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

A method of calibrating a patient monitoring system the method comprising:

• using an image detector of the patient monitoring system to obtain a set of calibration images comprising images of a calibration object at various orientations and locations;
• storing the obtained calibration images;
• processing the stored calibration images to calculate camera parameters defining the location and orientation of the image detector and parameters identifying internal characteristics of the image detector, and storing the calculated parameters;
• and
• subsequently re-calibrating the patient monitoring system by:
• obtaining an additional image of a calibration object using the image detector;
• detecting an apparent change in the internal characteristics of the image detector using the additional image obtained using the image detector and the stored camera parameters, and
• if an apparent change in the internal characteristics of an image detector is detected, obtaining a further set of calibration images using the image detector and recalculating the camera parameters for the image detector using the further set of calibration images;
• and
• if no apparent change in the internal characteristics of an image detector is detected, recalculating the camera parameters for the image detector using the stored set of calibration images and the additional image of the calibration object obtained by the image detector.