Stability in Frameless Stereotactic Radiotherapy

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The development of fractionated stereotactic radiotherapy (SRT) has been hampered by slow and cumbersome relocation techniques. The Medtronic Sofamor Danek (MSD) frameless system is promoted as a fast, relocatable system. It relies upon the integrity of a bite tray system that is molded to the patient's upper dentition. Precise relocation of the bite tray is the key to accurate external placement of the fiducial reference frame. The optimum construction method for a stable reproducible tray is not described. We undertook a study to identify factors, which might influence the integrity of the bite tray system. Reprosil Fast Set Putty was used to construct three bite tray conditions to include: teeth only, teeth and alveolar sulcus, and teeth, alveolar sulcus, and the hard palate. The influence of dentition was also assessed: full dentition, partial dentition, and edentulous. Stability of the bite tray system was tested by assessing minimum displacement forces from known locations around a centrally placed maxilla jig. Dentition is important to stability, with progressively less displacement forces required for partial and edentulous conditions. Extension of the surface area of the bite tray to include teeth, gums, and also hard palate substantially increased stability across all dental conditions. Impression techniques that incorporate the teeth, alveolar sulcus, and the hard palate, result in significantly increased stability and are recommended to ensure maximal resistance to deviation.

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