A data processing method for determining a treatment plan for radiation therapy treatment of at least two spatially separate targets by means of a treatment device constituted to treat the at least two targets by means of one or more sub-beams during a treatment time, the one or more sub-beams constituting at least one treatment beam which is to pass through the at least two targets in accordance with a treatment plan during the treatment time, the treatment device being further constituted to allow for simultaneous treatment of the at least two targets by at least two of the sub-beams at least during a time interval during the treatment time, the method comprising the following steps performed by a computer: - acquiring target data describing spatial information on the at least two spatially separate targets in a patient's body; - acquiring treatment beam constraint data which allow determination of potential treatment plans which can be realized by the treatment device and which potential treatment plans describe potential arrangements of the at least one treatment beam, referred to as potential treatment beam arrangements; - acquiring treatment beam criteria data describing criteria for the treatment of the at least two targets by the at least one treatment beam, the criteria comprising a radiation reduction criterion which describes that a treatment plan is allowed to be determined which describes a non-simultaneous treatment of the at least two targets at least for a time interval during the treatment time if this treatment plan describes that parts of the patient's body outside the at least two targets are less exposed to treatment radiation, in particular at least to a predetermined extent, compared to the case of a treatment plan which describes a simultaneous treatment of the at least two targets during the treatment time; and - determining the treatment plan which fulfils the radiation reduction criterion based on the target data, the treatment beam constraint data and the treatment beam criteria data. - acquiring target data describing spatial information on the at least two spatially separate targets in a patient's body; - acquiring treatment beam constraint data which allow determination of potential treatment plans which can be realized by the treatment device and which potential treatment plans describe potential arrangements of the at least one treatment beam, referred to as potential treatment beam arrangements; - acquiring treatment beam criteria data describing criteria for the treatment of the at least two targets by the at least one treatment beam, the criteria comprising a radiation reduction criterion which describes that a treatment plan is allowed to be determined which describes a non-simultaneous treatment of the at least two targets at least for a time interval during the treatment time if this treatment plan describes that parts of the patient's body outside the at least two targets are less exposed to treatment radiation, in particular at least to a predetermined extent, compared to the case of a treatment plan which describes a simultaneous treatment of the at least two targets during the treatment time; and - determining the treatment plan which fulfils the radiation reduction criterion based on the target data, the treatment beam constraint data and the treatment beam criteria data.