A computer-based method of providing a set of selected spots for use in ion radiotherapy optimization is proposed, the method comprising the steps of defining a number of positions in the volume, each position corresponding to a possible Bragg peak location, the positions being defined to provide coverage of the volume, for each position, for at least one beam direction, performing a ray trace to determine characteristics of a potential spot in the beam that will place its Bragg peak in the position and for each position, selecting zero, one or more of the potential spots, based on the characteristics, and including the one or more selected spots in the set of selected spots. The set of selected spots may be used in treatment planning. The invention also relates to computer program products and a computer system for performing the methods.