System for radiotherapy provided with a program for the calculation of the intensity of an electrons beam, the system comprising a mobile electrons accelerator, equipped with a conveyor adapted to convey an electrons beam with a presettable intensity on a zone of or internal to a patient, and constituting the target radiotherapy, an images acquisition apparatus, adapted to detect said zone, an integrated control unit, comprising a bidimensional or tridimensional images display and a computer, said computer being connected to said electrons accelerator and to said images acquisition apparatus, and an active sensing device of the radiation dose deposited by said electrons beam, that can be placed between the distal end of said conveyor and said zone, and that can be connected to said integrated control unit, said computer being equipped with a program for the calculation of an intensity of said electrons beam by which said zone can be irradiated, comprising the following steps: (i) setting a first intensity of said electrons beam, according to functioning and geometric characteristic parameters of said electrons accelerator and/or calculated by Monte Carlo method, and according to at least one predefined image of said zone, said intensity being emitted by said electrons accelerator in a first radiation; (ii) operating said electrons accelerator to emit a first electrons beam in said first radiation, corresponding to a fraction of said first intensity set in said step (i); (iii) detecting a real intensity of said first electrons beam during said first radiation, by said radiation sensing device placed in correspondence of said zone; (iv) acquiring at least one image of said zone by said images acquisition apparatus; (v) calculating an intermediate intensity of said electrons beam, according to said at least one image acquired in said step (iv) and according to said real intensity of said first electrons beam detected in said step (iii); and (vi) comparing said first intensity of said step (i) with said intermediate intensity of said step (v) to calculate a final intensity of said electrons beam, by which said zone can be to irradiated in a second radiation following said first radiation. characterized in that the calculation of said final intensity of said step (vi) is performed by the following equation: f i j k = g i j ∑ l = 1 k ρ l Δ z k Δ z where f(i,j,k) is the matrix that represents said final intensity of said step (vi) that can be emitted in said second radiation, index k represents the depth of the medium passed through, g(i,j,l) is the original matrix of the data related to said first intensity set in said step (i), ρ l is the electronic density of the water and Δz is the voxel dimension of said at least one image acquired in said step (iv) along the axis of said electrons beam and i and j represent the starting point of the path of the radiation on said zone. (i) setting a first intensity of said electrons beam, according to functioning and geometric characteristic parameters of said electrons accelerator and/or calculated by Monte Carlo method, and according to at least one predefined image of said zone, said intensity being emitted by said electrons accelerator in a first radiation; (ii) operating said electrons accelerator to emit a first electrons beam in said first radiation, corresponding to a fraction of said first intensity set in said step (i); (iii) detecting a real intensity of said first electrons beam during said first radiation, by said radiation sensing device placed in correspondence of said zone; (iv) acquiring at least one image of said zone by said images acquisition apparatus; (v) calculating an intermediate intensity of said electrons beam, according to said at least one image acquired in said step (iv) and according to said real intensity of said first electrons beam detected in said step (iii); and (vi) comparing said first intensity of said step (i) with said intermediate intensity of said step (v) to calculate a final intensity of said electrons beam, by which said zone can be to irradiated in a second radiation following said first radiation.