Method for planning an irradiation of a target volume and radiation therapy system

A method of planning an irradiation of a target volume (260) by means of particle therapy comprising the steps of: include obtaining an irradiation plan with at least two radiation fields (210, 210-1), each having a plurality of dose points (230, 230-1, ..., 230-4) in the target volume (260) and with different irradiation directions (211, 211 - -1) are associated, wherein the plurality of dose points (230, 230-1, ..., 230-4) for each of the at least two irradiation fields in isoenergy layers (240) are grouped with the same energy of particles, - for at least a selected radiation field (210, 210-1) of: determining such dosage points (230, 230-1, ..., 230-4) of the selected irradiation field (210, 210-1), in the area (239) no Another dose point (230, 230-1, ..., 230-4) of the remaining at least one irradiation field (210, 210-1) is, as isolated dose points (230-3) wherein the neighborhood is defined with respect to a geometric distance and / or with respect to a dosimetric distance, - for at least a selected radiation field (210, 210-1), depending on the specific dose isolated points (230-3), removing at least one iso-energy layer (240) from the irradiation plan.