A radiation treatment plan is determined for a treatment volume associated with a target and at least one organ-at-risk. For each delivery direction, a respective description is determined defining how an amount of radiant energy from a radiation source shall be distributed over a projection area, by the consecutive steps: [1] receiving a current fluence map defining a radiation dose to be delivered to the at least one target [2] receiving a current control-point sequence describing machine settings for a collimator associated with the radiation source, the machine settings defining at least one parameter for an output beam of radiation from the radiation source [3] determining an updated fluence map (FM) and an updated control-point sequence based on the current fluence map, the current control-point sequence and an objective function quantifying at least one quality of the radiation dose with respect to a predefined goal [4] determining a further updated control-point sequence based on the updated control-point sequence and the updated fluence map (FM), the further updated control-point sequence describing how, with respect to an error-minimizing function, to adjust the machine settings to approximate an amount of radiant energy defined by the updated control-point sequence and the updated fluence map (FM), [5] determining a further updated fluence map based on the updated fluence map (FM), the updated control-point sequence and the further updated control-point sequence [6] checking if a stopping criterion is fulfilled if so: stopping the process, and producing an output radiation treatment plan based on the further updated control-point sequence and otherwise: setting the further updated fluence map, or zero, to the current fluence map setting the further updated control-point sequence to the current control-point sequence and returning to step [3].
