The present invention relates to 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 describing an image of an anatomical body part; b) acquiring target region extent data comprising target region extent information describing a spatial extent of the target region in the medical image information; 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 describing a tissue type component of the target region assigned to each target region image particle in the target region, which component is represented at least partly by the image particle, and 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.