Disclosed is a method for determining a deviation of the position of an anatomical body part relative to a predetermined position defined by a radiation treatment plan. A monitoring image is taken of the anatomical body part at the instant that a predetermined control point defined in the treatment plan has been reached. The monitoring image is compared to a simulated image simulated from a planning image of the anatomical body from the perspective of a monitoring camera used to generate the monitoring image. The comparison allows for a determination of whether there is a positional deviation.