A method for creating a therapy plan for a particle therapy, in which a filter is used to adjust the depth of penetration of the particle beam and an immobilization device is used to immobilize a body region of a patient to be treated, includes adjusting the geometry of a deformable mass of a filter to the geometry of the immobilization device. The deformable filter mass is applied, at least partially, to the immobilization device. A planning data record is obtained with recordings of the filter, so that the properties of the filter are determined on the basis of the recordings and are used in the therapy plan.
