SU‐E‐J‐30: The Effect of Planning CT Slice Thickness On Evaluation of SRS Target Localization Accuracy

Abstract

Purpose:To evaluate the effect of planning CT slice thickness on SRS target localization accuracy on Varian Edge SRS treatment platform.Methods:A metal BB of diameter 5 mm was inserted in a Rando head phantom. Planning CTs were scanned with 0.8, 2 and 3 mm slice thickness respectively. The center of BB was setup as isocenter and tattoos were marked on phantom surface. The phantom was initially aligned using laser and tattoos, then a CBCT was acquired and 6‐degree of freedom (DOF) image registration was performed based on bony anatomy. The 6D couch correction was applied. A verification CBCT was taken to evaluate couch corrections. Three orthogonal pairs of images (MV‐MV, KV‐KV and MVKV), were also taken to verify the position of the phantom using a 2D‐3D registration algorithm. Winston‐Lutz tests were taken at 12 combinations of gantry, collimator and couch angles using 1×1cm MLC aperture and 10mm circular cone.Results:The vector lengths of the residual error measured with verification CBCT were 0.3, 0.2 and 0.2 mm for the planning CT of 0.8, 2 and 3 mm slice thicknesses respectively. Residual setup error vector lengths for 0.8, 2, and 3 mm CT measured by orthogonal pair were as following: a) MV‐MV: 0.2, 0.5, 0.5mm; b) KV‐KV: 0.2, 0.3, 0.3mm; c)MV‐KV: 0.1, 0.6, 0.3mm respectively. The vector lengths for the Winston‐Lutz tests were: a) MLC: 0.55, 0.32, and 0.2mm; b) CONE: 0.16, 0.17, and 0.29mm for slice thickness of 0.8, 2, and 3mm respectively.Conclusion:The residual setup error in this study measured by the 2D‐3D algorithm was comparable to that of the 3D‐3D algorithm. The planning CT slice thickness in the range of 0.8 to 2 mm has no significant impact on the evaluation of the residual setup error and the Result of Winston‐Lutz test.