PTV-based IMPT optimization incorporating planning risk volumes vs robust optimization

PURPOSE: Robust optimization leads to intensity-modulated proton therapy (IMPT) plans that are less sensitive to uncertainties and superior in terms of organs-at-risk (OARs) sparing, target dose coverage, and homogeneity compared to planning target volume (PTV)-based optimized plans. Robust optimization incorporates setup and range uncertainties, which implicitly adds margins to both targets and OARs and is also able to compensate for perturbations in dose distributions within targets and OARs caused by uncertainties. In contrast, the traditional PTV-based optimization considers only setup uncertainties and adds a margin only to targets but no margins to the OARs. It also ignores range uncertainty. The purpose of this work is to determine if robustly optimized plans are superior to PTV-based plans simply because the latter do not assign margins to OARs during optimization. METHODS: The authors retrospectively selected from their institutional database five patients with head and neck (H mean dose to the oral cavity: 22.3 robust vs 22.9 PTV+PRV vs 26.1 PTV Gy (RBE); maximum dose to 1% of the normal brain: 66.0 robust vs 68.0 PTV+PRV vs 69.3 PTV Gy (RBE)}. CONCLUSIONS: For H however, target dose robustness and homogeneity were comparable in the PTV+PRV-based and PTV-based optimizations. The same pattern held for the prostate case. The authors' data suggest that the superiority of robust optimization is not due simply to its inclusion of margins for OARs, but that this is due mainly to the ability of robust optimization to compensate for perturbations in dose distributions within target volumes and normal tissues caused by uncertainties.