A new concept for the integration of an imaging system into a medical linear accelerator for radiotherapy is presented, and its technical realization is described. An x-ray tube and a flat panel detector were installed along the treatment beam axis, such that the detector could not only be used for x-ray imaging of patients lying on the treatment couch, but also to measure the primary fluence of the therapy beam. The imaging system was synchronized with the linear accelerator by means of several hardware and software components that were developed as a part of this work in order to allow, for instance, the acquisition of fluoroscopic image sequences during beam delivery and the acquisition of volumetric cone beam CT information. Furthermore, external patient monitoring systems were integrated for respiration triggered imaging. Several applications and investigations about image quality, about the application of the images for the concepts of Adaptive Radiotherapy, and about the improvement of image quality were carried out using the new system. Furthermore, first feasibility studies with patients were performed, that combine imaging and therapy at the linac, in order to provide a higher precision for the beam delivery. Finally, the first commercial prototype, following the described concept, is presented
