J Setianegara¹*, H Li², Y Hao¹, (1) Department of Radiation Oncology, Washington University in St. Louis, St. Louis, MO 63110; (2) Department of Radiation Oncology, The University of Kansas Cancer Center, Kansas City, KS 66160

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  J Setianegara

PO-GePV-T-374 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To investigate the use of an extended-length phantom for tilt verification measurements (pitch, roll and yaw) of a Varian PitchPerfect 6DoF SRS/SBRT couch and compare its performance (tilt sensitivity) with a smaller Varian marker phantom (5cm smallest edge) that is currently used.

Methods: We selected the TG66 laser phantom as the extended-length (25cm) phantom for this study. CT images were obtained of both phantoms (extended-length and marker) and the isocenter was precisely marked at their geometric centers. Then, each phantom was mounted upon a PitchPerfect couch and aligned to the isocenter of the treatment room using CBCT. Once these phantoms were aligned, the couch’s pitch, roll and yaw were increased simultaneously in three 1.0° increments. Full-arc CBCTs were obtained for each 1.0°/1.0°/1.0° pitch/roll/yaw increments up to total tilts of 3.0°/3.0°/3.0°. In addition, AP-MV and RT-kV orthogonal images were obtained at each tilt increment. The resultant CBCT and orthogonal kV/MV images were then compared with DRRs that were generated from CT images.

Results: Table tilts of 1.0°/1.0°/1.0° will result in a drift of the edge of the blocks on both the orthogonal kV/MV images and the CBCT. Comparing between the axial scans of both phantoms, the Varian marker phantom was less sensitive, measuring an average edge displacement of 0.05±0.01cm, 0.12±0.02cm and 0.15±0.05cm for tilts of 1.0°/1.0°/1.0°, 2.0°/2.0°/2.0° and 3.0°/3.0°/3.0° respectively. The extended-length phantom showed a doubling of the tilt sensitivity with the corresponding displacements on the axial scans measured 0.10±0.02cm, 0.21±0.02cm and 0.31±0.02cm respectively. On the lateral radiograph, peripheral crosshair drifts of the extended-length phantom measured 0.25cm, 0.56cm and 0.86cm respectively while the edges were displaced by 0.60cm, 0.92cm and 1.18cm respectively.

Conclusion: We have demonstrated that the limitations of the Varian marker phantom and the increased sensitivity and suitability of extended-length phantoms for SRS/SBRT daily couch tilt verification measurements.

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