A method for calculating local part radiation doses in a radiotherapy system for applying a total radiation dose in a target volume with several beams is provided. The method includes determining at least one first control plane for controlling the dosing of the beams, determining at least one second control plane for controlling the positioning of the beams, and allocating beams to the first and second control planes. The method also includes, for at least one side of a first control plane, calculating in isolation of the corresponding local part radiation doses of all the beams allocated to the first control plane.