A radiotherapy apparatus comprises a rotatable gantry, supporting a source of therapeutic radiation and a source of diagnostic radiation. The two sources may be rotationally spaced apart around a rotation axis of the gantry, with at least one collimator associated with the source of therapeutic radiation and arranged to limit the cross-sectional area of a beam produced by that source. A control means may be arranged to conduct a treatment fraction using the apparatus by causing the apparatus to acquire images of a patient using the source of diagnostic radiation, retain those images at least temporarily, select a retained image acquired when the source of diagnostic radiation was at a rotational position corresponding to the instantaneous rotational position of the source of therapeutic radiation, and control the beam relative to the patient using information derived from the selected image.
