SU‐E‐T‐858: Expanding the Search Space of Multiobjective IMRT Optimization

Abstract

Purpose: To evaluate how more flexibility in the search space affects performance in multiobjective IMRT optimization. Method: Conventional inverse planning combines multiple objectives using a weighted sum. Typical structure‐specific objectives are characterized by a specific reference dose. Optimization with variable weights explores a region of feasible plan space and yields the Pareto front for that region. Fixed references doses and structure‐specific objectives can limit the region explored. Effects of varying weights, reference doses and segmentation of structures were explored. A multiobjective evolutionary algorithm (MOEA) was used to optimize plans for an example prostate case. Both the objective weights and reference doses were optimized in the MOEA and results were compared to optimizing only one parameter. In a second experiment, each voxel throughout structures were allowed different weights and reference doses. Plans were evaluated using decision objectives (OAR mean dose and target variance) which were distinct from the optimized parameters. Results: By optimizing both weighting and reference doses that define the penalty function for each structure, a set of IMRT plans was found that was superior to plans generated by only optimizing weights or by only using optimizing reference dose. Furthermore, when the search space was expanded so that every voxel in every structure was allowed its own reference dose and weight, plans were found that were superior to the best set of plans generated using structure‐specific parameters. Conclusion: Algorithms that donˈt allow all input parameters to be optimized or consider a voxel‐specific search space will not approach the best possible set of plans given the physical limitations of IMRT treatment when judged by clinically relevant and possibly non‐convex metrics. The set of plans generated using a limited search space can potentially be improved in all decision criteria by increasing the scope of the search space.