A radiation therapy device includes a radiation application apparatus, which is used to direct a therapeutic beam along a therapeutic beam center axis onto a target volume to be irradiated. The radiation therapy device also includes a diagnostic radiation source for diagnostic x-ray radiation, is the diagnostic radiation source being disposed eccentrically in relation to the therapeutic beam center axis in a rotatable manner on a rotation apparatus so that the diagnostic radiation source is operable to be rotated about the therapeutic beam center axis. The radiation therapy device further includes a detector for diagnostic x-ray radiation, which is used to detect the diagnostic x-ray radiation emitted from the diagnostic radiation source.