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Robust Optimization in Intensity-Modulated Proton Therapy for Base-Of-Skull Cancers

W Liu

W Liu*, R Mohan, X Zhu, D Grosshans, University of Texas, MD Anderson Cancer Ctr., Houston, TX

TH-C-144-4 Thursday 10:30AM - 12:30PM Room: 144

Purpose: Intensity-modulated proton therapy (IMPT) is highly sensitive to uncertainties in beam range and patient setup. Here we evaluated plan robustness and optimality generated by a robust optimization method for a population of base-of-skull cases. The method incorporates uncertainties directly in optimization algorithm to ensure clinical target volume (CTV) coverage to uncertainties.
Methods: Two IMPT plans were generated for each of 10 base-of-skull cases, one PTV-based conventionally optimized and the other CTV-based robustly optimized. For the conventionally optimized plans, uncertainties are considered with PTV margins, while delivering the prescribed dose to PTV. For the robustly optimized plans, optimization was guided to minimize deviation from the prescribed dose under extreme setup and range uncertainties, while delivering the prescribed dose to CTV. For each of these plans, we calculated dose distributions under various uncertainty scenarios. The root-mean-square-deviation-dose (RMSD) for each voxel was computed and the area under the RMSD-volume histogram (RVH) curves (AUC) was used to compare relative plan robustness. Data derived from the dose volume histogram (DVH) in the worst-case and nominal doses were used to evaluate plan optimality. Then the plan evaluation metrics were averaged and compared with paired t tests.
Results: Compared with the conventional optimization, robust optimization led to more robust dose distributions (smaller AUCs) for both targets and organs-at-risks (OARs). In the nominal dose, robust optimization resulted in similar D95% doses (D95% CTV: 63.3 versus 64.8 Gy[RBE] [p<0.01]) and D5% minus D95% (D5% - D95% CTV: 8.0 versus 7.1 Gy[RBE] [p<0.01) for targets and irradiation of OARs was less with robust optimization (brainstem V60: 0.076 versus 0.26 cm3 [p<0.01]). Results in the worst-case dose were consistent.
Conclusions: CTV-based robustly optimized IMPT for base-of-skull cancer provides a significantly more robust dose distribution to targets and OARs and better or equivalent sparing of OARs than PTV-based conventionally optimized IMPT.

Funding Support, Disclosures, and Conflict of Interest: Supported by National Cancer Institute Program Grant P01CA021239, by the University Cancer Foundation via the Institutional Research Grant program at The University of Texas MD Anderson Cancer Center, and by the National Cancer Institute of the National Institutes of Health under Career Award Number K25CA168984

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