SU-E-T-130: Improved Accuracy for Stereotactic Radiotherapy: An Image-Based Method for Submillimeter Alignment of Linear Accelerator Table Rotation About MV Isocenter

Purpose: Accurate alignment of the linear accelerator table with radiation isocenter is critical for several radiotherapy applications, including stereotactic radiotherapy. We developed a computerized image-based method to align the table rotation axis and the MV isocenter to sub-millimeter accuracy, and observed that alignment was stable over time. Methods: An 8 mm ball-bearing was placed within 0.2 mm of average radiation isocenter using an existing digital star shot alignment methodology. Electronic portal imaging device (EPID) images were acquired between table rotation of −90° to 90° in 30° intervals. A Matlab script was developed to calculate the center of rotation as well as the necessary adjustment to move the table rotational axis to MV isocenter. Adjustment was applied via torque to screws at the linac base. Stability of rotational alignment was measured with 37 measurements over 320 days on two linacs (Linac-A, Elekta Synergy S; Linac-B, Elekta Synergy). Results: Linac-A initially had table alignment inaccuracy greater than 2 mm with respect to MV isocenter. After adjustment, error was significantly decreased from 2.11+/−0.11 to 0.40+/−0.12 mm. This adjustment was stable over the course of 15 measurements over 231 days. Linac-B was not adjusted but tracked from time of commissioning with 20 measurements over 320 days. No discernible shift in couch characteristics was observed (mean error 1.40+/−0.25 mm). The greater variability for Linac-B may relate to the interchangeable two-piece couch, which allows more lateral movement than the one-piece Linac-A couch. Conclusion: Sub-millimeter isocenter alignment was achieved by applying a precision correction to the linac table base. The alignment on two linear accelerators was durable over the course of ten months as assessed by our computerized EPID method. Further investigation on other radiotherapy tables is warranted as this method can significantly improve the quality of stereotactic radiation and other precision techniques.