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

Abstract

We have developed a densitometric method for measuring the isocentric accuracyand the accuracy of marking the isocentre position for linear acceleratorbased radiosurgery with circular collimators and room lasers. Isocentric shotsare used to determine the accuracy of marking the isocentre position with roomlasers and star shots are used to determine the wobble of the gantry and tablerotation movement, the effect of gantry sag, the stereotactic collimatoralignment, and the minimal distance between gantry and table rotation axes.Since the method is based on densitometric measurements, beam spot stabilityis implicitly tested. The method developed is also suitable for qualityassurance and has proved to be useful in optimizing isocentric accuracy. Themethod is simple to perform and only requires a film box and film scanner forinstrumentation. Thus, the method has the potential to become widely availableand may therefore be useful in standardizing the description of linearaccelerator based radiosurgical systems.