Abstract BACKGROUND Tumor Treating Fields (TTFields) are known to exert anti-mitotic effects on cells. Numerical simulations investigating the electric field distribution within isolated cells have been reported. These studies have shown that during metaphase a uniform electric field forms within the rounded cells. This field is thought to disrupt spindle formation through alignment of the tubulin dimers with the field. Simulations also show that during cytokinesis, a non-uniform electric field forms at the furrow, leading to strong dielectrophoretic forces that further disrupt cell division. Cells in the tumor are densely packed. We used numerical simulations to investigate how the clustering of cells influences TTFields distribution. METHODS COMSOL was used to numerically simulate delivery of TTFields to clusters of round cells placed in a hexagonal arrangement. The influence of the distance between the cells on field distribution was investigated. The effect of pores in the cell membrane on field distribution was also investigated. RESULTS Placing round cells in clusters resulted in regions of highly non-uniform fields within the cells. Strong gradients in the electric field were also observed around pores placed in the membrane. Non-uniformity and gradients in the field could result in strong dielectrophoretic forces capable of disrupting key cellular structures such as the cytoskeleton and mitotic spindle, as well as cell membrane integrity. CONCLUSIONS The placement of cells in close proximity to one another creates gradients in the electric field, which could be associated with very strong dielectrophoretic forces that enhance the effects of TTFields on cells. Strong dielectrophoretic forces were also observed around the membrane pores. Previous studies have reported that TTFields increases membrane permeability [Chang et al Cell Death Discovery. 2018]. The strong dielectrophoretic forces in the membrane may provide a physical mechanism by which TTFields enhance membrane permeability.