The invention relates to a planning system 7 for planning a radiation therapy procedure, wherein the radiation therapy procedure includes rotating a radiation source 2, which is configured to emit a radiation beam 3 for treating a subject 4, along an arc around the subject, in order to apply the radiation beam to the subject in different treatment directions. A sequence of sub-arcs groups is provided, wherein a respective sub-arcs group comprises several sub-arcs of the arc, which are evenly distributed along the arc, wherein a treatment plan is generated by sequentially determining for each sub-arcs group treatment parameters, which define a property of the radiation beam, in the provided sequence. Determining the treatment parameters by using this sequence leads to an increased likelihood that really an optimal treatment plan is generated.
