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 configured to provide a fluence map, - a layered graph structure providing unit configured to repeatedly provide a layered graph structure, wherein the layered graph structure comprises layers with nodes, wherein the nodes of a layer represent realizable extensions ( b r) of an illumination distribution along a line, which are realizable by a radiation device, wherein the nodes of different layers represent realizable extensions ( b r) of an illumination distribution along different parallel lines and wherein nodes of adjacent layers are connected by graph edges, to which first weights ( e r) and regularization values ( P r) are assigned, wherein the regularization values ( P r) depend on deviations between the extensions ( b r, b r -1) of the illumination distributions represented by the nodes connected by the respective graph edges, - an extensions determination unit configured to repeatedly determine extensions ( b r) of an illumination distribution along the parallel lines for several illumination distributions, wherein the extensions determination unit is adapted to determine the extensions ( b r) 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 ( e r) and the regularization values ( P r) assigned to the graph edges connecting the nodes of the sequence, thereby determining a contour of the illumination distribution, - a second weights determination unit configured to repeatedly determine second weights ( w) defining the fluences of the illumination distributions, for which the extensions ( b r) have been determined, such that a deviation between the provided fluence map and a combination ( m) of the illumination distributions having the determined extensions ( b r) and the fluences defined by the second weights ( w) is minimized, wherein the layered graph structure providing unit is adapted to determine a deviation function ( f) being indicative of a deviation between the provided fluence map and the combination ( m) of the illumination distributions having the determined extensions ( b r) and the fluences defined by the second weights ( w) and to repeatedly determine the first weights ( e r) depending on the deviation function. - a fluence map providing unit configured to provide a fluence map, - a layered graph structure providing unit configured to repeatedly provide a layered graph structure, wherein the layered graph structure comprises layers with nodes, wherein the nodes of a layer represent realizable extensions ( b r) of an illumination distribution along a line, which are realizable by a radiation device, wherein the nodes of different layers represent realizable extensions ( b r) of an illumination distribution along different parallel lines and wherein nodes of adjacent layers are connected by graph edges, to which first weights ( e r) and regularization values ( P r) are assigned, wherein the regularization values ( P r) depend on deviations between the extensions ( b r, b r -1) of the illumination distributions represented by the nodes connected by the respective graph edges, - an extensions determination unit configured to repeatedly determine extensions ( b r) of an illumination distribution along the parallel lines for several illumination distributions, wherein the extensions determination unit is adapted to determine the extensions ( b r) 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 ( e r) and the regularization values ( P r) assigned to the graph edges connecting the nodes of the sequence, thereby determining a contour of the illumination distribution, - a second weights determination unit configured to repeatedly determine second weights ( w) defining the fluences of the illumination distributions, for which the extensions ( b r) have been determined, such that a deviation between the provided fluence map and a combination ( m) of the illumination distributions having the determined extensions ( b r) and the fluences defined by the second weights ( w) is minimized, wherein the layered graph structure providing unit is adapted to determine a deviation function ( f) being indicative of a deviation between the provided fluence map and the combination ( m) of the illumination distributions having the determined extensions ( b r) and the fluences defined by the second weights ( w) and to repeatedly determine the first weights ( e r) depending on the deviation function.