A method of evaluating the robustness of a radiotherapy treatment plan for ion based radiotherapy, comprises the steps of: obtaining information related to the incident energy of ions that will stop in each voxel of the treatment volume； calculating a quantity value representative of the incident energy of these ions； and using the quantity values calculated for the first and second portion as a measure of the quality of the radiotherapy treatment plan, in particular its robustness. The information may include the mean incident energy for all ions, and/or an indication of the distribution of the energy values for all the ions.
