Method of calibrating a patient monitoring system for use with a radiotherapy treatment apparatus

A method of verifying stored pre-calibrated parameters of a camera system for monitoring a patient during a radiotherapy treatment, wherein the camera system comprises an image detector configured to obtain an image of light reflected by an object, and wherein the stored pre-calibrated parameters, which are determined on the basis of processing a set of initial images obtained by the image detector, relate to location and orientation of the image detector relative to the object, and relate to internal characteristics of the image detector including a focal length, the method comprising:

• using the image detector to obtain an additional image of light reflected by the object;
• processing said additional image by using the stored parameters to determine location and orientation of the object relative to the image device;
• determining a back-projection of an expected appearance of the object based on the stored parameters onto the image plane of the image detector;
• and
• comparing the appearance of the object within the additional image obtained of the object with the back-projection of the expected appearance of the object.