Automatic measurement of beam‐positioning accuracy at off‐isocenter positions

PURPOSE: This study performed an automatic measurement of the off‐axis beam‐positioning accuracy at a single isocenter via the TrueBeam Developer mode and evaluated the beam‐positioning accuracy considering the effect of couch rotational errors. METHODS: TrueBeam STx and the Winston–Lutz test‐dedicated phantom, with a 3 mm diameter steel ball, were used in this study. The phantom was placed on the treatment couch, and the Winston–Lutz test was performed at the isocenter for four gantry angles (0°, 90°, 180°, and 270°) using an electronic portal imaging device. The phantom offset positions were at distances of 0, 25, 50, 75, and 100 mm from the isocenter along the superior–inferior, anterior–posterior, and left–right directions. Seventeen patterns of multileaf collimator‐shaped square fields of 10 × 10 mm(2) were created at the isocenter and off‐axis positions for each gantry angle. The beam‐positioning accuracy was evaluated with couch rotation along the yaw‐axis (0°, ± 0.5°, and ± 1.0°). RESULTS: The mean beam‐positioning errors at the isocenter and off‐isocenter distances (from the isocenter to ±100 mm) were 0.46–0.60, 0.44–0.91, and 0.42–1.11 mm for the couch angles of 0°, ±0.5°, and ±1°, respectively. The beam‐positioning errors increased as the distance from the isocenter and couch rotation increased. CONCLUSION: These findings suggest that the beam‐positioning accuracy at the isocenter and off‐isocenter positions can be evaluated quickly and automatically using the TrueBeam Developer mode. The proposed procedure is expected to contribute to an efficient evaluation of the beam‐positioning accuracy at off‐isocenter positions.