On isocentre adjustment and quality control in linear accelerator based radiosurgery with circular collimators and room lasers

We have developed a densitometric method for measuring the isocentric accur acy and the accuracy of marking the isocentre position for linear accelerat or based radiosurgery with circular collimators and room lasers. Isocentric shots are used to determine the accuracy of marking the isocentre position with room lasers and star shots are used to determine the wobble of the ga ntry and table rotation movement, the effect of gentry sag, the stereotacti c collimator alignment, and the minimal distance between gantry and table r otation axes. Since the method is based on densitometric measurements, beam spot stability is implicitly tested. The method developed is also suitable for quality assurance: and has proved to be useful in optimizing isocentri c accuracy. The method is simple to perform and only requires a film box an d film scanner for instrumentation. Thus, the method has the potential to b ecome widely available and may therefore be useful in standardizing the des cription of linear accelerator based radiosurgical systems.