1. A method for performing intensity-modulated ion therapy, the method comprising: obtaining a representation of a patient, the representation comprising information about structures within or on the patient; identifying a volume of interest in the representation of the patient; dividing the volume of interest into a plurality of sub-volumes; for each of the plurality of sub-volumes, setting a dose constraint; determining one or more ion treatment plans that satisfy the dose constraints for each of the plurality of sub-volumes; from the one or more ion treatment plans, selecting an ion treatment plan that satisfies treatment criteria; and delivering ions to the patient based on the selected ion treatment plan, wherein dividing the volume of interest into a plurality of sub-volumes comprises: dividing the volume of interest into a total number of voxels; identifying one or more features of interest; ordering the voxels according to increasing distance from a nearest feature of interest; for each of the plurality of sub-volumes, selecting a fractional value corresponding to a ratio of a size of the sub-volume to a size of the volume of interest; and for each of the plurality of sub-volumes, defining a sub-volume as a group of a number of consecutive voxels from the ordered voxels, wherein a ratio of the number of consecutive voxels to the total number of voxels is approximately equal to the fractional value for the sub-volume.