Evaluation of a head-repositioner and Z-plate system for improved accuracy of dose delivery

Radiation therapy requires accurate dose delivery to targets often identifiable only on computed tomography (CT) images. Translation between the isocenter localized on CT and laser setup for radiation treatment, and interfractional head repositioning are frequent sources of positioning error. The objective was to design a simple, accurate apparatus to eliminate these sources of error. System accuracy was confirmed with phantom and in vivo measurements. A head repositioner that fixates the maxilla via dental mold with fiducial marker Z-plates attached was fabricated to facilitate the connection between the isocenter on CT and laser treatment setup. A phantom study targeting steel balls randomly located within the head repositioner was performed. The center of each ball was marked on a transverse CT slice on which six points of the Z-plate were also visible. Based on the relative position of the six Z-plate points and the ball center, the laser setup position on each Z-plate and a top plate was calculated. Based on these setup marks, orthogonal port films, directed toward each target, were evaluated for accuracy without regard to visual setup. A similar procedure was followed to confirm accuracy of in vivo treatment setups in four dogs using implanted gold seeds. Sequential port films of three dogs were made to confirm interfractional accuracy. Phantom and in vivo measurements confirmed accuracy of 2 mm between isocenter on CT and the center of the treatment dose distribution. Port films confirmed similar accuracy for interfractional treatments. The system reliably connects CT target localization to accurate initial and interfractional radiation treatment setup.