Robustness Quantification and Worst-Case Robust Optimization in Intensity-Modulated Proton Therapy

Intensity-Modulated Proton Therapy (IMPT) is highly sensitive to uncertainties in beam range, patient setup and organ motion. Therefore, it is essential to evaluate the robustness of the IMPT plans against these uncertainties and design robustly optimized plans to improve the plan quality. The root-mean-square-dose volume histograms (RVH) measure the sensitivity of the dose to uncertainties and the areas under the RVH curve (AUCs) can be used to evaluate plan robustness. Results of our research have shown the following. In the worst case and nominal scenarios, robustly optimized plans have better target coverage, improved dose homogeneity, and lower or equivalent dose to organs at risk (OARs). Additionally, robust optimization provides significantly more robust dose distributions to targets and organs than conventional optimization in IMPT. Reduction of PTV and planning directly based on CTV provides better or equivalent OAR sparing. Also 4D robust optimization provides more respiratory-motion-insensitive plans compared to 3D robust optimization.

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