Images obtained by a camera system (10) arranged to obtain stereoscopic images of a patient (20) undergoing radio-therapy are processed by a modeling unit (56,58) which generates a model of the surface of a patient (20) being monitored. Additionally the patient monitoring system processes image data not utilized to generate a model of the surface of a patient being monitored to determine further information concerning the treatment of the patient (20). Such additional data can comprise data identifying the relative location of the patient and a treatment apparatus (16). This can be facilitated by providing a number or retro-reflective markers (30-40) on a treatment apparatus (16) and a mechanical couch (18) used to position the patient (20) relative to the treatment apparatus (16) and monitoring the presence and location of the markers in the portions of the images obtained by the stereoscopic camera (10).