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); and 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; and 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), characterized in that the tip length is determined based on adapting the predetermined spatial distribution to fit the planned coverage, wherein the predetermined spatial distribution has been previously determined by executing a simulation of the flow rate of a flow of the medical substance through the catheter and a given tip length resulting in that, for the given tip length, a ratio between the semi-axes of the ellipsoidal predetermined spatial distribution is constant for flow rates of a medical substance less than a critical flow rate, and wherein the predetermined spatial distribution approaches a spherical geometry with decreasing flow rate, and wherein an increase in the flow rate leads to an increase in the length of the longitudinal semi-axis and a decrease in the length of the transverse axis of the predetermined spatial distribution, wherein a tip length which is associated with a ratio between the semi-axes of the ellipsoidal predetermined spatial distribution which best fits the planned coverage is selected, as the tip length of the catheter, from a table of associated pairs of flow rates and tip lengths. a) acquiring target region geometry data comprising target region geometry information describing the geometry of the target region (10, 11); and 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; and 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),