A method to assist in the selection of optimum beam orientations for radiation therapy when a planning treatment volume (PTV) is adjacent to one or more organs-at-risk (OARs). A mathematical analysis of the boundaries between the PTV and OARs allows the definition of a continuum of pairs of gantry and table angles whose beam orientations have planes that are essentially parallel to those boundaries, and can, therefore, separate the PTV from the OARs when a multi-leaf collimator is used in the therapy. The Radiation Oncologist can then select one or more pairs of gantry and table angles from the continuum as input to a beam optimization step. The selected angles can deliver highly uniform dose to the PTV, while minimizing the radiation dose to the OARs.