Program Information
A Modified Winston-Lutz Test for Single Isocenter Multiple Targets SRS QA
C Song1*, K Xue2 , R Li3 , T Xue4 , (1) St. Luke's Hospital, Bethlehem, PA, (2) Emmaus High School, Emmaus, PA, (3) St. Luke's Hospital, Bethlehem, PA, (4) St. Luke's Hospital, Bethlehem, PA
Presentations
SU-E-T-45 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:When treating multiple targets with a single isocenter, the distances between some targets and the isocenter may be large. The gantry, couch and collimator angle errors, as well as the isocenter errors, are important for this kind of SRS treatment. We developed a Modified Winston-Lutz (MWL) test to check the combined errors of the isocenter, the aforementioned angles, and MLC position and alignment.
Methods:We use a phantom cube (VisionRt SRS QA phantom) containing 5 ceramic spheres with diameters of 7.5 mm. The maximum distance between two spheres is 8.0 cm, with the central sphere at the center of the phantom. The treatment machine is a TrueBeam STx with HD-MLC. The spheres are contoured on CT images using the Varian Eclipse planning system. We generated eight fields with combinations of gantry angles of 0, 90, 180 and 270, couch angles of 0, 90 and 270, and collimator angles of 0, 45, 90, 270 and 315. The MLC apertures of four fields are fitted to spheres 1, 3 and 5 with 2 mm margins, and the other four fields are fitted to spheres 2 and 4. After CBCT alignment of the phantom, the eight fields are imaged with the MV port imager.
Results:The MLC aperture off-center errors of the central sphere and the maximum off-center errors of the other four spheres are measured on the port images for each field. After eight tests, the errors of the central sphere on each field range from 0.0 mm to 0.6 mm. The maximum errors of the other spheres on each field range from 0.5 mm to 1.0 mm.
Conclusion:We have developed an MWL test for single isocenter multiple target SRS treatments. On the TrueBeam, the errors of targets within 4 cm of the isocenter are 1 mm or less.
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