A radiotherapy device includes a therapeutic radiation source for providing a beam for irradiating a moving target volume and a beam correcting apparatus for directing the beam onto the target volume. The beam correcting apparatus includes a collimator having a collimator aperture for delimiting the beam, a positioning apparatus for positioning the target volume relative to the beam, and a controller for selecting a position of the collimator aperture relative to the beam and for selecting a position of the positioning apparatus such that when the beam is correctively adjusted, the movement of the target volume in a first movement direction is compensated for by displacing the collimator aperture, and the movement of the target volume in a second movement direction is compensated for by repositioning the target volume with the positioning apparatus.
