A monitoring system for use with radiotherapy apparatus comprising a target surface 36a having one or more three-dimensional projections 38 provided thereon, each projection 38 having a multiplicity of planar side surfaces, a stereoscopic camera 14 operable to obtain images of the target surface 36a, and a processing module 200 operable to process images obtained by the stereoscopic camera 14 of the target surface 36a together with data identifying the position and orientation of the stereoscopic camera 14 relative to a defined point in space to determine the positions relative to the defined point in space of the planar side surfaces defined by the at least one or more three-dimensional projections 36a. There may be a plurality of target surfaces 36a, which may be arranged on an object which is mounted on the patient.
