In order to assess the achievability of treatment goals for a radiation therapy treatment, an estimation unit generates a treatment plan for the radiation therapy treatment only on the basis of treatment goals relating to a target structure to obtain a first dose distribution, (i) segments the body region into a plurality of concentric shells (301a, 301b) surrounding the target structure (302) and determines a mean radiation dose assigned to each shell (301a, 301b) in accordance with the first dose distribution, and (iii) checks whether a further dose distribution can be determined which fulfills the treatment goals relating to at least one structure at risk and which is configured such that the mean radiation dose assigned to each shell (301a, 301b) in accordance with the further dose distribution corresponds to the mean radiation dose assigned to the same shell (301a, 301b) in accordance with the first dose distribution.
