1. A device for determining illumination distributions to be directed to a target region for performing intensity modulated radiation therapy, the device comprising: a fluence map providing unit for providing a fluence map, a layered graph structure providing unit for providing a layered graph structure, wherein the layered graph structure comprises layers with nodes, wherein the nodes of a layer represent realizable extensions of an illumination distribution along a line, which are realizable by a radiation device, wherein the nodes of different layers represent realizable extensions of an illumination distribution along different parallel lines and wherein nodes of adjacent layers are connected by graph edges, to which first weights and regularization values are assigned, wherein the regularization values depend on deviations between the extensions of the illumination distributions represented by the nodes connected by the respective graph edges, an extensions determination unit for determining extensions of an illumination distribution along the parallel lines for several illumination distributions, wherein the extensions determination unit is adapted to determine the extensions for an illumination distribution by determining a sequence of nodes connected by the graph edges through the layered graph structure, which optimizes a cost function that depends on a combination of the first weights and the regularization values assigned to the graph edges connecting the nodes of the sequence, thereby determining a contour of the illumination distribution, a second weights determination unit for determining second weights defining the fluences of the illumination distributions, for which the extensions have been determined, such that a deviation between the provided fluence map and a combination of the illumination distributions having the determined extensions and the fluences defined by the second weights is minimized.