A radiotherapy apparatus is disclosed, comprising a fixed support and a gantry comprising a chassis part and a source part, the chassis part being rotateably attached to the fixed support to allow rotation thereof about a generally horizontal axis, and the source part being connected to the chassis part via a rotateable connection allowing the source part to rotate relative to the chassis part around a transverse axis, being aligned transverse to and intersecting with the horizontal axis； the source part comprising a source of therapeutic radiation directed towards the intersection of the transverse axis and the horizontal axis； the chassis part and the source part together defining an annular ring that encircles the horizontal axis. In this way, the radiotherapy apparatus can provide the full usual range of treatments, but can also adapt itself to adopt a non-coplanar geometry when required, for example to treat difficult locations in the head and neck. The chassis part can comprise a source of diagnostic radiation and a detector for the imaging radiation, to allow the patient to be monitored prior to, during, and/or after treatment. The source of diagnostic radiation preferably emits a beam in a direction transverse to the direction of the therapeutic radiation so as to prevent scattering from the structure of the therapeutic source and to this end, the diagnostic beam can be emitted along the transverse axis. The detector can be moveable between a first position lying within the diagnostic beam and a second position that lies within the therapeutic beam in at least one relative position of the chassis part and source part； this permits the same detector to be used for either beam.L'invention concerne un appareil de radiothérapie comprenant un support fixe et un support mobile comprenant une partie châssis et une partie source, la partie châssis étant fixée de façon rotative au support fixe pour permettre la rotation de celui-ci autour d'un axe généralement horizo
