Radiation therapy plan determination system for determining a radiation therapy plan for a radiation therapy system comprising a multi-leaf collimator (MLC) , wherein the MLC comprises a plurality of moveable leaves for shaping an aperture of the MLC such that a radiation beam is shaped by the aperture before being provided to a patient , wherein the radiation therapy plan determination system comprises: - a therapy system characteristics providing unit for providing characteristics of the radiation therapy system , wherein the characteristics comprise possible leaf positions defining possible apertures of the MLC and possible radiation fluence values that are providable by the radiation therapy system , - a planning objectives providing unit for providing planning objectives, wherein the planning objectives are indicative of a desired therapeutic radiation dose distribution that should be provided to the patient , - an optimization function providing unit for providing an optimization function being indicative of a deviation of a radiation dose distribution from the planning objectives, wherein the radiation dose distribution depends on a sequence of possible apertures defined by the possible leaf positions and possible radiation fluence values, and wherein the optimization function is further indicative of an uncertainty of the radiation dose distribution at edges of the possible apertures, - a therapy plan optimization unit for determining an optimized therapy plan, wherein the therapy plan optimization unit is adapted to determine a sequence of possible apertures and possible radiation fluence values for which the optimization function is optimized, wherein the sequence of optimized possible apertures and optimized possible fluence values defines the optimized therapy plan. - a therapy system characteristics providing unit for providing characteristics of the radiation therapy system , wherein the characteristics comprise possible leaf positions defining possible apertures of the MLC and possible radiation fluence values that are providable by the radiation therapy system , - a planning objectives providing unit for providing planning objectives, wherein the planning objectives are indicative of a desired therapeutic radiation dose distribution that should be provided to the patient , - an optimization function providing unit for providing an optimization function being indicative of a deviation of a radiation dose distribution from the planning objectives, wherein the radiation dose distribution depends on a sequence of possible apertures defined by the possible leaf positions and possible radiation fluence values, and wherein the optimization function is further indicative of an uncertainty of the radiation dose distribution at edges of the possible apertures, - a therapy plan optimization unit for determining an optimized therapy plan, wherein the therapy plan optimization unit is adapted to determine a sequence of possible apertures and possible radiation fluence values for which the optimization function is optimized, wherein the sequence of optimized possible apertures and optimized possible fluence values defines the optimized therapy plan. <EOS>