1. A method for permitting manipulation of an achievable dose distribution estimate deliverable by a radiation delivery apparatus for proposed treatment of a subject, the achievable dose distribution estimate defined over a three-dimensional range of voxels with a dose value for each voxel, the method comprising: (a) determining a dose modification voxel for which it is desired to modify the dose value and determining a corresponding magnitude of desired dose modification; (b) receiving the achievable dose distribution estimate, the dose modification voxel and the corresponding magnitude of desired dose modification as input to a processor; (c) for each of a plurality of beams: (i) characterizing, using the processor, each beam as a two-dimensional array of beamlets, wherein each beamlet is associated with a corresponding intensity value and a ray line representing a projection of the beamlet into space; and (ii) identifying, using the processor, one or more dose-change beamlets which have associated ray lines that intersect the dose modification voxel; (d) modifying, using the processor, the intensity values of the one or more identified dose-change beamlets based on the magnitude of desired dose modification, wherein modifying the intensity values of the one or more identified dose-change beamlets comprises, for each of the one or more identified dose-change beamlets, modifying the intensity value of each identified dose-change beamlet by a corresponding amount, the corresponding amount based on the magnitude of the desired dose modification; (e) updating, using the processor, the achievable dose distribution estimate to account for the modified intensity values of the identified dose-change beamlets; and (f) outputting, from the processor, the updated achievable dose distribution estimate.