A method and an apparatus for planning a treatment beam aimed at at least one target region are provided. Planning parameters having associated predetermined parameter values are for performing a treatment that is to be applied with the treatment beam. The method includes defining a first predetermined parameter value of the planning parameters that is suitable for the irradiation of a treatment region that includes at least one target region. The method also includes ascertaining, based on the first predetermined parameter value, a first probability distribution in relation to an optimizable variable that is prespecified based on deviations to be expected when carrying out the irradiation.
