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 , - 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 configured to be detectable by the radiation imaging device , 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 . - 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 , - 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 configured to be detectable by the radiation imaging device , 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 configured to be detectable by the radiation imaging device , 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 . <EOS>