The invention relates to a system for planning a radiation therapy treatment, wherein radiation delivered to a region of a patient comprises a plurality of individually controllable radiation contributions. The system obtains a first treatment plan generated in accordance with values of parameters quantifying an amount of radiation provided by the radiation components and resulting in a first dose distribution in the region of the patient body, obtains an instruction to change a radiation dose delivered to at least one volume element, and directly calculates, for each of the radiation components, a change of the amount of radiation provided by the radiation component based on the instruction and based on the contribution of the radiation component to the radiation dose delivered to the at least one volume element. On the basis of the calculated changes, a second treatment plan is determined.
