo, T. Hashimoto, K. Inomata, T. Shiraishi, A. Miyanaga, and S. Hayashi, Appl. Phys. Lett. 60, 816 (1992). 12 E. Stoffels, A. J. Flikweert, W. W. Stoffels, and G. M. W. Kroesen, Plasma Sources Sci. Technol. 11, 383 (2002). 13 E. A. Ratovitski, X. Cheng, D. Yan, J. H. Sherman, J. Canady, B. Trink, and M. Keidar, Plasma Processes Polym. 11, 1128 (2014). 14 X. Yan, Z. Xiong, F. Zou, S. Zhao, X. Lu, G. Yang, and K. K. Ostrikov, Plasma Processes Polym. 9, 59 (2012). 15 H. J. Ahn, K. I. Kim, G. Kim, E. Moon, S. S. Yang, and J. S. Lee, PLoS One 6, e28154 (2011). 16 O. Volotskova, T. S. Hawley, M. A. Stepp, and M. Keidar, Sci. Rep. 2, 636 (2012). 17 E. Stoffels, S. Yukinori, and D. B. Graves, IEEE Trans. Plasma Sci. 36, 1441 (2008). 18 M. Keidar, R. Walk, A. Shashurin, P. Srinivasan, A. Sandler, S. Dasgupta, and B. Trink, Br. J. Cancer 105, 1295 (2011). 19 B. Halliwell, Am. J. Med. 91, S14 (1991). 20 R. P. Patel, J. McAndrew, H. Sellak, C. R. White, H. Jo, B. A. Freeman, and V. M. Darley-Usmar, Biochim. Biophys. Acta, Bioenerg. 1411, 385 (1999). 21 M. Thiyagarajan, I. Alexeff, S. Parameswaran, and S. Beebe, IEEE Trans. Plasma Sci. 33, 322 (2005). 22 J. Y. Kim, J. Ballato, P. Foy, T. Hawkins, Y. Wei, J. Li, and S. O. Kim, Biosens. Bioelectron. 28, 333 (2011). 23 R. M. Walk, J. A. Snyder, P. Srinivasan, J. Kirsch, S. O. Diaz, F. C. Blanco, and A. D. Sandler, J. Pediatr. Surg. 48, 67 (2013). 24 M. Vandamme et al., Int. J. Cancer 130, 2185 (2011). 25 M. Vandamme, E. Robert, S. Dozias, J. Sobilo, S. Lerondel, A. Le Pape, and J. M. Pouvesle, Plasma Med. 1, 27 (2011). 26 J. K€ oritzer, V. Boxhammer, A. Sch€afer, T. Shimizu, T. G. Kl€ampfl, Y. F. Li, and J. Schlegel, PLoS One 8, e64498 (2013). 27 X. Cheng, J. Sherman, W. Murphy, E. Ratovitski, J. Canady, and M. Keidar, PLoS One 9, e98652 (2014). 28 D. Yan, J. Sherman, X. Cheng, E. Ratovitski, J. Canady, and M. Keidar, Appl. Phys. Lett. 105, 224101 (2014). 29 A. Siu, O. Volotskova, X. Cheng, S. Khalsa, K. Bian, F. Murad, M. Keidar, and J. Sherman, PLoS One (unpublished).

Use of cold atmospheric plasma in the treatment of cancer.

Cold atmospheric plasma (CAP) is an emerging modality for the treatment of solid tumors. In-vitro experiments have demonstrated that with increasing d...
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