Irradiating a moving target volume

A method for determining an effective radiation dose distribution at an irradiation of a moving target volume (102) having a plurality of volume elements, comprising the steps of: Detecting first position data of the plurality of volume elements in a first movement phase of the movable target volume (102); Detecting further position data of the plurality of volume elements in at least one other motion phase of the moving target volume (102); Determining transformation parameters by transforming the first position data in the further position data; during the irradiation of a halftone dot detecting the current motion phase of the moving target volume (102); Assigning grid points (222), the moving target volume (102) was at the irradiation in the first phase of movement, to a first sub-irradiation plan; Assigning further raster points (222a), wherein the irradiation, the movable target volume (102) in the at least was a further movement phase, at least one further sub-irradiation plan; and Determining the effective dose for at least one of the plurality of volume elements, in each case from the contribution of the halftone dots (222, 222a) of the first sub-irradiation plan and the at least one further sub-irradiation plan, using the transformation parameters.