AbstractID: 9341 Title: Quality Assurance in Stereotactic Radiosurgery (QUASIR) A quality assurance system has been developed for stereotactic radiosurgery and radiotherapy, to fulfill three goals: First to evaluate the geometrical error within imaging, second to simulate the complete chain of a treatment beginning with the imaging and ending with the application of dose and third to verify calculated dose distributions in three dimensions with high spatial resolution and accuracy. A water filled cylindrical imaging phantom has been manufactured in which equally spaced rods are placed. It can be mounted in a stereotactic frame in different orientations to be imaged using CT and MRI. The distances of the rod positions are analyzed and a reference grid can be positioned in order to calculate displacement vectors. To measure 2D dose distributions a drum scanner has been investigated and modified. The spectral emission of the light and the filters together with the efficiency of the red CCD channel were matched and balanced with the absorption spectra of the radiochromic film used. Software has been designed for evaluating the film data, for calibration, and for correction of temperature and darkening. A stack of films can be used to verify calculated dose distributions within an accuracy of up to 3 % (double irradiation technique) and resolutions down to ~ 1/100 mm in film plane.A spherical phantom with shells of different tissue equivalent materials and inserts for measurements using different detectors has been developed to simulate complex treatment situations within the human skull, which allows the complete simulation of the patient’s treatment.