Author: Nellie Brovold, Wenbo Gu, Huan Amanda Liu, Lin Ma 👨🔬
Affiliation: University of Pennsylvania, Mayo Clinic 🌍
Purpose: To develop a low-cost comprehensive QA phantom and software for single-iso multi-target SRS Linac machines. This phantom can perform 1) Winston-Lutz QA test for on-axis and off-axis targets, and 2) QA for surface-guided radiotherapy (SGRT) system.
Methods: The phantom was constructed by embedding six metal BBs into a piece of head-shaped Styrofoam. The center BB serves as target for on-axis Winston-Lutz test (at isocenter), other BBs for off-axis Winston-Lutz test, and Styrofoam head provides an anthropomorphic surface to QA SGRT system for couch-kick SRS fields. To eliminate the impact of setup errors from off-axis WL test results, we proposed a new workflow. First, we take a pair of KV OBI images at each QA session and calculate 3D coordinates of BBs. Then a new multi-target Winston-Lutz plan is generated based on current BB positions to rule out the impact of setup errors.
Results: The BB identification algorithm was first benchmarked using the SunNuclear MultiMet-WL Cube phantom. Each BB’s position (x,y,z) was calculated using four equations derived from two kV images (2 equations per each) over-determinedly. The calculated positions were accurate within 0.3 mm of vendor specifications. Subsequently, we constructed a custom phantom for under $10 and developed a software to streamline the Winston-Lutz process. The software is organized into three tabs: “BB Identification,” “Generate MLC Plan,” and “Analyze W-L Results.” Users load a pair of kV images in the first tab to identify BB coordinates, which are then used to create a Winston-Lutz plan in the second tab. The third tab allows users to analyze MV port film results.
Conclusion: We designed an SRS machine QA phantom with off-axis targets for Winston-Lutz tests and an anthropomorphic surface for SGRT QA. A user-friendly software was developed to streamline the QA process using this phantom.