binned into 10 respiratory phases based on liver position. For all phases, the binary mask of each volumetric time frame at phase i were summated prior to the normalization to the total number of dynamic volumes at phase i (resulting in a total of 10 probability distribution maps). For each phase i, V 90, i and the SI translational range (R SI, i) of the liver position were evaluated. For each subject, the optimal gating window phase (P w) and the gating window durations (D w) was determined with two approaches based on (1) R SI (P w Z phases with minimum R SI , D w Z neighboring phases of P w with R SI < 2mm); (2) dice similarity coefficient (DSC) (P w Z phase of maximum V 90 (V 90,max), D W Z neighboring phases of P w with DSC > 0.9). The mean R SI of the phase of V 90,max for all VOIs, and the mean minimum R SI (R SI,min) were also reported. Comparison between RSI and DSC approaches was performed using signed rank test. Results: Comparing between approaches, similar P w at exhalation was obtained using R SI (50AE11%) and DSC (LK:46AE24%, pZ1; RK:54AE19%, pZ0.69). The mean V 90,max (LKZ156AE34mm 3 , RKZ149AE26mm 3) was close to the mean organ volume (LKZ160AE34mm 3 ; RKZ154AE27mm 3). The mean R SI of the phase at V 90,max (LKZ2.48AE1.92mm, pZ0.38; RKZ2.41AE1.91mm, pZ1) was similar to R SI,min (2.06AE1.99mm). The choice of gating window using DSC was preferable because a highly repeatable volumetric coverage was obtained, while such volume information cannot be fully revealed by the R SI approach. A smaller D w was obtained using RSI (14AE5%) then DSC (LK:42AE23%, pZ0.016; RK:60AE31%, pZ0.016), though insignificant, might suggest a less effective gated-RT delivery based simply on SI displacement. Conclusion: Larger D w was obtained based on DSC approach allows a more effective gated-RT delivery with the overlapping volume taken into consideration.