Three-dimensional accuracy and interfractional reproducibility of patient fixation and positioning using a stereotactic head mask system

Purpose: Conformal radiotherapy in the head and neck region requires precis e and reproducible patient setup. The definition of safety margins around t he clinical target volume has to take into account uncertainties of fixatio n and positioning. Data are presented to quantify the involved uncertaintie s for the system used. Methods and Materials: Interfractional reproducibility of fixation and posi tioning of a target point in the brain was evaluated by biplanar firms. 118 film pairs obtained at 52 fractions in 4 patients were analyzed. The setup was verified at the actual treatment table position by diagnostic X-ray un its aligned to the isocenter and by a stereotactic X-ray localization techn ique. The stereotactic coordinates of the treated isocenter, of fiducials o n the mask, and of implanted internal markers within the patient were measu red to determine systematic and random errors. The data are corrected for u ncertainty of the localization method. Results: Displacements in target point positioning were 0.35 +/- 0.41 mm, 1 .22 +/- 0.25 mm, and -0.74 +/- 0.32 mm in the x, y, and z direction, respec tively. The reproducibility of the fixation of the patient's head within th e mask was 0.48 mm (x), 0.67 mm (y), and 0.72 mm (z). Rotational uncertaint ies around an axis parallel to the x, y, and z axis were 0.72 degrees, 0.43 degrees, and 0.70 degrees, respectively. A simulation, based on the acquir ed data, yields a typical radial overall uncertainty for positioning and fi xation of 1.80 +/- 0.60 mm. Conclusions: The applied setup technique showed to be highly reproducible. The data suggest that for the applied technique, a safety margin between cl inical and planning target volume of 1-2 mm along one axis is sufficient fo r a target at the base of skull. (C) 2001 Elsevier Science Inc.