BACKGROUND Tumor microtubes (TMs) are ultralong membrane protrusions of tumor cells in astrocytic gliomas, including glioblastomas. TMs are used as routes for brain invasion and for cells to interconnect over long distances resulting in a functional network that allows multicellular communication. This network mediates resistance against the cytotoxicity of radiation and chemotherapy. One explanation for TM network protection is a better homeostasis of calcium ions that would otherwise increase to toxic intracellular levels in response to these therapies. Our working hypothesis is that interfering with the integrity of the glioblastoma cell network is key to a potential breakthrough in glioma therapy. Many cellular structures are polarized and composed of charged elements and are thus potential subjects to electrical forces; this might also influence the complex intercellular calcium waves (ICWs) that are characteristic for glioma networks. We were therefore interested in the effect of TTF on glioma network maintenance. MATERIAL AND METHODS To examine the effect of TTF on glioma TMs we have established a 2D in vitro glioma model using glioblastoma stem cells (GBSCs) grown in high-glucose medium and a 3D model using glioma tumor organoids. Both models reliably reproduce functionality and complexity of morphological features we observe in our mouse model. We analyzed the disruption of tumor network complexity and disruption of functionality by measuring intercellular calcium waves. Tumor cell death and proliferation was investigated in the 2D in vitro glioma model using the inovitroTM-System. RESULTS A peculiar “cricked-TM” phenotype that rarely (0.2% ±0.14) occurred under standard or control conditions was observed in TTF-treated cells (16.22% ±5.12). Cell number was reduced by 75% in two lines of GBSCs after 5 days of TTF exposure; predominantly TM-rich GBSCs (> 4 TMs) were affected. This reduction in tumor cell number corresponded with an increase in cell death (0.3% ±0.09 in untreated cells; 1.4% ±0.45 at day 5 of TTF exposure). The frequency of intercellular calcium transients, a measurement for calcium wave frequency in the glioma networks, was instantly reduced after TTF exposure to 58% ±20.42 of control levels in the primary GBSC 2D culture, and to 57.78% ±12.34 in tumor organoids derived from 3 glioblastoma patients. CONCLUSION This data suggests a potential effect of TTF application on tumor cell networks, at least in vitro. Interestingly, particularly those glioblastoma cells that have so far been proven to be resistant to radio- and chemotherapy appeared to be affected. We will confirm the observed effects of TTFs on tumor cell calcium signaling in our in vivo chronic cranial window mouse model. We anticipate that the results of our project will provide important insights into the underlying mechanism of TTF therapy.