A method of evaluating a radiotherapy treatment plan for ion based radiotherapy, is proposed, comprising the steps ofdetermining multiple elastic scattering of ions for scattering angles in a first angular interval having an upper limit at a selected cut-off angle by means of a model for Coulomb scattering；determining multiple elastic scattering of ions for scattering angles in a second angular interval having a lower limit at the selected cut-off angle；determining the scattering for angles in a range comprising at least a part of the first angular interval and at least a part of the second angular interval, based on the results obtained for the first and second angular interval, respectively.The method avoids the double counting of particles at large scattering angles that occurs when using conventional methods.
