Abstract: A radiation therapy imager provides images of a patient while being treated while on a radiation therapy machine for purposes of verification and monitoring of position and alignment, and for shaping of the radiation field. The radiation therapy imager has a video camera mounted on the gantry of the radiation therapy machine and positioned diametrically opposite the treatment head. An elongated light box is disposed over the camera to exclude ambient light from the camera and a fluoroscopic plate is positioned at the distal end of the light box, i.e., the end remote of the camera. The plate is aligned with and facing the head to produce a fluoroscopic image in response to the treatment radiation that is applied from the head through the patient. A mirror in the light box is oriented to reflect the image to the camera to permit monitoring on a viewing screen. The light box can be retracted from under the treatment table either by telescoping the light box down or by swinging the light box to one side.