1. A method for performing intensity-modulated ion therapy, the method comprising: obtaining positional information of structures within or on the patient; identifying a feature of interest to be targeted by the ion therapy, the feature of interest comprising at least a portion of a structure within or on the patient; identifying a volume of interest that includes the feature of interest, the volume of interest being larger than the feature of interest; dividing the volume of interest into a plurality of sub-volumes such that the feature of interest comprises one or more sub-volumes; setting a dose constraint for individual sub-volumes; generating one or more ion treatment plans that satisfy each of the dose constraints of the individual sub-volumes; and delivering ions to the patient using a selected ion treatment plan from the one or more generated ion treatment plans, wherein individual sub-volumes comprise a group of voxels that are within a range of defined distances from the feature of interest, the range of defined distances from the feature of interest being different for different sub-volumes.