A data processing method performed by a computer for monitoring the position of a patient, comprising the steps of: acquiring a 3D image dataset of the patient; acquiring an initial real image of the patient from a medical imaging system having a known and fixed location in space, thus defining a known viewing direction of the initial real image in space, the real image being taken at a first point in time; —performing 2D/3D registration by calculating a simulated image from the 3D image dataset which matches the initial real image best, the simulated image having a viewing direction onto the 3D image dataset; storing an initial similarity measure value for the pair of the initial real image and the calculated simulated image; —acquiring a subsequent real image from the medical imaging system taken at a second point in time later than the first point in time; calculating a subsequent similarity measure value for the pair of the subsequent real image and the calculated simulated image; and outputting an indication signal if the difference between the initial similarity measure value and the subsequent similarity measure value fulfills a predetermined criterion.