This invention provides a device for checking the performance of an image-guided radiation therapy (IGRT) apparatus. The device (referred to here as a phantom) has a central body with detectable markers, rotatably suspended on a ball joint so that the pitch, roll, and yaw may be adjusted. The body is secured against a base plate, which in turn may be positioned laterally, longitudinally, and vertically within the patient treatment area. Thus, the phantom can be adjusted through six degrees of freedom so as to simulate patient positioning. To perform quality control, the phantom is secured at a predetermined offset, and the position is detected by the IGRT apparatus. The robotic couch is then allowed to compensate, a second measurement is made. The measured values are compared with the predetermined offset to assess both the accuracy in detecting the position of the phantom, and the accuracy of the mechanical correction.