A method of calibrating a stereoscopic camera system for use with a radio therapy treatment apparatus, the radio therapy apparatus having an iso-centre which defines an origin of the radio therapy treatment apparatus co-ordinate system, the method comprising: i) utilizing a stored reference 3D computer model of a surface of a calibration phantom positioned at the iso-centre of the radio therapy treatment apparatus with the centre of the stored reference 3D computer model coincident with the centre of the calibration phantom; ii) positioning the calibration phantom in the radio therapy treatment apparatus setup with the phantom's centre at an estimated location for the iso-centre, iii) utilizing the stereoscopic camera system to obtain images of the calibration phantom; iv) irradiating the positioned calibration phantom using the radio therapy treatment apparatus to obtain irradiation images of the positioned calibration phantom; and v) determining a first transformation required to transform the position and orientation of the positioned calibration phantom into the co-ordinate system of the radio therapy treatment apparatus in order to make the location of the centre of the positioned calibration phantom coincident with the iso-centre of the radio therapy treatment apparatus, by analysing the images obtained in step iv) vi) processing images obtained by the stereoscopic camera system in step iii) to generate a 3D computer model of the surface of the positioned calibration phantom, vii) determining a second transformation required to transform the generated 3D computer model of the surface of the positioned calibration phantom into alignment with the surface of the stored reference 3D computer model of the calibration phantom, in order to make the current origin for the co-ordinate system for the generated 3D computer model of the surface of the positioned calibration phantom coincide with the centre of the stored reference 3D computer model of the calibration phantom, and viii) applying a sum of the first and second transformations to transform the positions and orientations of the generated 3D computer model obtained by the stereoscopic camera system into corresponding positions and orientations in the co-ordinate system of the radio therapy treatment apparatus. i) utilizing a stored reference 3D computer model of a surface of a calibration phantom positioned at the iso-centre of the radio therapy treatment apparatus with the centre of the stored reference 3D computer model coincident with the centre of the calibration phantom; ii) positioning the calibration phantom in the radio therapy treatment apparatus setup with the phantom's centre at an estimated location for the iso-centre, iii) utilizing the stereoscopic camera system to obtain images of the calibration phantom; iv) irradiating the positioned calibration phantom using the radio therapy treatment apparatus to obtain irradiation images of the positioned calibration phantom; and v) determining a first transformation required to transform the position and orientation of the positioned calibration phantom into the co-ordinate system of the radio therapy treatment apparatus in order to make the location of the centre of the positioned calibration phantom coincident with the iso-centre of the radio therapy treatment apparatus, by analysing the images obtained in step iv) vi) processing images obtained by the stereoscopic camera system in step iii) to generate a 3D computer model of the surface of the positioned calibration phantom, vii) determining a second transformation required to transform the generated 3D computer model of the surface of the positioned calibration phantom into alignment with the surface of the stored reference 3D computer model of the calibration phantom, in order to make the current origin for the co-ordinate system for the generated 3D computer model of the surface of the positioned calibration phantom coincide with the centre of the stored reference 3D computer model of the calibration phantom, and viii) applying a sum of the first and second transformations to transform the positions and orientations of the generated 3D computer model obtained by the stereoscopic camera system into corresponding positions and orientations in the co-ordinate system of the radio therapy treatment apparatus.