Systems and methods are provided for determining an angular trajectory for dynamically rotating a multileaf collimator during arc therapy. According to various embodiments, a suitable collimator trajectory may be determined based on the reduction or minimization of a residual unblocked area residing between a planning target volume and leaves of the multileaf collimator in the beam's eye view over the set of control points corresponding to an arc therapy plan. Various example methods are provided for determining collimator trajectories based on whitespace reduction, and for providing quantitative measures of whitespace optimization associated with a given trajectory. In some embodiments, the whitespace may be calculated using terms that account for the overlap of a planning target volume with an organ at risk of exposure.