Abstract: A triple imager arrangement is mounted onto a linear accelerator, providing pre- or post-treatment images of a patient, for verification and monitoring of patient position. Left and right diagnostic radiation sources mounted onto the treatment head. The triple imager arrangement is mounted on the gantry counterweight, with a portal imager and left and right diagnostic image detectors. Image-forming plates of the portal imager and the diagnostic imagers are positioned perpendicular to the therapy beam central axis, and perpendicular to the central axes of the diagnostic sources. The image forming plates are the same given distance X from the respective source. The resulting image information can be used in conjunction with an assistance module for assisting the linear accelerator therapist in accurate adjustment of the patient position.