A data processing method of determining a trajectory of a medical instrument to be inserted into a patient's body for treatment of a target region in the patient's body, the method being executed by a computer and comprising the following steps: a) acquiring medical image data comprising medical image information representing an image of an anatomical body part; and b) acquiring (S5, S6) target region extent data comprising target region extent information describing a spatial extent of the target region in the medical image information, the spatial extent being defined as a number of image particles comprised in the target region called target region image particles, an image particle being a discrete unit for displaying the medical image information, characterised by c) determining, based on the target region extent data, whether the spatial extent of the target region in the medical image information comprises more than one image particle; d) if it is determined that the target region comprises more than a predetermined number of image particles, determining, based on the medical image data, image particle weight data comprising image particle weight information describing a weight represented by a numeric value describing a tissue type component of the target region, which numeric value is assigned to each target region image particle in the target region and depends on the number of tissue type components assigned to the respective image particle, which component is represented at least partly by the image particle, and e) determining, based on the image particle weight data and the medical image data, instrument trajectory data comprising instrument trajectory information describing a trajectory of the medical instrument along which the instrument is to be inserted into the patient's body, wherein determining the instrument trajectory data comprises determining an entry point at which the medical instrument is to be inserted into the patient's body, wherein the entry point is defined to lie on the surface of a trajectory volume including all possible trajectories from any possible entry point towards the target region, wherein an individual weight is assigned to each target region image particle for each tissue type component assigned to it, in particular for each surface or volume of which the target region image particle is a member, and a unique sum of those weights is assigned to the target region image particle, the sum representing the specific combination of tissue types, wherein the instrument trajectory data is determined based on that sum, wherein a target end point of the trajectory is determined to lie in a reduced target region comprising the target region image particles having the highest sum of weights. a) acquiring medical image data comprising medical image information representing an image of an anatomical body part; and b) acquiring (S5, S6) target region extent data comprising target region extent information describing a spatial extent of the target region in the medical image information, the spatial extent being defined as a number of image particles comprised in the target region called target region image particles, an image particle being a discrete unit for displaying the medical image information, characterised by c) determining, based on the target region extent data, whether the spatial extent of the target region in the medical image information comprises more than one image particle; d) if it is determined that the target region comprises more than a predetermined number of image particles, determining, based on the medical image data, image particle weight data comprising image particle weight information describing a weight represented by a numeric value describing a tissue type component of the target region, which numeric value is assigned to each target region image particle in the target region and depends on the number of tissue type components assigned to the respective image particle, which component is represented at least partly by the image particle, and e) determining, based on the image particle weight data and the medical image data, instrument trajectory data comprising instrument trajectory information describing a trajectory of the medical instrument along which the instrument is to be inserted into the patient's body,