In order to increase the accuracy of the calibration of an imaging system of a radiation treatment system operable to generate a treatment beam, an imaging device generates first data representing a reticle disposed in a beam path of the treatment beam. A first transformation between at least two dimensions of a coordinate system of a radiotherapy device of the radiation treatment system and a two-dimensional (2D) image coordinate system for the imaging device is determined based on the first data. The imaging device generates second data representing a phantom including a plurality of markers. A position of the phantom in the coordinate system of the radiotherapy device is determined based on the second data and the first transformation. A second transformation between three dimensions of the coordinate system of the radiotherapy device and the 2D image coordinate system for the imaging device is determined based on the second data and the determined position of the phantom.