A method for optimizing a treatment plan for use in radiation therapy of a patient volume, wherein the treatment plan defines a set of instructions to be executed by a treatment machine, the method being performed in a treatment planning system and comprising the steps of: obtaining a first quality function, the first quality function yielding an output value based on an input treatment plan, the first quality function being based on one or more dose-volume measures or compositions thereof, wherein the first quality function constitutes an approximation of a second quality function; obtaining a first pair of input values and a slope indicator, the pair of input values associating a value of the first quality function to a value of a score; constructing a score function that maps values of the first quality function to values of the score by fitting a curve to the first pair of input values and the slope indicator; and optimizing the treatment plan with respect to the score function, wherein the optimization is a computer controlled iterative optimization which comprises varying the treatment plan such that the value of the score function is either improved or constrained to a feasible range of score values, wherein the optimization is divided in stages, and the following steps are performed after each stage: calculating at least one pair of calculated values, each pair of calculated values associating a value of the first quality function to a value of the second quality function; updating the first quality function based on a discrepancy between the first quality function and the second quality function at the pairs of calculated values; and adjusting a smoothing parameter for the first quality function, the smoothing parameter for the first quality function controlling deviation between the first quality function and the second quality function and further controlling how smooth the first quality function is. obtaining a first quality function, the first quality function yielding an output value based on an input treatment plan, the first quality function being based on one or more dose-volume measures or compositions thereof, wherein the first quality function constitutes an approximation of a second quality function; obtaining a first pair of input values and a slope indicator, the pair of input values associating a value of the first quality function to a value of a score; constructing a score function that maps values of the first quality function to values of the score by fitting a curve to the first pair of input values and the slope indicator; and optimizing the treatment plan with respect to the score function, wherein the optimization is a computer controlled iterative optimization which comprises varying the treatment plan such that the value of the score function is either improved or constrained to a feasible range of score values, wherein the optimization is divided in stages, and the following steps are performed after each stage: calculating at least one pair of calculated values, each pair of calculated values associating a value of the first quality function to a value of the second quality function; updating the first quality function based on a discrepancy between the first quality function and the second quality function at the pairs of calculated values; and adjusting a smoothing parameter for the first quality function, the smoothing parameter for the first quality function controlling deviation between the first quality function and the second quality function and further controlling how smooth the first quality function is. calculating at least one pair of calculated values, each pair of calculated values associating a value of the first quality function to a value of the second quality function; updating the first quality function based on a discrepancy between the first quality function and the second quality function at the pairs of calculated values; and adjusting a smoothing parameter for the first quality function, the smoothing parameter for the first quality function controlling deviation between the first quality function and the second quality function and further controlling how smooth the first quality function is.