A data processing method for planning an infusion of a medical substance into a target region (10, 11) in an anatomical body part comprising at least a part of the brain, wherein the medical substance is to be infused by means of a catheter for performing the infusion, the method comprising: a) acquiring target region geometry data comprising target region geometry information describing the geometry of the target region (10, 11); b) acquiring target coverage data comprising target coverage information describing a planned coverage of the target region (10, 11) by the medical substance, wherein the target coverage data is acquired based on the target region geometry data; c) acquiring relationship data comprising relationship information which describes a relationship between tip lengths of the catheter and an ellipsoidal predetermined spatial distribution (10', 11') of the medical substance, wherein the tip length of the catheter is the length of a portion located at the distal end of the catheter comprising outlets for the medical substance for exiting the catheter, wherein the proximal end of the portion is defined as the point at which the diameter of the catheter decreases in the distal direction; d) determining infusion setup data comprising infusion setup information which describes the tip length, based on the relationship data and the target coverage data and based on a comparison of the predetermined spatial distribution (10', 11') of the medical substance with the planned coverage of the target region (10, 11) with the medical substance and determining the tip length for which the predetermined spatial distribution (10', 11') best fits the planned coverage (10, 11), assuming that a ratio between the tip length and a length of a longitudinal axis of the predetermined spatial distribution has a value of 0.16. a) acquiring target region geometry data comprising target region geometry information describing the geometry of the target region (10, 11); b) acquiring target coverage data comprising target coverage information describing a planned coverage of the target region (10, 11) by the medical substance, wherein the target coverage data is acquired based on the target region geometry data; c) acquiring relationship data comprising relationship information which describes a relationship between tip lengths of the catheter and an ellipsoidal predetermined spatial distribution (10', 11') of the medical substance, wherein the tip length of the catheter is the length of a portion located at the distal end of the catheter comprising outlets for the medical substance for exiting the catheter, wherein the proximal end of the portion is defined as the point at which the diameter of the catheter decreases in the distal direction; d) determining infusion setup data comprising infusion setup information which describes the tip length, based on the relationship data and the target coverage data and based on a comparison of the predetermined spatial distribution (10', 11') of the medical substance with the planned coverage of the target region (10, 11) with the medical substance and determining the tip length for which the predetermined spatial distribution (10', 11') best fits the planned coverage (10, 11), assuming that a ratio between the tip length and a length of a longitudinal axis of the predetermined spatial distribution has a value of 0.16.