Abstract: A device for accurately locate the tumor in a radiation therapy machine (RTM) based on the patient's CT image is disclosed. The device immobilizes a patient in a repeatable location when a CT scan is made and the radiation therapy is given. The device includes a base plate to support the patient, on which there are: a pair of pins to constrain the armpits of the patient, a pair of shoulder pads to constrain the shoulders of the patient, a headrest and head pads to constrain the head of the patient, markers, and a jaw restrainer secured to the head pads. Coordinate transformations will be used to locate the tumor in the RTM.