The present invention relates to a method for positioning an arrangement of at least one beam position relative to a patient's body when the patient is placed for treatment by means of a treatment device which can emit treatment radiation, wherein the at least one beam position describes at least one position of at least one treatment beam, wherein the arrangement is referred to as the treatment arrangement and the treatment beam comprises the treatment radiation, wherein the method is executed by a computer and comprises the following steps: providing treatment beam absorption data which describe the absorption properties of the at least part of the body with respect to absorbing the treatment radiation, and the relative position between the at least part of the body and the treatment arrangement; providing treatment beam data which describe radiation properties of the at least one treatment beam; providing condition data which describe a condition for treating the at least part of the body; determining an optimum relative position between the treatment arrangement and the at least part of the body on the basis of the condition data, the treatment beam data and the treatment beam absorption data.