Patient position reproducibility in fractionated stereotactically guided conformal radiotherapy using the BrainLab (R) mask system

Purpose: Dedicated mask systems nowadays allow the use of stereotactic radi otherapy in fractionated regimes, therefore combining the advantages of hig h precision radiotherapy with the biological benefit of fractionation. Ther efore the knowledge of institution specific isocenter accuracy is essential for decision-making about margins to be allowed to form the planning targe t volume. Patients and Method: Measurements of isocenter deviations during fractionat ed treatments were performed in 33 patients using the simulator Simulix-xy (Oldelft) in connection with the BrainLab(R) angiographic Localizer-box as well as port-films. In both cases repeated images were overlaid by use of a natomical Landmarks with a methodical accuracy in the order of 0.5 mm. Results: Both methods yield random isocenter deviations of Less then 2 mm ( standard deviation) in all three directions and no significant systematic d eviations. These values are in the order of the accuracy of the method, obt ained by comparison of two independent investigators, as well as they are c omparable with the literature. Conclusions: The accuracy of Less than 2 mm indicates safety margins of 3-4 mm as sufficient for clinical routine to cover the target in 95.5% of all set-ups (2 SD).