A method of determining a radiation therapy treatment plan including a medical data processing method for determining a target set comprising irradiation targets in an anatomical body part of a patient's body for radiation therapy treatment by means of a treatment device constituted to treat the irradiating targets by means of one or more sub-beams, the one or more sub-beams constituting at least one treatment beam which is to pass through at least one of the irradiation targets in accordance with a treatment plan during the treatment time, the method comprising the following steps and being constituted to be executed by a computer: a) acquiring (S1.1) critical area data describing spatial information on anatomical regions, irradiation of which regions is to be avoided during radiation therapy treatment; b) acquiring (S1.2) target data describing spatial information on the irradiation targets; c) acquiring (S1.3) treatment beam constraint data which allows determination of potential treatment beam arrangements which can be realized by the treatment device and which potential treatment beam arrangements describe potential arrangements of the at least one treatment beam; d) acquiring treatment beam criteria data (S1.4) describing criteria for the treatment of the irradiation 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 radiation treatment of the irradiation 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 irradiation targets, referred to as outside body parts, are less exposed to treatment radiation at least to a predetermined extent, compared to the case of a treatment plan which describes a simultaneous treatment of the irradiation targets and the outside body parts; and e) determining, based on the critical area data, the target data, the treatment beam constraint data and the treatment beam criteria data, target set data describing a target set of irradiation targets and spatial information on at least one angular irradiation region comprising the target set and at least one angular non-irradiation region, irradiation of which is to be suppressed, wherein the target set data is determined based on at least one of information about a spatial relationship between one of the irradiation targets and a nearest neighbouring one of the irradiation targets and information about the envisaged radiotherapy treatment, wherein the treatment plan is determined based on the target set data and wherein the treatment plan fulfils the radiation reduction criterion based on the target data, the treatment beam constraint data and the treatment beam criteria data, and a constraint mask is determined based on the target set data, the constraint mask prescribing, for determining the treatment plan, which parts of the anatomical body part are to be determined as lying within an irradiation region and which parts of the anatomical body part are to be determined as lying in a non-irradiation region. a) acquiring (S1.1) critical area data describing spatial information on anatomical regions, irradiation of which regions is to be avoided during radiation therapy treatment; b) acquiring (S1.2) target data describing spatial information on the irradiation targets; c) acquiring (S1.3) treatment beam constraint data which allows determination of potential treatment beam arrangements which can be realized by the treatment device and which potential treatment beam arrangements describe potential arrangements of the at least one treatment beam; d) acquiring treatment beam criteria data (S1.4) describing criteria for the treatment of the irradiation 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 radiation treatment of the irradiation 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 irradiation targets, referred to as outside body parts, are less exposed to treatment radiation at least to a predetermined extent, compared to the case of a treatment plan which describes a simultaneous treatment of the irradiation targets and the outside body parts; and e) determining, based on the critical area data, the target data, the treatment beam constraint data and the treatment beam criteria data, target set data describing a target set of irradiation targets and spatial information on at least one angular irradiation region comprising the target set and at least one angular non-irradiation region, irradiation of which is to be suppressed, wherein the target set data is determined based on at least one of information about a spatial relationship between one of the irradiation targets and a nearest neighbouring one of the irradiation targets and information about the envisaged radiotherapy treatment, wherein the treatment plan is determined based on the target set data and wherein the treatment plan fulfils the radiation reduction criterion based on the target data, the treatment beam constraint data and the treatment beam criteria data, and