Immobilization system for repeated use in imaging and treating of brain tumors

There is disclosed a patient immobilization system for repeated use in imaging and radiation therapy of a patient, the system comprising an immobilization plate, side rails mounted on the plate with mask studs outwardly projecting therefrom engaging a thermoplastic immobilization mask with at least two anchor bars holding the mask on said studs, wherein the improvement comprises a relatively rigid plastic arch having two ventricle leg portions and an apex, the lower ends of each of the leg portions being mounted on a respective one of the anchor bars; a flap of material outwardly projecting from the mask to a point adjacent the arch; a fixation member for mounting the outwardly projecting flap of material to the arch whereby the mask can be tightly held between the bars and the side rails on the studs and at a third point to the arch. Contrast material is provided at the apex of the arch and in each of the leg portions such that the x, y, and z axes can be shown in three-dimensional images of the patient.