1. A method for creating a therapy plan for a particle therapy, the method comprising: adjusting the geometry of a deformable filter mass of a filter to the geometry of an immobilization device; applying, at least partially, the deformable filter mass to the immobilization device; obtaining a planning data record with a recording of the filter; and determining properties of the filter to be used for the therapy plan using the planning data record, wherein the filter is used to adjust the depth of penetration of a particle beam, and wherein the immobilization device is used to immobilize a body region of a patient to be treated.