It is provided a method for determining a distribution of spots for use with ion beam therapy for providing the spots in a target volume, wherein each spot represents a collection of ions of a specific energy level and of a specific size at a specific lateral location. The method is performed in a treatment planning system and comprises the steps of: dividing the target volume in a plurality of target sections based on a user configuration comprising at least one spot size strategy defining a maximum spot size at the location of a Bragg peak； assigning a spot size strategy to each one of the target sections based on the location of the respective target section； and determining, within each target section, spots in accordance with its spot size strategy.
