In intensity modulated radiation therapy (IMRT) the uniform dose distribution has been replaced by non-uniform dose distributions. Radiobiologically, "one" still assumes that the local dose response is identical to that from a uniform dose of the same identity. This project deals with the radiation induced bystander effect (RIBE) in nearby but unirradiated cells. The objective is to unravel the molecular mechanisms of RIBE through in-vitro research on MCF-7/6 breast cancer cells in monolayer, that are exposed to steep dose gradients.