A method of identifying portions of a model of a surface of a patient which can be used to identify to position of a patient is provided. A pre-existing model of the surface of a patient is utilised to generate simulated images of the projection of patterns of light onto the surface of the patient. The simulated images are then processed to generate a further model of the surface. Any differences between the model generated by processing the simulated images and the original model surface identifies portions of the surface which cannot be reliably modelled and hence portions of a surface which should not be used to monitor the positioning of a patient. The invention has particular application for identifying the reliability and accuracy of generated models of the surface of a patient used for monitoring and positioning a patient undergoing radiotherapy.
