Radiotherapy apparatus comprising: a source of therapeutic radiation, a first detector for the therapeutic radiation； a support for locating a patient between the source and the detector； a second detector for deriving an internal image of the patient and a controller in receipt of the detectors output. The control apparatus is adapted to: use the output of the first detector to reconstruct a volume image representing the radiation dose delivered to the patient by the source； and display the volume image against a background of an image derived from the second detector. The first detector may be flat-panel scintillation detector. The volume image may be a difference image showing a difference between a pre-planned dose and a delivered dose derived from the first detector. The second detector may comprise a source of diagnostic radiation of lower energy than therapeutic-radiation. The second detector may comprise a magnetic resonance imager or possibly an ultrasound apparatus. The second detector image may be a two-dimensional radiograph. The second detector image may be a section taken from one of several volume images created using the output of the second detector, each depicting the patient at a different phase of a breathing cycle.
