A calibration sheet is located at a first position where its surface substantially corresponds to the expected position of a patient surface lying on a mechanical couch during treatment. Images of the calibration sheet are obtained and processed to ascertain relative locations and orientations of the image detectors obtaining the images and optical distortion parameters indicative of optical distortions present in the obtained images of the calibration sheet. The calibration sheet is then re-located to a known position relative to the iso-centre of the treatment apparatus and images of the re-located calibration sheet are obtained and processed to determine a transform corresponding to the relocation of the calibration sheet from the first position to the iso-centre of the treatment apparatus. Data indicative of optical distortions present in the images and data indicative of the locations and orientations of the image detectors relative to the iso-centre of the treatment apparatus are stored.