There is presented a method 100 and apparatus 200 to measure the surface of the patient (thorax and abdominal regions), e.g., during therapy delivery and (if necessary) while imaging. Together with biomechanical considerations the position of internal structures of the patient, such as an organ, and optionally a tumor in an organ, is inferred from the measured patient surface. In case the patient breaths and thus the organ and/or tumor moves, the position may be determined, which may be advantageous during, e.g., radiation therapy, since it enables that whenever the tumor is at the right position according to the radiation therapy plan, the radiation is switched on. In a specific embodiment, a finite element model is employed.