1. A radiation inverse treatment planning system for a linear accelerator, comprising: a radiation source, configured for delivering individual radiotherapeutic beams, wherein the beam is comprised of a plurality of beamlets, each individual radiotherapeutic beam having a predetermined location and incidence angle inside and/or outside a target area, a size and a shape, at least a data bus system, a memory coupled to the data bus system, wherein the memory comprises a computer usable program code, and a processing unit coupled to the data bus system, wherein the processing unit is configured to execute the computer usable program code to pre-compute a dictionary composed of a set of individual possible radiotherapeutic beams including, for each individual radiotherapeutic beam its location and its incident angle, associate a weight to each individual radiotherapeutic beam, based on one or more constraints, wherein the processing unit executes the computer usable program code to find the sparsest subset of individual radiotherapeutic weighted beams, so as to satisfy said one or more constraints.