The present invention concerns a method, a system, a measurement tool and computer readable medium for determining a predetermined position of a positioning system in a radiation therapy system comprising a radiation therapy unit relatively a fixed radiation focus point of the radiation therapy unit. The method comprises positioning (73) a diode element, obtaining (75) measurement data at each of a predetermined number of measurement points from a radiation intensity curve, determining (76) a three-dimensional centre coordinate of the obtained coordinates, and determining (76) the predetermined position of a patient positioning unit (20) relatively to a fixed focus point of a radiation unit (10) using the determined three-dimensional centre coordinate.