Room: AAPM ePoster Library
Purpose: To develop a quality assurance (QA) method to verify isocenter congruence between 3D surface imaging system (3DSI), MV and kV imaging and the radiation isocenter. In addition, radiation isocenter walkouts both at on- and off-isocenter locations are evaluated using single phantom setup.
Methods: Eight image sets obtained on our stereotactic linac are used for this study. The isocenter of 3DSI is aligned to the radiation isocenter by performing isocenter calibration using the AlignRT isocube phantom (VisionRT, UK). Then orthogonal MV and kV images at four gantry angles, and Winston-Lutz test images of all five BBs and also focused only on the central BB are automatically acquired using the Varian TrueBeam Developer mode (Varian, CA). The isocenter shifts of 3DSI, MV and kV images from the radiation isocenter are calculated using the 3DSI software. The radiation isocenter walkout is determined using the RIT software (Radiological Imaging Technology, CO). Radiation isocenter walkouts for the off-isocenter BB's are determined using an algorithm developed in Matlab (Mathworks, MA). This algorithm analyzes the locations of the BBs in each MV image, and then determines deviations between the expected and measured positions.
Results: The mean and standard deviation of radiation isocenter walkouts for the central BB is 0.2±0.1mm and for the off-isocenter BB's are between 0.2±0.5mm – 0.4±0.4mm with a maximum of 1.1mm for the most offset BB. The maximum 3D isocenter shifts of MV, kV, and 3DSI from the radiation isocenter are 0.3, 0.4, and 0.4mm respectively.
Conclusion: Our QA method allows comprehensive verification of isocenter congruence of multiple imaging systems relative to the radiation isocenter with a single phantom setup. Additionally, the radiation isocenter walkouts, including off-isocenter locations, are calculated for multi-lesion SRS treatment with a single isocenter. This QA can be performed periodically or on a stereotactic radiosurgery (SRS) treatment day.