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PURPOSE To provide a method for optimizing the multi-leaf collimator angle trajectory in volumetric modulated arc therapy (VMAT) in order to make VMAT delivery and planning more efficient. METHODS Static uence maps are optimized at a 10-degree spacing around the patient. Sliding window delivery time of each of these fields is computed for a large set of possible collimator orientations. An optimal trajectory, which selects a collimator angle for each field and assures that the collimator angles do not differ excessively between adjacent fields, is computed by solving a network flow model of a shortest path problem. RESULTS For four clinical cases (two brains, an anal, and a spine) we demonstrate time reductions from 6%-32% (average: 24%) for the optimal static angle versus the worst static angle. Further reductions from 3%-17% (average 9:25%) are achievable when dynamic collimator trajectories are allowed. CONCLUSIONS Dynamic collimator trajectories, which can be computed with an efficient linear programming formulation, can improve the efficiency of VMAT delivery. This article is protected by copyright. All rights reserved.