Pretreatment SBRT Imaging Correlates Equally Well With Multiphase 4DCT and Averaging of 4DCT Simulation

locations. Simulations of combined setup errors showed an increased cord dose sensitivity to rotational errors when combined with translational error. Conclusions: This study is the most detailed analysis of the effect of rotational setup errors in isolation, and when combined with translational setup errors, on the spinal cord dose distribution and mapped cord surface dose. The effects were more significant with greater distance between the isocenter and the 90% isodose on the PRV surface. A linear model was designed to predict dosimetric impact of rotational setup error which can be used to prospectively identify treatment plans that are more likely to be sensitive to even minor rotational setup errors. Author Disclosure: A. Fotouhi Ghiam: None. H. Keller: None. M. Sharpe: None. B. Millar: None. P. Chung: None. D. Jaffray: None. A. Sahgal: None. D. Letourneau: None.