Comparison of the 4D cumulative dose (average) DVH for the GTV when incorporating different numbers of respiratory phases into the calculations (Case 1). By incorporating additional phases, the accuracy of the dose calculation improved. However, the use of just two phases (0% and 50%, the maximum inspiration and maximum expiration respectively based on diaphragm motion) provides a reasonable approximation. The dose difference for the same volume coverage between each of the three averaged DVH curves is less than 0.5 Gy. The lowest GTV coverage occurred at 30%, which is shown in this figure too for reference.