We consider an approach to multicriteria radiation therapy optimization where the clinical treatment plan is selected from a representationof the set of Pareto optimal treatment plans in the form of a discrete setof plans and their combinations. The approximate nature of this representation implies that a selected plan in general has an approximationerror with respect to Pareto optimality. To assess and, if necessary, improve the quality of such plans, a technique is suggested that eliminatesthe approximation error of a given treatment plan by a projection ontothe Pareto surface. A more elaborate form of projection is also suggested that requires the projected solution to be not only as good asthe input plan in terms of objective function values, but also equallygood or better with respect to the three-dimensional dose distribution.The versatility of the suggested technique is demonstrated by application to planning for step-and-shoot and sliding window delivery ofintensity-modulated radiation therapy, and planning for spot-scanneddelivery of intensity-modulated proton therapy. Our numerical resultsshow that the proposed projections generally lead to improved sparingof organs at risk and a higher degree of dose conformity compared towhen projections are not performed.
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