The present invention provides a patient positioning device, which enables the latest advances in Stereotactic Radiation Therapy. The geometry, which incorporates at least one window or lateral cut-out, is designed to allow clear treatment access to the patient during non-coplanar treatment regimes through the use of windows or lateral cut-outs. A laterally positionable diaphragm compression device allows the full use of the geometry. The development of a stand-alone device and a couch top insert device allows for a maximum number of protocol methods while minimizing the skin surface radiation dose to the patient.