A computer-implemented method for generating a volumetric modulated arc therapy, VMAT, treatment plan that can be used for providing radiation therapy to a patient, the method comprising: receiving, from a user, information indicating a plurality of treatment obj ectives; providing a library including a plurality of alternative plans for a VMAT treatment, the plurality of alternative plans representing a treatment space defined by the plurality of treatment objectives, each alternative plan having an associated outcome for the plurality of treatment objectives; generating a graphical user interface having a plurality of navigation controls operable by the user to navigate within the treatment space, the graphical user interface presenting a dynamically updated current outcome based in part on the outcomes associated with the alternative plans in the library and in part on current settings of the navigation controls; receiving user input via the graphical user interface, the user input indicating that the current outcome is a satisfactory outcome; selecting, based on the satisfactory outcome, a candidate plan from the plurality of alternative plans in the library; and performing a machine parameter optimization on the candidate plan, using the satisfactory outcome, to generate a deliverable plan; characterized in that providing the library includes: receiving, from the user, a seed plan; creating, from the seed plan, a plurality of optimized plans, wherein each optimized plan is optimized with respect to a different one of the treatment objectives by adjusting machine parameters of the seed plan; and including each of the optimized plans as one of the alternative plans in the library. receiving, from a user, information indicating a plurality of treatment obj ectives; providing a library including a plurality of alternative plans for a VMAT treatment, the plurality of alternative plans representing a treatment space defined by the plurality of treatment objectives, each alternative plan having an associated outcome for the plurality of treatment objectives; generating a graphical user interface having a plurality of navigation controls operable by the user to navigate within the treatment space, the graphical user interface presenting a dynamically updated current outcome based in part on the outcomes associated with the alternative plans in the library and in part on current settings of the navigation controls; receiving user input via the graphical user interface, the user input indicating that the current outcome is a satisfactory outcome; selecting, based on the satisfactory outcome, a candidate plan from the plurality of alternative plans in the library; and performing a machine parameter optimization on the candidate plan, using the satisfactory outcome, to generate a deliverable plan; characterized in that providing the library includes: receiving, from the user, a seed plan; creating, from the seed plan, a plurality of optimized plans, wherein each optimized plan is optimized with respect to a different one of the treatment objectives by adjusting machine parameters of the seed plan; and including each of the optimized plans as one of the alternative plans in the library. receiving, from the user, a seed plan; creating, from the seed plan, a plurality of optimized plans, wherein each optimized plan is optimized with respect to a different one of the treatment objectives by adjusting machine parameters of the seed plan; and including each of the optimized plans as one of the alternative plans in the library. <EOS>