According to a first aspect, it is presented a method for determining a treatment plan comprising a distribution of spots for use with ion beam therapy for providing the spots in a target volume. The method comprises the steps of: selecting energy layers to be used in the treatment plan； determining a number of spot sizes to use； generating, for each energy layer, one copy of the energy layer for each spot size to use and populating each copy with spots of the spot size for that copy； optimizing spots of all copies of all energy layers, by repeatedly varying a weight of at least a subset of the spots and calculating an effect on a performance measurement, wherein the performance measurement is calculated by combining a plurality of evaluation criteria, comprising a first criterion related to total treatment time and a second criterion related to a desired dose distribution.
