Radiation therapy system with o-ring linac and rotatable treatment couch

A radiation therapy system comprising:

• a stationary support structure fixed to a support surface external to the radiation therapy system;
• a rotatable gantry with a treatment-delivering radiation source and at least one X-ray imager mounted thereon, wherein the treatment-delivering radiation source directs a treatment beam through a treatment isocenter of the radiation therapy system and wherein the rotatable gantry is rotatably coupled to the support structure and is configured to rotate about a bore of the radiation therapy system;
• and
• a couch-positioning system for rotating a couch about a center of rotation, wherein the couch-positioning system is rotatably coupled to the support surface and the center of rotation is vertically aligned with the treatment isocenter.