A data processing method performed by a computer for planning trajectories of at least two catheters (6, 7), wherein each trajectory ends at a target point which is associated with a corresponding catheter which is one of said catheters and located within a brain which is in turn located in a cranium, the method comprising the steps of: a) acquiring an initial brain image dataset which represents a three-dimensional image of at least a part of the brain; b) acquiring an initial planning dataset which represents initial trajectories of the catheters (6, 7); and c) acquiring a patient orientation dataset which represents the orientation of the cranium; the method being characterized by the steps of: d) acquiring a first burr hole dataset which represents the size and location of a first burr hole in the cranium for a first catheter; e) calculating a first brain shift dataset from the initial brain image dataset, the patient orientation dataset and the first burr hole dataset, wherein the first brain shift dataset at least represents a brain shift which would be caused by the first burr hole; f) calculating an updated brain image dataset from the initial brain image dataset and the first brain shift dataset; g) updating the trajectory of the first catheter on the basis of the updated brain image dataset; h) acquiring a subsequent burr hole dataset which represents the size and location of a subsequent burr hole in the cranium for a subsequent catheter; i) calculating a subsequent brain shift dataset from the updated brain image dataset, the patient orientation dataset and the subsequent burr hole dataset, wherein the subsequent brain shift dataset at least represents a brain shift which would be caused by the subsequent burr hole; j) updating the updated brain image dataset on the basis of the subsequent brain shift dataset; and k) updating the trajectory of the subsequent catheter and all preceding catheters on the basis of the re-updated brain image dataset, wherein a preceding catheter is a catheter whose trajectory has been updated in a preceding step of the method. a) acquiring an initial brain image dataset which represents a three-dimensional image of at least a part of the brain; b) acquiring an initial planning dataset which represents initial trajectories of the catheters (6, 7); and c) acquiring a patient orientation dataset which represents the orientation of the cranium; the method being characterized by the steps of: d) acquiring a first burr hole dataset which represents the size and location of a first burr hole in the cranium for a first catheter; e) calculating a first brain shift dataset from the initial brain image dataset, the patient orientation dataset and the first burr hole dataset, wherein the first brain shift dataset at least represents a brain shift which would be caused by the first burr hole; f) calculating an updated brain image dataset from the initial brain image dataset and the first brain shift dataset; g) updating the trajectory of the first catheter on the basis of the updated brain image dataset; h) acquiring a subsequent burr hole dataset which represents the size and location of a subsequent burr hole in the cranium for a subsequent catheter; i) calculating a subsequent brain shift dataset from the updated brain image dataset, the patient orientation dataset and the subsequent burr hole dataset, wherein the subsequent brain shift dataset at least represents a brain shift which would be caused by the subsequent burr hole; j) updating the updated brain image dataset on the basis of the subsequent brain shift dataset; and k) updating the trajectory of the subsequent catheter and all preceding catheters on the basis of the re-updated brain image dataset, wherein a preceding catheter is a catheter whose trajectory has been updated in a preceding step of the method.