Disclosed is a computer-implemented method of determining an entry region for inserting elongated medical instruments such as trocars or laparoscopes into an anatomical body part such as the abdomen. The determination is made on the basis of a patient image and a known or assumed geometry of the medical instruments and a known position of the target region, and risk structures which shall be avoided by the medical instrument are determined either by user interaction from the patient image or automatically by matching the patient image with an atlas defining the position of such risk structures in an image-based model of the anatomical body part. On the basis of the position of the determined entry region, a robotic device may be controlled to attain a position which is suitable for automatic insertion of the medical instruments into the anatomical body part.