Determining a time-optimized treatment plan for a particle irradiation system by specifying a boundary condition

The invention relates to a method and a device (10) for determining a radiation treatment plan for a particle irradiation unit (20). In this case, depending on a target volume of the treatment plan (6) within an examination object (14; 18) with the particle irradiation unit (20) irradiated with a particle beam (16). The treatment plan is determined, in order to apply the energy of the particle beam (16) in accordance with a predetermined dose distribution in the target volume (6), wherein the target volume (6) and the predetermined dose distribution are given. In addition, a boundary condition for at least one of isoenergy layers (7-9) is specified and the irradiation plan additionally determined so that the boundary condition for the at least one iso-energy layer is maintained. Here, the constraint comprises at least one of the following conditions: • a minimum threshold energy, • a maximum energy limit, • a minimum dot number, • a minimum total number of particles, • a minimum total dose, • a minimum dose contribution to be administered a total dose, • a minimal contribution to the objective function, which is calculated to determine the treatment plan, and • a minimum dose compensation error, wherein the dose compensation error indicates an error that occurs through the non-irradiating the respective isoenergy layer despite appropriate compensation by the irradiated isoenergy layers.