SU‐E‐T‐196: A Quality Assurance Method for Stereotactic Radiosurgical Cones Output Factor Check Using Kodak EDR2® Film

Abstract

Purpose: In the wake of the recent negative publicity for Radiation Oncology, there has been an increase in interest in quality assurance methodology and the frequency of which the methods are performed. Stereotactic radiosurgery procedures are performed in clinics using small cone sizes that introduce complex dosimetry calculations. Measuring small cones output factors are challenging due to the small field size, presence of high dose gradient, as well as lack of spatial resolution of the ion chambers, and diode detectors. The purpose of this study was to compare current clinical cone output factors to the measurement of these same cones using a method of EDR2 film and the RIT Technology high resolution film scanning system.Methods: Kodak EDR2 film was exposed to 200cGy 6MV x‐rays at a depth of 1.5 cm in solid water phantom. The following cones were measured for the BrainLab Novalis linear accelerator: 4, 6, 7.5, 10, 12.5, 15, 17.5, and 20 mm. The cones measured for the Varian Trilogy linear accelerator: 4, 7.5, 10, 12.5, 15, 17.5, 20, 25, and 30 mm. A 10×10 cm2 field size reference field was exposed at the same depth, on the same film, to negate film and developer discrepancies. Using the RIT Technology high resolution film scanning system the maximum film optical density for each cone was measured and relative output factors were calculated. Results: Cone output factor values relative to the 10×10 cm2 reference field for the Novalis linear accelerator differed from the commissioned values by an average 4.14%±2.78% (range of −0.5% to 8.2%). The values for the Trilogy linear accelerator cones differed by 2.77%±2.53% (ranged from −0.50% to 7.00%) Conclusions: EDR2 film dosimetry is a useful tool for checking output factors for SRS small cones on a more frequent basis as an implemented quality assurance method.