A method for determining a range of radiation is provided. The method includes defining a target volume to be irradiated using a plurality of voxels; determining, without exposing the target volume to radiation, radiation-attenuating properties that are associated with individual voxels of the plurality of voxels; deriving a range datum from the radiation-attenuating properties; and changing the range datum of a first voxel if the range datum of the first voxel differs from the range datum of two adjacent voxels.