SU-E-T-563: A Fast and Quantative Picket-Fence Test of a Submillimeter Patient Positioning System for Stereotactic Radiosurgery

Abstract

Purpose: Picket-fence test is a qualitative TG142-recommended quality assurance (QA) test for multileaf collimators. In study, we adopted the same concept and developed a fast but quantatitive QA test for an automatic patient positioning system that requires submilleter accuracy for a radiosurgical treatment. Methods: A piece of radiochromic film was first placed inside a spherical solid water phantom and then irradiated with a sequenence of linearly placed shots of same collimator size (e.g. 4-mm) via the Leskell Gamma Knife Perfexion system (PFX). The shots were positioned with either equal or non-equal gaps of approximately 4 mm to 8 mm depending on the location of the region of interest. A pattern recognization program was developed and then applied to measure the gap spacing between two adjacent shots. The measured distance was then compared with the initial preset values for the test. Results: By introducing variable systematic and random shifts of 0.1 mm to 0.5 mm to the shot sequence, the maximum gap variation from the described test was found to be 0.35 mm or less. On average the positioning uncertainty for the PFX delivery system was found to be 0.1±0.2 mm. No significant difference in the positioning uncertainty was noted for the centrally aligned shot sequence locations versus the peripherally aligned shot sequence locations. Conclusion: A new quantitative picket-fence type test was developed and demonstrated for routine QA of the submillimeter PFX patient positioning sytem. This test also enables independent verification of any patient-specific shot positioning for a critical treatment such as a tumor in brainstem. Dr Ma is currently on the board of international society of stereotactic Radiosurgery