A radiotherapy system, comprising - a couch, - a gantry which is configured to be rotatable around a gantry axis and having a radiation source, - at least one radiation imaging device (330, 340), - a calibration system comprising - at least one first optical detection means mounted at the gantry, - at least one second optical detection means fixed in a surrounding area of the couch and/or the gantry, - a set of first fiducial markers which are selectively attachable at the couch at defined positions and configured to be detectable by the first optical detection means, and - a phantom which is selectively attachable at the couch at a defined position, wherein the phantom comprises a set of second fiducial markers configured to be detectable by the second optical detection means and a set of third fiducial markers (331, 341) configured to be detectable by the radiation imaging device (330, 340), and - a controlling means which is configured to selectively activate the radiation source and rotate the gantry, and, for one or more rotational positions of the gantry, to determine a point of intersection of a beam axis of the radiation source and the gantry axis by linking detection data of at least the first optical detection means, the second optical detection means and/or the radiation imaging device (330, 340). - a couch, - a gantry which is configured to be rotatable around a gantry axis and having a radiation source, - at least one radiation imaging device (330, 340), - a calibration system comprising - at least one first optical detection means mounted at the gantry, - at least one second optical detection means fixed in a surrounding area of the couch and/or the gantry, - a set of first fiducial markers which are selectively attachable at the couch at defined positions and configured to be detectable by the first optical detection means, and - a phantom which is selectively attachable at the couch at a defined position, wherein the phantom comprises a set of second fiducial markers configured to be detectable by the second optical detection means and a set of third fiducial markers (331, 341) configured to be detectable by the radiation imaging device (330, 340), and - at least one first optical detection means mounted at the gantry, - at least one second optical detection means fixed in a surrounding area of the couch and/or the gantry, - a set of first fiducial markers which are selectively attachable at the couch at defined positions and configured to be detectable by the first optical detection means, and - a phantom which is selectively attachable at the couch at a defined position, wherein the phantom comprises a set of second fiducial markers configured to be detectable by the second optical detection means and a set of third fiducial markers (331, 341) configured to be detectable by the radiation imaging device (330, 340), and - a controlling means which is configured to selectively activate the radiation source and rotate the gantry, and, for one or more rotational positions of the gantry, to determine a point of intersection of a beam axis of the radiation source and the gantry axis by linking detection data of at least the first optical detection means, the second optical detection means and/or the radiation imaging device (330, 340).