Background: For high precision radiotherapy of the neurocranium a precise, reproducible positioning technique is the basic prerequisite. The aim of this study was to assess the influence of a modification of the commercially available stereotactical BrainLab-head mask system on accuracy in patient positioning during fractionated radiotherapy.
Material and methods: 29 patients were treated with stereotactic radiotherapy of the head. Immobilization was provided by a two layer thermoplastic mask system (BrainLab). 18 of these patients received an additional custom made fixation either of the upper jaw (OKF) or of the mandibula (UKF). The positioning accuracy was assessed by measurements of the shifting of anatomical landmarks in relation to the rigid mask system on biplanar simulator films using a digital imaging system. Before each measurement a fine adjustment of the simulator to an optical ring system was performed. The reference radiographs were done just before CT-planning. During a 2-7 weeks lasting course of radiotherapy displacement measurements in relation to the reference images for all three dimensions (z, y and x) were done once a week. In 29 patients 844 measurements were analyzed.
Results: An additional jaw fixation improves the reproducibility of patient positioning significantly in all three spatial dimensions. The standard deviation in lateral direction (x) was 0.6 mm with jaw fixation vs. 0.7 mm without jaw fixation (p < 0.001); in longitudinal direction (z) (measured in 0 degree radiographs) 0.5 mm vs. 1.3 mm (p < 0.001); in longitudinal direction (measured in 90 degrees radiographs) 0.5 mm vs. 1.5 mm (p < 0.001); in vertical direction (y) 0.6 mm vs. 0.9 mm (p = 0.001). No significant differences in standard deviations were found comparing OKF (n = 14) with UKF (n = 4).
Conclusion: A significant improvement in reposition accuracy using an additional, individually formed jaw fixation can be acquired. The variability of positioning can be reduced especially in the z-direction. A further reduction of the safety margin around the target volume--especially in benign tumors--is possible by improved fixation technique.