Purpose: Conformal radiotherapy in the head and neck region requires precise and reproducible patient setup. The definition of safety margins around the clinical target volume has to take into account uncertainties of fixation and positioning. Data are presented to quantify the involved uncertainties for the system used.
Methods and materials: Interfractional reproducibility of fixation and positioning of a target point in the brain was evaluated by biplanar films. 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 units aligned to the isocenter and by a stereotactic X-ray localization technique. The stereotactic coordinates of the treated isocenter, of fiducials on the mask, and of implanted internal markers within the patient were measured to determine systematic and random errors. The data are corrected for uncertainty 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, respectively. The reproducibility of the fixation of the patient's head within the mask was 0.48 mm (x), 0.67 mm (y), and 0.72 mm (z). Rotational uncertainties 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 acquired data, yields a typical radial overall uncertainty for positioning and fixation 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 clinical and planning target volume of 1-2 mm along one axis is sufficient for a target at the base of skull.