Radiotherapy apparatus with calibration

A radiotherapy apparatus is disclosed, having an imaging system 2 and a therapeutic radiation source 7, an imageable volume into which therapeutic radiation may be provided, a first imaging means for providing an image resulting from the imaging system and a second imaging means 7b for providing an image resulting from the therapeutic radiation (e.g. portal imaging), and a patient support 19 moveable relative to the volume, wherein the patient support 19 is provided with a calibration portion 23 comprising at least one object of a material capable of appearing in the image produced by the first imaging means 2 and at least one object of a material capable of appearing in the image produced by the second imaging means 7b, the at least one object being fixed in a pre-determined position relative to the calibration portion 23 and the calibration portion being fixed in a pre-determined position relative to the patient support 19, the patient support 19 being moveable so that the at least one object portion may optionally be positioned within or outside the volume. A room and method are also claimed.