Some embodiments are directed to a calibration method including a calibration phantom positioned on an adjustable table on the surface of a mechanical couch, with the phantom's center at an estimated location for the iso-center of a radio therapy treatment apparatus. The calibration phantom is then irradiated using the apparatus, and the relative location of the center of the calibration phantom and the iso-center of the apparatus is determined by analyzing images of the irradiation. The calibration phantom is then repositioned by the mechanical couch applying an offset corresponding to the determined relative location of the center of the calibration phantom and the iso-center of the apparatus to the calibration phantom. Images of the relocated calibration phantom are obtained, and the images are processed to set the co-ordinate system of a 3D camera system relative to the iso-center of the apparatus.